How to Blueprint Engines: Connecting Rods Guide

Connecting rods are placed under more stress than any other component in the engine. Selecting the strongest rod for a performance target is absolutely essential. You have a variety of materials to choose from: powdered metal, forged steel, aluminum, titanium, billet steel, and aluminum. Connecting rods are offered in H- and I-beam configurations, and you have weight, balance, and dimensional factors to consider. Connecting rod bolts are also placed under enormous stress and they must not fail. As we all know, if a connecting rod fails, the engine can be turned to junk in a fraction of a second. So choose wisely and do not select inexpensive rods.

 


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Rod Types

Connecting rods are available in a variety of materials and design processes. Older production rods for passenger car engine applications were typically made of cast iron. For high-performance production in select engines, rods were commonly made of forged steel. Today, the majority of OEM production rods are made of powdered metal (often referred to as PM rods), with select applications using forged-steel rods.

Powdered-Metal Rods

Powdered-metal rods are made in a similar way as casting or forging. A specialized powdered mixture of alloys is placed into a mold, heated (to melt and flow), and then pressurized. The process results in a surprisingly strong product that requires only big-end and small-end honing and bolt thread tapping (no additional exterior finishing is required). Instead of having separate rod body and cap (as with cast or forged rods), PM rods are manufactured as one piece.

A series of machining steps creates the pin bore, crank bore, and bolt faces. The cap is also separated by a cut, faced and attached in order to finalize the big-end dimension.

 

A billet connecting rod provides far greater strength than a cast-iron rod.

A billet connecting rod provides far greater strength than a cast-iron rod.

 

After the powder mix is formed in a die, the mix is heated to more than 1,500 degrees F and forged with more than 750 tons of pressure. The powdered-metal forging is then heat treated and finishmachined. Rods are seen here on their way into the heat-treating oven. (Photo Courtesy Howards Cams)

After the powder mix is formed in a die, the mix is heated to more than 1,500 degrees F and forged with more than 750 tons of pressure. The powdered-metal forging is then heat treated and finishmachined. Rods are seen here on their way into the heat-treating oven. (Photo Courtesy Howards Cams)

 

Once a PM connecting rod is formed in a mold, the cap is created by breaking it off in a holding fixture. This is also referred to as a snapped-cap design. While this process creates a very precise cap-to-rod mating, a PM rod cannot be resized by traditional methods. If resizing is required, it possible to simply hone the big ends larger and select an oversize-OD bearing.

Once a PM connecting rod is formed in a mold, the cap is created by breaking it off in a holding fixture. This is also referred to as a snapped-cap design. While this process creates a very precise cap-to-rod mating, a PM rod cannot be resized by traditional methods. If resizing is required, it possible to simply hone the big ends larger and select an oversize-OD bearing.

 

This close-up of a powdered-metal “cracked cap” rod’s cap mating surface shows the irregularity of the surface. The two fractured surfaces align perfectly when the cap is installed, providing precise cap register.

This close-up of a powdered-metal “cracked cap” rod’s cap mating surface shows the irregularity of the surface. The two fractured surfaces align perfectly when the cap is installed, providing precise cap register.

 

Close-up of a powdered-metal crackedcap rod-mating surface. When dealing with powdered-metal rods, be careful. do not disturb the irregular mating surfaces. If you blast them or touch them with a file, or drop the rod onto the floor, you ruin the mirror-image mating surfaces, and the rod becomes worthless. The profile of these mating surfaces (on rod saddle and cap) must remain absolutely intact and undisturbed.

Close-up of a powdered-metal crackedcap rod-mating surface. When dealing with powdered-metal rods, be careful. do not disturb the irregular mating surfaces. If you blast them or touch them with a file, or drop the rod onto the floor, you ruin the mirror-image mating surfaces, and the rod becomes worthless. The profile of these mating surfaces (on rod saddle and cap) must remain absolutely intact and undisturbed.

 

A powder-forged rod eliminates most of the machining and forming steps, since the initial forged shape is extremely close to all final dimensions. Fracturing the rod end creates the cap. The rod is held in a fixture, the parting line area is scored, and the cap is literally snapped off.

Unlike a rod and cap that have each been machined flat at their mating surfaces, PM rods have uneven surfaces at the mating areas. The benefit of this is that it creates a perfect mating between rod and cap, since no material is lost during the separation. The cap fits to the rod precisely (mirror-image surfaces). The cap is now dedicated to its original rod, and when mated and the bolts are tightened the parting line is invisible to the naked eye. This provides perfect cap-to-rod alignment, with no wiggle room and no need for locating slots or keys.

Aside from saving on production costs, the irregular mating surfaces created when the cap is severed provides a precision mating of cap to rod that perfectly aligns the cap during assembly. The metallurgy of the rod prevents elongation of the bore during the fracturing process, so there’s no concern for creating an out-of-round, big-end bore. Also, due to the compacted powdered metal construction, no stress cracks or weak areas are created when the fracture takes place.

The irregular mating fracture surfaces provide an “intimate interlock” between rod and cap. This virtually eliminates cap shift (rotation of the cap relative to the rod) and lateral movement of the cap relative to the rod. Cap shift can lead to accelerated wear of bearing surfaces and, in extreme cases, bearing seizure. Lateral movement can result in high shear stress on connecting rod bolts at high engine speed (RPM).

OEM PM rods are usually okay for up to about 400 hp. Beyond that, move up to forged rods. The downside of PM rods is that they should not be reconditioned in the traditional manner. In fact, resurfacing the mating ends reduces big-end diameter and creates an out-ofround condition, and then they need to be rehoned round to the specified diameter. PM rods don’t have enough material to cut in this manner. However, depending on the engine application, you can simply hone the big ends to an oversized bore dimension and then install oversizeOD bearings. If a PM rod is damaged, you often need to pitch it and buy another. They’re not designed to be reconditioned.

 

Because the cap-to-rod mating surfaces are precise mirror images, once the cap is fully tightened, the parting lines virtually disappear.

Because the cap-to-rod mating surfaces are precise mirror images, once the cap is fully tightened, the parting lines virtually disappear.

 

Although PM rods have traditionally been used only in OEM engines, you’re now also starting to see high-performance aftermarket PM rods. Howards Cams, as an example, teamed up with GKN and now offers forged, powdered-metal rod technology with an extremely dense and non-directional grain structure.

A high-tech base powder is blended with select alloy elements. Melting, atomizing, and annealing are controlled to exacting standards. The metal mix is compacted (in dies) under tremendous pressure, at more than 1,500 degrees F. Hot forging with a 750-ton press finalizes the structure of the metal.

This new generation of PM rods represents a hybrid of PM and forging. Although OEM-level PM rods typically withstand about 400 hp, Howard’s new PM rods with 5/16-inch ARP 2000 rod bolts are capable of withstanding more than 585 hp, but they require L19 rod bolts. These rods have even survived at more than 800 hp.

The OEM PMs are a practical choice for a street engine up to about 500 hp. I recently built a 5.3L iron-block LS engine that I overbored to 327 ci and assembled using stock OEM powder-metal rods on a stock crank. In carbureted form (single 650-cfm), the engine pulled 425 hp and had no issues whatsoever. If you’re planning on any high-performance build, forged aftermarket rods from Scat, Eagle, Lunati, Callies, Crower, Oliver, etc. are good choices. If you’re going to use PM rods, you shouldn’t expect to pull more than 400 hp and your engine will be reliable.

Reconditioned PM Rods

A major concern when trying to recondition rods with a cracked cap in the traditional manner relates to the irregular mating surfaces of the rod and cap. This irregular surface provides an accurate locating of the cap to the rod, preventing any misalignment of the cap during assembly.

However, if the mating surfaces are machined or ground flat to reduce the rod’s large-end bore in preparation for rehoning to size, all centering ability is lost. This destroys the unique cap-torod interlock. Since no interlock mating remains because there are no positioning tangs to use, it’s possible to install the cap slightly off-center, due to the small tolerance range of the bolts to the cap’s bolt holes. As a result, the cap might be installed off-center left-to-right (laterally), or at an angle relative to the axis of the large-end bore. If only for this reason, it is not advisable to reface the rod and cap mating surfaces.

Another reason that resizing these rods can create a problem is the relatively thin cap material. Once the mating surfaces are ground flat, the new smaller and non-round large-end bore may require so much enlargement in order to create a round hole that the cap material may be reduced enough to create a potential weak area. Note that in the process of creating flat mating surfaces, it may be necessary to reduce the mating surfaces by .040 inch or more, which could result in a combined reduction of the hole by .080 inch or more. Precious little cap material may be left after resizing.

If resizing is necessary, do not disturb the irregular cracked mating surfaces. Instead, hone the big end to an oversize dimension to accommodate fitting oversized-OD rod bearings. However, oversized bearings are not available for all cracked cap applications. You may find them with a standard-size ID and a .010 inch larger OD; or in an undersized ID (to accommodate an undersized ground crank) and an oversized OD, to accommodate an enlarged connectingrod big end.

If an oversized OD bearing is not available for your particular application, replace it with a new rod. In some cases a new rod may not be available as a separate piece. A case in point is the Dodge/ Plymouth 2.0 SOHC and DOHC engines, where a complete rod/piston/pin assembly must be purchased from the automaker because individual rods or pistons are not offered. So, in a situation where a rod’s big end has been distorted and an oversize OD bearing is not available, you are forced to pay for a complete rod/ piston/pin assembly, even if the original piston is perfectly serviceable.

 

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High-quality aftermarket connecting rods, whether forged or billet, commonly are supplied with high-grade, high-tensilestrength rod bolts. Generally, rod manufacturers supply a torque specification as well as a rod bolt-stretch range, providing a choice of method for you to follow. Even when using OEM cast, forged, or powdermetal rods, if you’re using quality aftermarket rod bolts that are supplied with a stretch recommendation, tightening by stretch (rather than torque or torque-plusangle) is possible.

High-quality aftermarket connecting rods, whether forged or billet, commonly are supplied with high-grade, high-tensilestrength rod bolts. Generally, rod manufacturers supply a torque specification as well as a rod bolt-stretch range, providing a choice of method for you to follow. Even when using OEM cast, forged, or powdermetal rods, if you’re using quality aftermarket rod bolts that are supplied with a stretch recommendation, tightening by stretch (rather than torque or torque-plusangle) is possible.

 

Forged Rods

Forged steel rods start as an ingot, or billet, of alloy steel. In fact, performance aftermarket forged rods are usually made from 4340 chrome-moly steel. A steel ingot is usually heated in an oven to about 2,200 degrees F during conventional steel forging, at which point the steel is very formable. The ingot is then placed in a forging die and squeezed into the approximate shape of the desired profile. A hammering or pressing process performs this squeezing with as much as 240,000 pounds of pressure. This increases the strength of the alloy by compacting, tightening, and aligning the molecules. The size of the ingot is much larger than required in the die, so it starts with an ingot that weighs about twice as much as the desired final product. During the forging/compacting process, the excess material is forced out of the die at its mating lines. This excess is later sheared off in a trimming die. Depending on the manufacturer, the rods may be induction hardened, shot peened and/or cryogenically stress relieved, and heat treated. Individual manufacturers often employ their own proprietary formulas.

 

Each rod bolt is first positioned on the tool’s anvils and the gauge is zeroed. This provides a reference point (while the bolt is in its relaxed state). A GearHead stretch gauge is shown here. Notice the convenient thumbhole on the billet housing. During test fitting, final assembly, and any future teardowns, always keep the rod bolts with their original rod (keep them organized), so that you can remeasure each bolt to determine if it’s still usable and hasn’t lost some of its original elasticity.

Each rod bolt is first positioned on the tool’s anvils and the gauge is zeroed. This provides a reference point (while the bolt is in its relaxed state). A GearHead stretch gauge is shown here. Notice the convenient thumbhole on the billet housing. During test fitting, final assembly, and any future teardowns, always keep the rod bolts with their original rod (keep them organized), so that you can remeasure each bolt to determine if it’s still usable and hasn’t lost some of its original elasticity.

 

I-beam rods have a recess on the beam faces. The strength of H-beam versus I-beam rods is often debated, as well as the theoretical characteristics of each beam design to sling parasitic oil during engine operation. For extreme engine speed and for increased cylinder pressure applications, such as nitrous/forced induction, H-beam designs are often recommended. Major rod manufacturers usually offer both styles.

I-beam rods have a recess on the beam faces. The strength of H-beam versus I-beam rods is often debated, as well as the theoretical characteristics of each beam design to sling parasitic oil during engine operation. For extreme engine speed and for increased cylinder pressure applications, such as nitrous/forced induction, H-beam designs are often recommended. Major rod manufacturers usually offer both styles.

 

Unlike OEM cast or forged connecting rods, high-quality aftermarket performance rods are manufactured to a much higher level of precision and uniformity. Note that this H-beam rod’s cap has strengthening ribs but no weighty “balance pad.” Today’s quality performance rods are made to such tight dimensional and weight tolerances that it’s extremely rare that any balance-correction work is needed, hence the absence of extra metal (that is, the pad) on the rod cap. Balance pads are common on older OEM rods, providing an area of mass that can be reduced in order to properly balance a set of rods.

Unlike OEM cast or forged connecting rods, high-quality aftermarket performance rods are manufactured to a much higher level of precision and uniformity. Note that this H-beam rod’s cap has strengthening ribs but no weighty “balance pad.” Today’s quality performance rods are made to such tight dimensional and weight tolerances that it’s extremely rare that any balance-correction work is needed, hence the absence of extra metal (that is, the pad) on the rod cap. Balance pads are common on older OEM rods, providing an area of mass that can be reduced in order to properly balance a set of rods.

 

The forged rods are rough shaped and must be trimmed then quenched and tempered. Before being machined, they should be tempered because the process can alter the shape of the part. The rods can deform by as much as .060 inch. Although the process may differ from manufacturer to manufacturer, the rod is typically quenched in a glycol solution. Once it has been quenched, the rod is finish machined to attain its final shape. Then the rod is put through stress relieving so it is resistant to the formation of stress cracks. These parts are baked in an oven at 400 to 600 degrees F to remove stresses that occurred during the machining process. During this process, carefully controlled heating and cool-down cycles properly cure the metal. The rod is then final-machined for small- and big-end bore size. And last, the final surface hardness is set.

When examining forged rods, you may find what looks like a large parting line. In fact, this is not a parting line. This line was established when the excess steel was pushed out of the die, and then it was cut off after forging. Ultimately, final machining was performed but this line remained. In some cases, hot-hammering or pressing squeezes the malleable steel out of the die, and then machining removes the excess. As a result, no evidence of a trim line remains.

In other cases, the trim area may not have been machined as closely, so you can faintly see the line in the trim area. Forged parts may slightly show that a die was used during the manufacturing process. Some evidence of a trim area is common and does not create any issues. Most quality aftermarket forged rods are machined with high precision and, in the process, any parting line traces are eliminated.

 
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Aluminum Rods

Forged aluminum rods are usually made from 7075 or 7075-T6 aluminum alloy. They may be made from forged flat blanks or forged aluminum that has been extruded. A common belief is that aluminum rods have a relatively short lifespan (due to fatigue) and are not suitable for street applications where routine teardown doesn’t take place. But that is not true. Aluminum rods can be used for the street.

 

This is a BME aluminum rod for a small-block Chevy. Note the precision polished surfaces (this eliminates stress risers and aids in tossing-off parasitic oil-cling). Also note the precision-cut mirror image serrated mating surfaces for cap-to-rod registering. (Photo Courtesy Bill Miller Engineering)

This is a BME aluminum rod for a small-block Chevy. Note the precision polished surfaces (this eliminates stress risers and aids in tossing-off parasitic oil-cling). Also note the precision-cut mirror image serrated mating surfaces for cap-to-rod registering. (Photo Courtesy Bill Miller Engineering)

 

Billet Rods

Billet rods come in alloy steel and aluminum. With the capabilities of today’s CNC machining, it’s now possible to machine connecting rods from raw stock. However, the stock is actually a dense-grain steel that has been made by a forging process. So in reality, billet steel rods use forged steel that is then CNC machined to final state.

Billet rods are more expensive than forged rods due to the higher cost of the steel alloy and the machining time. Since these rods are CNC machined, they are manufactured to very precise weight, dimensions, and specifications.

Titanium Rods

Titanium has an incredible strength-to-weight ratio. It is billet-machined from Ti6AL4V stock, and is about 33 percent lighter than a comparably sized forged steel rod. As an example, a complete titanium rod may be lighter than only the big end of a comparably sized steel rod. Lighter reciprocating weight translates into quicker revs and more power because of reduced parasitic mass. Titanium rods are much more expensive than forged steel.

These rods reduce rotating mass, which is a discernible advantage at engine speeds in excess of 5,000 rpm or so. In a race engine that has real benefits, but in a street engine, it’s really a waste of money. Also, titanium is a fragile material that is sensitive to scratches. Small scratches in the surface can lead to stress cracks that can lead to rod failure.

Titanium from a friction/machining standpoint is rather “gummy” and often galls when rubbed. The concern comes at the rod’s big-end sides. To prevent this condition, titanium rods must be polished and/or coated with a hard-surface coating, such as chromium nitride (see Chapter 6 for more details).

Relative Material Cost

Forged or billet steel is suited for the vast majority of street and race applications. Where further weight reduction is desired, forged/billet aluminum rods are available at a higher cost.

For high-RPM engines where weight savings are really critical, titanium rods are often the best option but they tend to fatigue more than steel over time. In racing use, they need to be replaced more frequently compared to steel rods.

Titanium rods are also very expensive, which is a major factor when you’re on a real-world budget.

Aluminum rods are lighter than steel and nearly as light as titanium but cost less than titanium and more than steel. Aluminum rods tend to be more on the chunky side and generally require more block clearance.

 

Center-to-Center Length

Center-to-center (CTC) length is the actual distance from the center of the rod pin bore to the center of the rod bearing bore. (See Chapter 5 for details on calculating this dimension.)

Rod length is a factor in determining the combination required to achieve a specific stroke, relative to the block deck height.

When blueprinting, you want an equal TDC location for each piston to obtain an equal compression ratio in each bore location. During test fitting, install the crank, rods, and pistons with bearings, but without rings. Slowly rotate the crank to bring each piston to TDC and measure the distance from the top of the piston’s compression deck to the block deck surface.

 

A rod bore gauge (the gauge shown here is on a Sunnen rod-honing machine) allows accurate and quick measurement of the rod-bearing bore diameter as well as allowing checking for bore runout. After calibrating the bore gauge to the desired bore diameter specification, the rod’s big end is placed onto the gauge to verify bore diameter. This reveals whether the bore is correct, undersized, or oversized.

A rod bore gauge (the gauge shown here is on a Sunnen rod-honing machine) allows accurate and quick measurement of the rod-bearing bore diameter as well as allowing checking for bore runout. After calibrating the bore gauge to the desired bore diameter specification, the rod’s big end is placed onto the gauge to verify bore diameter. This reveals whether the bore is correct, undersized, or oversized.

 

Slight deviations in tolerances of the crank, rods, and pistons may result in differences in TDC height. By swapping rods to other cylinder locations, you can optimize the components. For example, mix and match the rods/pistons until you obtain the most equalized dimensions. Yes, this is time consuming and nit-picky, but that’s part of blueprinting: the attempt to optimize all dimensions, weights, and clearances.

 

Piston-to-Rod Clearance

The small end of the rod must not touch any part of the piston. On the bench, test assemble each rod to its piston with wrist pin and check the clearance between the top of the rod to the underside of the piston. This generally isn’t an issue unless you’re running OEM stock rods with big balance pads at the top of the rod and non-stock pistons. Pivot the rod small end on the wrist pin and make sure that there is adequate clearance at the underside of the piston. Even considering thermal expansion, you should have at least .080-inch clearance.

Also check the clearance between the rod small end and the piston pin bosses (where the rod slides on the wrist pin). Even if you have clearance on the bench, that’s no guarantee that you will have clearance when the package is installed. In many engine designs, the rod beam is not centered under the piston (slight offset) when the rod is positioned onto the crank.

Test install the rod/piston onto the installed crankshaft. Rotate the engine block on your stand upside-down. Work the rod back and forth to see how close the small end of the rod gets to the pin bosses. You should have at least .060- to .080-inch clearance. If the small end touches the pin boss, mill a bit from the pin boss or narrow the rod small end to accommodate this.

Clearance issues more commonly occur when using aluminum rods because they are thicker. Never have the crankshaft balanced until all prefitting and clearancing has been accomplished.

 

Rod-to-Block Clearance

This should only be a concern when using a stroker crankshaft (or possibly when using thicker aluminum rods). With the crank installed and the pistons/ rods test installed, slowly rotate the crank to inspect rod big-end clearance at the bottom of all cylinders. If rods touch or if the clearance is too tight, mark the block and grind material to obtain clearance. Generally speaking, rod-to-block clearance should be at least .080 inch.

 

Rod-to-Camshaft Clearance

As the crank rotates and the rods travel toward TDC, the big ends of the rods approach the camshaft. Especially with a long-stroke crank, high-lift cam, and/or thicker steel or aluminum rods, there is a concern that the rods might hit the cam lobes. During test fitting, slowly rotate the crank and watch for rod-to-cam interference. If you feel resistance, stop. Using a slim flashlight, you may be able to visually check for clearance.

The camshaft rotates at half the speed of the crankshaft, so be sure to check through at least four crankshaft revolutions. An aid in checking clearance is to apply a strip of .125-inch-thick clay to the side of the rods, at the lower beam to about the rod cap parting line. Be sure to clean the rod surface to remove any oils before attaching the clay.

When you rotate the crank, any area tighter than the thickness of the clay imprints and provides a witness mark. If you find any contact marks, use a razor blade to cut out the section of clay and carefully measure the compressed clay thickness. You want about .060 inch of clearance. If the rod dead-stops against cam lobes, remove rod material at the contact area, re-install, reclay, and recheck clearance.

If more clearance is required, you have two choices: remove material from the rod or change to a camshaft with a smaller base circle. If you change to a cam with smaller base circle, you need longer pushrods. Obtaining such a cam was once a real hassle, requiring much expense and time. But with today’s CNC capabilities, most cam manufacturers can produce what you need relatively quickly.

If you opt to relieve the rods, do it carefully to avoid weakening the rod bolt’s female threaded areas. Also, the rod needs to be free of sharp edges and grind/scratch marks. After grinding the necessary relief, reassemble and recheck clearance. The ground area must be carefully polished to remove any potential stress risers.

Again, never perform crank balancing until after all clearancing has been verified. Remember to mark all rods and their caps with bore location numbers. Once a rod has been fit and clearance verified to a specific bore location, keep it in that location.

 

Rod Bearing Clearance

Measure the crankshaft rod journal diameter with a micrometer. Don’t just rely on the published specs that came with the crank. Record this diameter for every rod journal. Never assume that all journals were ground identically. Install a pair of new rod bearings to the rod and cap. Make sure that the rod and cap saddles are clean and dry first—no oil should be between the bearing and saddle. Install the rod cap and, in a rod vise, tighten both rod bolts to the specified value (whether you’re using torque or tightening by stretch).

Measure the crankshaft’s rod journal using a micrometer and record the reading. Install the rod bearings in the connecting rod and fully tighten the rod cap to spec. Calibrate a dial bore gauge to the recorded diameter of the rod journal. Use the bore gauge to check the installed bearing diameter in the rod. Any difference in the bore gauge (plus or minus) reveals the existing oil clearance. Be sure to measure the bearing diameter at the 12-o’clock and 6-o’clock positions, at 90 degrees to the parting line.

Assuming that you’ve already verified that the rod big-end bore is within specification, if oil clearance is too tight or too loose, undersize or oversize bearings are available for most applications.

Be sure to inspect rod-bearing edge clearance to the crankshaft fillets. Rather than trying to view this with the crankshaft installed to the block, position the crankshaft on a pair of clean V-blocks on a bench. Install the rod bearings to the rod saddle and cap. Before installing the rod to the crankshaft, use a felt-tip marker to paint the edge of the bearing that faces the journal fillet.

Assemble the rod to the crankshaft’s rod journal. Position the rod fully against the fillet and rotate the rod back and forth against the fillet. Remove the rod from the crankshaft and inspect the marked edge for witness marks that indicate whether the bearing was rubbing against the fillet. If so, the bearing edge can be carefully relieved with a bearing scraper tool.

 

Rod Side Clearance

Side clearance refers to the front and back clearance of a pair of rod big ends on the crankshaft’s rod journal. With the crank installed in the block, with a pair of rods and pistons installed, and with the rods on a shared journal, push the rod big ends apart (shoving both against their fillets and creating a gap between them). Use a feeler gauge to measure clearance. An acceptable range for side clearance is around .014 to .020 inch for steel rods, and perhaps a bit more for aluminum rods (considering theoretical expansion rates), around .017 to .022 inch. Always check to see what the manufacturer recommends.

 

Never assume that rod side clearance is correct, whether you’re dealing with new or used cranks and/or rods. Always check side clearance by inserting a clean feeler gauge between the two rod big ends. Be sure to pull the rods apart by hand while inserting the gauge. Always refer to side clearance specifications for the engine at hand, but in general terms, you should have at least .012 inch or so clearance.

Never assume that rod side clearance is correct, whether you’re dealing with new or used cranks and/or rods. Always check side clearance by inserting a clean feeler gauge between the two rod big ends. Be sure to pull the rods apart by hand while inserting the gauge. Always refer to side clearance specifications for the engine at hand, but in general terms, you should have at least .012 inch or so clearance.

 

Beam Design

Connecting rods are usually of I- or H-beam construction. The designations refer to the shape of the rod beam cross section. I-beam rods have a smooth, solid surface at the beam sides and a recess along each side of the beam faces. H-beam rods have flat, solid surfaces on the beam faces and grooves (recesses) along the beam sides.

Which style is better? In theory H-beam rods are stronger but, in reality, an H-beam rod can be lighter while being as strong as an I-beam rod. From a windage standpoint (oil clinging to the rod during engine operation) the I-beam is theoretically better. However, there are exceptions because specialty or “oil shedding” coatings can be applied to promote less parasitic oil cling to any style rod. In many cases, choosing between I-beam and H-beam boils down to manufacturer availability and/ or engine builder preference.

 

All connecting rod big ends feature a flat-machined surface on one side and a chamfered edge on the opposite side. A flat side is shown in this example. The chamfered side always faces the crankshaft journal fillet, and the flat sides always face each other (rod to rod) on the shared journal. Also, never swap rod caps among your rod set. The original cap that was mated to the rod by the manufacturer must always remain with the same rod. Rod manufacturers usually help by laseretch matching numbers on both the rod and the cap to ease identification.

All connecting rod big ends feature a flat-machined surface on one side and a chamfered edge on the opposite side. A flat side is shown in this example. The chamfered side always faces the crankshaft journal fillet, and the flat sides always face each other (rod to rod) on the shared journal. Also, never swap rod caps among your rod set. The original cap that was mated to the rod by the manufacturer must always remain with the same rod. Rod manufacturers usually help by laseretch matching numbers on both the rod and the cap to ease identification.

 

Example of an X-beam rod. This is one of Howards Cams new Extreme Duty X-beam rods for import turbo applications. Notice the lightening groves on both the beam faces and sides. This provides reduced reciprocating weight while maintaining rod strength. (Photo Courtesy Howards Cams)

Example of an X-beam rod. This is one of Howards Cams new Extreme Duty X-beam rods for import turbo applications. Notice the lightening groves on both the beam faces and sides. This provides reduced reciprocating weight while maintaining rod strength. (Photo Courtesy Howards Cams)

 

Shown here is the chamfered side of the same rod big end. Notice how pronounced the chamfer is on this side. The chamfered side must face the crank fillet (the area where the rod journal meets the crank’s counterweight base).

Shown here is the chamfered side of the same rod big end. Notice how pronounced the chamfer is on this side. The chamfered side must face the crank fillet (the area where the rod journal meets the crank’s counterweight base).

 

Each rod bolt is first positioned on the tool’s anvils and the gauge is zeroed. This provides a reference point (while the bolt is in its relaxed state). A GearHead stretch gauge is shown here. Notice the convenient thumbhole on the billet housing. During test fitting, final assembly, and during any future teardowns, always keep the rod bolts with their original rod (keep them organized), so that you can remeasure each bolt to determine if it’s still usable and hasn’t lost some of its original elasticity.

Most performance connecting rods feature floating pin designs, which allow the wrist pin to freely rotate in both the piston pin bore and the rod small-end bore. The rod big end has a bronze bushing.

 

Another style of rod is the X-beam, which is available for various automotive gas-engine applications as well. The X-beam is sort of a mix of both I-beam and H-beam, and it has weight-saving grooves on the beam faces and the beam sides. This provides a substantial weight savings, while also increasing the beam surface area, offering lighter weight while retaining strength.

 

Rod Bolt Tightening

One of the easiest and least expensive ways to ensure longevity of connecting rods and rod bearings is to use only the highest-quality rod bolts. That means that you should buy ARP, A1, or another performance brand.

Rod bolts can be tightened in one of three ways: using a torque-plus-angle method, torque application, or monitoring bolt stretch.

Regardless of which tightening method you choose, it’s a good idea to take advantage of a stretch gauge. Regardless of what type of rod bolts you have (OEM or aftermarket), first measure and record each rod bolt’s overall free length (when new and uninstalled). Be sure to record which rod each bolt will be installed to (cylinder number-1, -2, etc.). During any future engine teardowns (or when the opportunity arises), remeasure each bolt’s free length and compare it to the original (new) free length that you recorded. If the bolt has elongated (stretched) by more than .0005 inch, replace the bolt because it has begun to lose its elastic properties. Never assume that a used rod bolt is still serviceable.

Torque-Plus-Angle Tightening

The torque-plus-angle method is only to be used on OEM rod bolts, when the specifications call for torque-plusangle, for a specific engine application. If you’re using aftermarket performance rod bolts, in the majority (if not all) cases, the manufacturer provides a torque specification and a stretch range (giving you a choice of tightening methods).

Torque Spec Tightening

If you intend to use the torque spec method, bolt manufacturers usually provide two torque values: one with engine oil as a lubricant and one with a specific moly lube. Torque values are always a bit lower with moly because moly decreases thread and underhead friction (if you use moly but tighten to the oil spec, you run the risk of overtightening). Moly is preferred because it greatly reduces friction and provides a more accurate (and consistent) torque value. When you lubricate the rod bolt prior to installation, be sure to apply lube on the bolt threads and to the underside of the rod bolt head.

Bolt Stretch Monitoring

Remember that bolts are designed to stretch when they enter their elastic state (you want this “alive” elastic state in order to provide sufficient clamping force). However, if the rod bolt is overtightened and exceeds its designed elastic state, the bolt immediately weakens. If a rod bolt has been stretched beyond its elastic range, it must be replaced.

If you’re blueprinting an engine, you are most likely building a high-performance or race engine and should use aftermarket high-performance connecting rod bolts. This tightening process is used by all major race engine builders.

 

Each rod bolt is first positioned on the tool’s anvils and the gauge is zeroed. This provides a reference point (while the bolt is in its relaxed state). A GearHead stretch gauge is shown here. Notice the convenient thumbhole on the billet housing. During test fitting, final assembly, and during any future teardowns, always keep the rod bolts with their original rod (keep them organized), so that you can remeasure each bolt to determine if it’s still usable and hasn’t lost some of its original elasticity.

Each rod bolt is first positioned on the tool’s anvils and the gauge is zeroed. This provides a reference point (while the bolt is in its relaxed state). A GearHead stretch gauge is shown here. Notice the convenient thumbhole on the billet housing. During test fitting, final assembly, and during any future teardowns, always keep the rod bolts with their original rod (keep them organized), so that you can remeasure each bolt to determine if it’s still usable and hasn’t lost some of its original elasticity.

 

Today’s high-quality aftermarket performance connecting rod bolts always have a dimple at both ends of the bolt (at the head and at the tip of the shank). These dimples provide precise locating contacts for a stretch gauge.

Today’s high-quality aftermarket performance connecting rod bolts always have a dimple at both ends of the bolt (at the head and at the tip of the shank). These dimples provide precise locating contacts for a stretch gauge.

 

Once the gauge has been zeroed (on a relaxed rod bolt), the bolt is lubed and tightened with a torque wrench to the rod or bolt manufacturer’s torque spec. Then using the stretch gauge (still in its zeroed state from the original bolt free-length check), the bolt is checked to find out how far it has stretched. In this example, the rod bolt has stretched by .005 inch. Although a more time-consuming procedure than simply tightening to a specified torque value, achieving precise bolt clamping load is worth the trouble, especially for a race engine that represents a huge investment. It’s important to understand that bolts stretch as they are tightened. By tightening the bolt to its optimum clamping load while staying within its elastic range, many race engine builders prefer to tighten by monitoring bolt stretch, rather than a specific torque value. Monitoring bolt stretch eliminates the potential variables of tightening-by-torque, such as thread friction, bolt underhead contact friction, and torque wrench calibration/accuracy.

Once the gauge has been zeroed (on a relaxed rod bolt), the bolt is lubed and tightened with a torque wrench to the rod or bolt manufacturer’s torque spec. Then using the stretch gauge (still in its zeroed state from the original bolt free-length check), the bolt is checked to find out how far it has stretched. In this example, the rod bolt has stretched by .005 inch. Although a more time-consuming procedure than simply tightening to a specified torque value, achieving precise bolt clamping load is worth the trouble, especially for a race engine that represents a huge investment. It’s important to understand that bolts stretch as they are tightened. By tightening the bolt to its optimum clamping load while staying within its elastic range, many race engine builders prefer to tighten by monitoring bolt stretch, rather than a specific torque value. Monitoring bolt stretch eliminates the potential variables of tightening-by-torque, such as thread friction, bolt underhead contact friction, and torque wrench calibration/accuracy.

 

You should not just tighten the rod bolts to reach a particular torque value. Instead use a precision-tightening technique for the connecting rod bolts. This requires you to use a bolt stretch gauge during the process. The torque values for aftermarket connecting rod bolts may vary by as much as 10 ft-lbs from batch to batch because of differences in heat treating and materials. The bolt needs to be measured so you attain peak bolt strength and big-end roundness. Measuring bolt stretch allows you to accurately attain the target clamping loads rather than just using a torque wrench.

Granted, when dealing with production engines that utilize torque-plusangle specifications (for example, 20 ft-lbs followed by a 90-degree additional turn) follow the OEM procedure. Tightening connecting rod bolts using the stretch-monitoring method really only applies to performance engines that have aftermarket high-performance rod bolts and the bolt manufacturer has provided a stretch value reference.

There is some debate among engine builders regarding the validity of measuring rod bolt stretch, due to potential compression of the rod material as the rod cap is clamped to the rod. Although this may occur, a stretch gauge remains the optimum method of accurately determining connecting-rod bolt clamping load.

 
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Tightening Method Variations

Think of a bolt and other similar fasteners as a high-resistance spring. To get the best clamping force from a bolt, you must not overtighten it or you exceed the yield point. When tightened below the yield point, the bolt provides consistent and accurate clamping force that you need for a high-performance engine build. You can damage the rod threads through overtorquing, and bolts can have unequal clamping force. This can lead to failure of the bolt and make the bolt bore out of round.

OEM rod bolts commonly provide a tensile strength of approximately 150,000 to 160,000 psi. Variances in bolt production can greatly affect the tolerances, and as a result, peak bolt stretch travel can occur from .003 to .006 inch. Use a bolt-stretch gauge and the torque spec to reach the ideal bolt stretch for an application. If you simply use the torque spec, you may end up with unequal rodbolt clamping force.

Manufacturers produce premiumquality rod bolts to achieve far tighter tensile-strength tolerances. ARP calculates stretch and yield point for all bolts and its bolt packages include all specification data so you can safely and correctly attain the correct clamping force. The instructions state the particular bolt stretch for each bolt. ARP states that base tensile rating is 190,000 psi. Actual ratings are significantly higher for some specific products.

You must always strive for consistent and accurate torque of the connecting rod bolts. You must use the same method for tightening bolts at all stages of the engine build process. Different or inaccurate tightening methods can produce unequal or inadequate bolt tightness. In turn, this can damage the big-end bore shape. For instance, if one technician uses one torque value to recondition the connecting rods using torque value alone while another mechanic employs the bolt-stretch method, out-of-round bores can be the final outcome. The two methods produce different frictional variances. Thus, a higher clamping load may be reached using the stretch method, in contrast to using only bolt torque (without regard to actual bolt stretch). Also, only 80 percent of the torque can be exerted on a bolt without lubricant because of increased friction.

 

This rod cap has been drawn from its rod. Before removing the rod from the tool, I temporarily and loosely replaced the rod bolts, simply to ease the task of keeping the cap with its rod (makes it easier to avoid cap mismatch mistakes).

This rod cap has been drawn from its rod. Before removing the rod from the tool, I temporarily and loosely replaced the rod bolts, simply to ease the task of keeping the cap with its rod (makes it easier to avoid cap mismatch mistakes).

 

As you can see, variances can be caused by how the rod bolts were stretched (basing the results on torque value or bolt stretch), and how thread lubrication varies (if the bolts were tightened first with one type of lube then tightened subsequently with a different type).

When engines are hand-built, this close attention to bolt stretch and torque values is indispensable for achieving the highest-quality build. A methodical engine-build that is aiming for the highest level of accuracy should use the stretch method, rather than the torque method.

The Friction Factor

If a bolt is tightened by addressing only torque value, you may not necessarily achieve the desired preload due to the variable of friction. Consider the material of the connecting rod itself, which is beyond your control. As you tighten the bolt, the head of the bolt tends to embed itself into the rod, which slightly compresses the material of the connecting rod cap. The hardness of the rod cap material varies among OEM rods and aftermarket-forged billet steel, titanium, and aluminum rods. By monitoring bolt stretch, you eliminate this variable in the pursuit of achieving the required clamping force.

 

A connecting rod vise is definitely worth the investment. When removing a rod cap from a new rod, or when tightening rod bolts with the rod off the crank you need to secure the rod. Using a common bench vise can cause serious damage to the rod; the creation of burrs, gouges, or worse can lead to stress cracks and failures. A dedicated, purpose-designed rod vise allows securing of the rod without damage.

A connecting rod vise is definitely worth the investment. When removing a rod cap from a new rod, or when tightening rod bolts with the rod off the crank you need to secure the rod. Using a common bench vise can cause serious damage to the rod; the creation of burrs, gouges, or worse can lead to stress cracks and failures. A dedicated, purpose-designed rod vise allows securing of the rod without damage.

 

A dedicated “rod cap splitter” is another specialty tool to consider. With the need to remove a rod cap on a previously assembled rod, the alltoo-common approach is to first loosen the rod bolts, and then smack the rod bolt heads with a plastic hammer to dislodge the cap. That method is crude and always has the potential for dropping the rod. A rod cap splitter such as this one makes rod cap separation easy and precise. The rod big end is placed onto the tool’s split collar. The spindle is then rotated, moving the collar halves away from each other. This smoothly draws the cap from the rod.

A dedicated “rod cap splitter” is another specialty tool to consider. With the need to remove a rod cap on a previously assembled rod, the alltoo-common approach is to first loosen the rod bolts, and then smack the rod bolt heads with a plastic hammer to dislodge the cap. That method is crude and always has the potential for dropping the rod. A rod cap splitter such as this one makes rod cap separation easy and precise. The rod big end is placed onto the tool’s split collar. The spindle is then rotated, moving the collar halves away from each other. This smoothly draws the cap from the rod.

 

Simply put, by using the stretch-monitoring method, you eliminate the variables associated with thread friction and rod material hardness. If the manufacturer’s specified amount of stretch for a given rod bolt is .005 to .006 inch, and you tighten by a measured stretch of .0055 inch, for example, you know that you’ve achieved the needed clamping force while neither under-tightening nor over-tightening.

Torque Requirements

As you tighten any threaded fastener, you fight friction in thread engagement. The underside of the bolt head or nut rubs against its mating surface. As a result, a significant amount of torque energy is lost. Although you may apply the specified amount of torque, according to the torque wrench, you really have no idea exactly how much clamping force is being generated. This is another example of why it makes sense to tighten critical rod bolts by using the stretch method.

When you rely on torque to achieve clamping load, you maximize your efforts by minimizing frictional losses. It’s critical to make sure that the threaded holes in the block are clean and free of debris, burrs, and other contaminants.

In order to clean female threads in the engine block, do not use a cutting tap.

They are designed to create threads by removing material. In order to clean existing threaded holes, start with a cleaning procedure (such as hot-tanking the block). Never assume that all threaded holes are clean by simply washing the block.

Hand-clean each holes with solvent, a rifle brush, and compressed air (always wear safety glasses). If thread condition is suspect, you may use a forming tap to recondition the threads. A forming tap is designed to reform the threads, in contrast to a cutting tap, which removes material. Special forming taps are available through machinist specialty supply sources and engine rebuilding tool sources.

 

While a fairly expensive tool, a digital torque/angle wrench provides accuracy and work time efficiency; there’s no need to stop and change to an angle wrench. If you perform OEM torque/angle tightening on a regular basis, consider buying one of these.

While a fairly expensive tool, a digital torque/angle wrench provides accuracy and work time efficiency; there’s no need to stop and change to an angle wrench. If you perform OEM torque/angle tightening on a regular basis, consider buying one of these.

 

With the push of a button, you can quickly set your torque value (in ft-lbs, in-lbs, or Nm).

With the push of a button, you can quickly set your torque value (in ft-lbs, in-lbs, or Nm).

 

With the push of another button (changing mode), you quickly set your angle adjustment. One nice feature of this type of wrench (Snap-On shown) is that you can actually ratchet-tighten even in the angle mode without losing the reference point. Pretty slick.

With the push of another button (changing mode), you quickly set your angle adjustment. One nice feature of this type of wrench (Snap-On shown) is that you can actually ratchet-tighten even in the angle mode without losing the reference point. Pretty slick.

 

If the rod bolt-tightening specifications call for a torque-plus-angle (typical for many of today’s OEM rod bolts), a digital torque/ angle wrench makes the job easy. Press a button, set the torque, and tighten until the wrench beeps. Push another button to set the required angle, and tighten again until the wrench beeps. Shown here is rod bolt tightening (with new OEM rod bolts), on a GM LS engine, using a Snap-On digital torque/angle wrench.

If the rod bolt-tightening specifications call for a torque-plus-angle (typical for many of today’s OEM rod bolts), a digital torque/ angle wrench makes the job easy. Press a button, set the torque, and tighten until the wrench beeps. Push another button to set the required angle, and tighten again until the wrench beeps. Shown here is rod bolt tightening (with new OEM rod bolts), on a GM LS engine, using a Snap-On digital torque/angle wrench.

 

If your OEM rod bolts call for torque plus angle tightening, first use a torque wrench to tighten to the torque specification. Then continue to rotate the bolt for the specified number of degrees. Inexpensive metal or plastic angle gauges are available that attach to the head of a ratchet or breaker bar. This simple adapter, attached between the ratchet and socket, allows visual monitoring of tightening angle. With the tool engaged to the fastener, zero the gauge. As you continue to tighten, simply observe the gauge needle. Once it reaches the desired angle, stop.

If your OEM rod bolts call for torque plus angle tightening, first use a torque wrench to tighten to the torque specification. Then continue to rotate the bolt for the specified number of degrees. Inexpensive metal or plastic angle gauges are available that attach to the head of a ratchet or breaker bar. This simple adapter, attached between the ratchet and socket, allows visual monitoring of tightening angle. With the tool engaged to the fastener, zero the gauge. As you continue to tighten, simply observe the gauge needle. Once it reaches the desired angle, stop.

 

If you tighten with a torque wrench, always make sure that you grip the handle at its center. Also, do your best to keep the wrench as level as possible (90 degrees to the bolt), and brace the wrench head with your other hand. Tighten slowly/gradually until you reach the desired torque value. Avoid quick pulls and yanks that can result in slight over tightening.

If you tighten with a torque wrench, always make sure that you grip the handle at its center. Also, do your best to keep the wrench as level as possible (90 degrees to the bolt), and brace the wrench head with your other hand. Tighten slowly/gradually until you reach the desired torque value. Avoid quick pulls and yanks that can result in slight over tightening.

 

Specially developed fastener assembly lubricant is designed to drastically reduce friction (at the threads and bolt head underside) during torquing or tightening via stretch. Do not use a different type of lubricant. Rod bolt tightening is a critical aspect of engine assembly, and it’s no place to cut corners!

Specially developed fastener assembly lubricant is designed to drastically reduce friction (at the threads and bolt head underside) during torquing or tightening via stretch. Do not use a different type of lubricant. Rod bolt tightening is a critical aspect of engine assembly, and it’s no place to cut corners!

 

When applying assembly lube to the rod bolts, apply an even coating to the bolt threads.

When applying assembly lube to the rod bolts, apply an even coating to the bolt threads.

 

Also apply a coating of assembly lube to the underside of the bolt head. Remember, friction occurs between threads and between the bolt head and the rod cap.

Also apply a coating of assembly lube to the underside of the bolt head. Remember, friction occurs between threads and between the bolt head and the rod cap.

 

When using high-performance aftermarket cylinder head bolts and main cap bolts, always use the thread lubricant specified by the bolt manufacturer, and follow the torque value recommendations. There is a great difference in lubricity between engine oil and moly-type thread lubricants. A moly lube provides much less frictional resistance.

If a bolt is specified for, say, 60 ft-lbs of torque with oil, but the threads are lubed with moly, you overtighten the bolt. If the bolt is specified at 60 ft-lbs with moly and you use oil instead, you undertighten. Pay strict attention to the instructions included with any set of aftermarket head or main bolts.

 

Connecting Rod Inspection

In almost all circumstances, I recommend installing new connecting rod bolts for high-performance builds or stock rebuilds. It’s simply not worth taking the risk with used and possibly overstretched bolts. Whether you’re installing new or used connecting rods, you should always inspect each one for center-to-center length, big-end bore diameter, small-end bore diameter, and any out-of-round bigend bore. For used rods, also check for rod bend, rod twist, and cracks.

When measuring the big-end bore for diameter and out-of-round, the cap must be fully installed, with rod bolts fully tightened to the torque (or bolt stretch) specified by the manufacturer. Also, always use the specified bolt thread lubricant. This is especially important if you’re tightening to achieve torque value.

 

The same rod bore gauge is used to check the rod’s pin bore. If using aftermarket performance full-floating rods, diameter is based on recommended oil clearance to the wrist pins.

The same rod bore gauge is used to check the rod’s pin bore. If using aftermarket performance full-floating rods, diameter is based on recommended oil clearance to the wrist pins.

 
Each connecting rod (especially used) should always be checked for rod deformation, using a rod alignment stand. Here a rod is checked for bend. Note that the upper anvil is resting on top of the wrist pin. With the rod placed onto the tool-centering fixture, use a feeler gauge between the wrist pin surface and the upper checking base.

Each connecting rod (especially used) should always be checked for rod deformation, using a rod alignment stand. Here a rod is checked for bend. Note that the upper anvil is resting on top of the wrist pin. With the rod placed onto the tool-centering fixture, use a feeler gauge between the wrist pin surface and the upper checking base.

 
The same fixture is used to inspect the rod for twist, with the upper anvil contacting the side of the rod pin. Any measured bend or twist beyond .003 inch is cause for concern.

The same fixture is used to inspect the rod for twist, with the upper anvil contacting the side of the rod pin. Any measured bend or twist beyond .003 inch is cause for concern.

 

Torque value (and clamping load) differs between oil and a moly-type lubricant. If you’re using OEM rod bolts that are specified for tightening by a torqueplus-angle method, the following procedure must be followed.

After measuring the big-end bores for out-of-round (and if the deviation is less than .002 inch), the bores can be reconditioned by grinding material from the mating surfaces of the rod and cap, then reassembled and honed round to size.

PM rods have a unique, irregular mating surface that must not be disturbed. If a PM rod is out of round by no more than .002 inch, the big-end bore may be honed to oversize, providing that oversize-OD rod bearings are available for the application. Otherwise, the rods cannot be reused and must be replaced.

Certain aftermarket performance rods may have a serrated mating surface that registers the cap to the rod. This design also cannot be reconditioned by grinding material from the mating surfaces. Honing to a slight oversize and using oversize bearings is required. If any rod big-end bores are out of round by more than .002 inch, the rods should be discarded and replaced.

 

Written by Mike Mavrigian and Posted with Permission of CarTechBooks

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