Pistons must withstand tremendous heat and cylinder pressures during their four-stroke cycle. Pistons take quite a beating, especially in high-performance or racing applications. High-compression ratios and/or cylinder pressure boosting with nitrous injection, turbocharging, or supercharging places greater demands on the piston. Add to this the possibility of detonation, and you’re asking too much from these slugs. For any serious build, it’s imperative to use forged or billet pistons. Cast pistons, even today’s hypereutectic pistons, simply do not hold up to extreme demands.
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Current-day OEM pistons are most commonly hypereutectic, which means they are precision-cast pistons with a high silica content. Hypereutectic pistons are much stronger and more stable than the cast pistons of old, and are generally good for up to about 400 hp.
A step up is forged or billet pistons for much-improved strength. Forged aluminum pistons start as a dense forging and are then CNC machined to final shape. Billet pistons begin as a dense-alloy billet block and are fully CNC machined to the final product. Billets are more expensive, mostly due to the waste of material that’s machined away during manufacturing.
If you’re building a performance engine from scratch, going to forged or billet simply makes sense. They’re stronger and you have a much wider range of sizes and configurations available. If you’re upgrading an OEM engine with hypereutectics, or if you’re overboring and/or changing compression ratio, altering stroke, etc., forged and billet pistons are readily available in any configuration. Hypereutectic pistons are primarily available as direct replacements for rebuilds.
If you plan to boost cylinder pressure with nitrous or forced induction, you must upgrade to forged or billet pistons. They withstand combustion temperatures better than cast or hypereutectic pistons.
The following are features commonly found on performance pistons.
Vertical Gas Ports
These small, vertical holes in the piston deck (around the perimeter area of the piston deck) allow combustion pressure to directly enter behind the top ring (on the power stroke). This pressurizes the area behind the top ring for improved ring sealing (the pressure pushes the top ring against the cylinder wall). During operation other than the power stroke, the top ring experiences normal pressure and less drag.
Drag racing is the most common application.
Lateral Gas Ports
These very small slots are milled into the top (roof) of the top ring grooves, providing a path for combustion pressure to push the top rings out against the cylinder wall for improved ring seal.
This type of gas porting is most commonly used in circle track applications.
Contact Reduction Grooves
Some pistons have a series of narrow circumferential grooves machined around the piston area between the top ring groove and the piston deck. According to JE Pistons, the purpose is to reduce the amount of contact area against the cylinder wall when the piston “rocks over” during the transition from TDC. Contact reduction disrupts the flame travel, reducing the chance of detonation.
This radiused groove is machined into the piston land between the top and second ring grooves. This groove provides added volume for residual combustion gas that blows past the top ring. This reduces pressure between the top and second rings. The benefit: It reduces ring flutter and helps top ring seal.
Double Pin Oilers
These large oil holes are milled into the floor of the oil ring groove. They deliver additional oil to the piston’s wrist pin.
Oil Squirt Notch
Oil squirters are small nozzles secured to the block that squirt pressurized oil from the galley to the underside of the pistons for additional cooling. They have a notch machined into the bottom of the piston skirt to provide clearance for the squirter nozzle to avoid contact between the nozzle and piston.
These small oil holes are drilled into the underside of the pin boss (on each side of the boss) and provide additional splash oiling to the wrist pin.
Pin Lock Grooves
Any piston designed for full-floating wrist pins must be secured so the wrist pin doesn’t walk out of its bore. Floating pins are secured with pin locks, which are inserted into the pin bore after the pin has been installed. A groove is machined into the entrance of each side of the pin bore to accept wire locks or flat-wound spiral locks.
Piston-to-wall clearance specifically refers to the clearance between the widest area of the piston skirt to the cylinder wall. If clearance is too tight, the pistons can scrub against the cylinder wall and possibly seize. If clearance is too great, the pistons rock back and forth excessively, which degrades ring seal and can result in piston skirt cracking as the piston skirts slap the walls.
All pistons expand when operated at full temperature. This must be taken into account in order to arrive at the ideal spec. Desired clearance is based on the piston and bore diameter under operating temperature. That’s why you may hear piston slap noise when the engine is fired cold. The noise goes away once the pistons warm up.
Factors that affect wall clearance include actual cylinder wall thickness, including how the cylinder bore changes geometry during engine operation. Piston compression distance (CD), piston material (specific alloy mix and density), piston dome thickness, and the operation of the engine are all factors contributing to bore shape. This is critical if the engine is subjected to short runs in drag racing, run for long periods at lower RPM on the street, or run for extended periods in oval track or road racing.
In very general terms, .001 inch of clearance per inch of bore diameter is accepted. But this always depends on the specific alloy and density of the piston material. Performance and racing piston manufacturers use their own alloy formulas, so always follow the piston manufacturer’s specified wall clearance.
Typical small-bock engines have a wall clearance of about .004 inch. Typical big-block engines call for about .005- inch clearance. Whatever clearance is listed for naturally aspirated engines, you add about .001 inch if the engine uses a high-pressure, forced-induction system because the pistons run hotter and therefore grow in diameter under operating temperatures.
Again, follow the piston manufacturer’s clearance specs. JE, Diamond, Ross, Probe, Wiseco, and others have performed the research and development to determine how their material reacts to temperature and pressure. Don’t make the mistake of thinking that you know more than they do.
Another general rule is that the higher the anticipated engine speed (RPM), the greater the required clearance. In simple terms, if the manufacturer says that the wall clearance range for a particular piston is, say, .003 to .005 inch, the .003-inch clearance is better suited to a street application, while the higher end of the range is better for high-RPM use. In a race engine, where clearance is set a bit wide, any cold piston slap noise isn’t the concern it might be in a street engine.
Before final-honing your cylinder bores for proper piston clearance, never assume that the pistons are manufactured to the specific diameter. In most cases, however, top-quality pistons are often precisely made to the correct specification because of today’s CNC machining and tight manufacturing tolerances. Always measure each piston’s diameter for two reasons: to verify consistency piston to piston and to determine the required honed-bore diameter in order to achieve desired wall clearance.
Never measure a piston diameter anywhere near the ring grooves. Always measure the widest area of the skirt. Manufacturer instructions may tell you to measure exactly .500 inch above the bottom of the skirt, while some pistons must be measured directly under the oil ring groove. It depends on the profile of the piston.
Pistons are not usually perfectly round from top to bottom. The upper area (ring groove area) is round, but the skirt is likely machined to an eccentric (oval) shape. This out-of-round condition might only be .020 inch or so, but the widest skirt area is located 90 degrees to the piston pin bore centerline (the cam profile). This oval profile exists in order to compensate for the piston’s rate of expansion in specific areas of the piston. Due to varying material thickness in the cross-section, different areas of the piston are expected to react at different rates (determined by design). Always measure piston diameter exactly where the piston manufacturer tells you to measure.
Piston Skirt to Crankshaft Clearance
When using components with non-stock dimensions, check clearance, which includes stroker cranks and different-length rods. Also check clearance between the bottom of the piston skirts and the crank counterweights, primarily as the piston approaches BDC. Most high-performance piston bottoms have a relief to add clearance and to reduce weight.
During test fitting, slowly rotate the crank and perform a visual check for this clearance. As long as you have any clearance, you should be okay but, in general, a minimum of about .060 inch provides a safe margin.
Specialty coatings and treatments aid in piston performance and longevity. See Chapter 18 for in-depth information about coatings.
Piston pins (often referred to as wrist pins) provide the critical link between the piston and connecting rod. Here I discuss pin design, clearances, and locating methods for floating pins.
Pin Bore Design
Piston pin bores are commonly (but not always) slightly offset from the piston-diameter centerline. The reason for this is to compensate for friction resulting from the thrust side of the piston as it travels through the cylinder bore. The angle of the connecting rod changes as the crank rotates. When the rod is at its greatest angle, it pushes against one side of the piston harder than the other side.
By offsetting the piston pin bore toward the thrust side, friction is slightly increased but piston skirt slap noise is reduced. If you turn the piston 180 degrees, with the offset toward the minor thrust side (the side with less thrust force), friction is slightly reduced but noise increases. One of the main reasons that some pistons have an offset pin bore is to reduce slap noise.
The piston is offset from the small end of the rod slightly in order to soften the loading. Instead of one entire side of the piston abruptly smacking against the cylinder wall during the rocking transition before and after TDC, the offset reduces the impact, allowing the piston skirt to make contact first, followed by the rest of the piston side.
The thrust side is where greater force and pressure is placed on the piston and the cylinder wall. It’s fairly common for some race builders to reverse the piston to reduce friction, since slap noise (cold start) isn’t a concern.
Pin Bore Clearance
On a full-floating pin design in which the wrist pin floats on the rod and the piston pin bore, oil clearance from the pin to the piston bore is generally about .0008 to .001 inch. When you purchase performance pistons, the pins are usually included and already provide the necessary oil clearance, but it never hurts to measure for yourself just to verify.
A retention device (or pin lock) is required to prevent the pin from walking out of its bore into the cylinder wall. Many pistons use a full-floating pin in which the pin floats in the rod’s small end and in the piston’s pin bore. There are three options, depending on the piston design: wire locks, spiral locks, or buttons.
Wire locks are tempered roundwire clips with an end gap. Spiral locks are made of tempered, flat-wound steel that is spirally wrapped (similar to a spring that is fully compressed). Pistons designed for wire or spiral locks usually have one or two small notches around the perimeter of the pin bore to provide access with a small, flat-blade screwdriver that makes removal easier. Wire locks and spiral locks function the same: After the pin is inserted in the piston, the lock snaps into a groove at the end of each side of the piston pin bore.
Depending on piston pin bore design, the piston requires one wire lock per side, one spiral lock per side, or two spiral locks per side. These provide a positive stop for the pin. Installing and removing these snap-in tensioned locks isn’t that difficult, but dealing with spiral locks requires practice. After installing a couple of them, you get the hang of it.
When installing a spiral lock, first gently spread it apart so that it isn’t mashed together. Insert one end of the spiral into the groove while pushing the rest of the lock into the groove in a counterclockwise direction. Once it’s fully inserted, it snaps in place.
The third method of piston retention is the use of buttons. These work well when the piston pin is short (still providing enough support through the pin bore but recessed farther at the ends). A soft material slug (softer than the cylinder wall) is finger inserted into each end of the pin bore. Once the piston is inside the cylinder bore, the buttons are captive.
Button materials include nylon, Delrin, and even aluminum. Buttons are great for racers who routinely perform engine teardowns at the track (no spring clips to mess with), but due to the eventual wear on the button faces, they’re not intended for long-term or street use.
Most pistons have three ring grooves: the top compression groove, a second compression groove, and an oil ring package groove. Ring grooves are machined to a very precise dimension and surface finished to allow the rings to seal properly and to slide in and out. It’s extremely critical to avoid disturbing this surface finish. If you plan to glassbead blast a piston to clean or to soften custom-machined, valve-pocket surfaces, carefully mask off the ring grooves. Ring “lands” refer to the outer-diameter surface of the piston at the ring areas.
The required CD of the piston pin bore can be such that the pin bore encroaches into the oil-ring groove area, and this condition is very common with short-CD pistons. Fortunately, the floor of the ring groove has an open area above each side of the pin bore. In order to provide uniform support for the oil ring package, a support rail is installed to the floor of the oil ring groove before the oil ring package.
The support rail has a small dimple that protrudes downward. When the support rail is installed, this dimple must be positioned at this open space, above the pin bore. The dimple prevents the support rail from rotating out of position. When required, support rails are supplied with the pistons. The end gap of these rails is fairly wide, so even if the compression rings require file fitting, you don’t have to mess with the support rail gap.
You don’t want your pistons smacking into the head combustion-chamber quench area. Even slight contact between the piston and the head eventually (if not quickly) destroys the rod bearings, piston wrist pin bore, etc. Compensate for thermal and dynamic growth of the rod and piston to avoid this degradation. Other contributing factors are connecting rod material and its rate of expansion, piston mass, and piston speed.
If you’re using forged steel connecting rods, minimum piston-to-head clearance should be around .035 inch in a typical small-block and about .045 inch in a big-block. If you’re using aluminum rods, add about .010 inch more clearance. Remember to factor-in the compressed head-gasket thickness, which can vary widely depending on the gasket (composite, multi-layer steel, copper, etc.).
Piston-to-valve clearance isn’t so much an issue at full lift when the cam lobe is at peak height because the piston has already moved into the bore when the valve is at full lift. Camshaft duration is the real issue, along with radial clearance of the valve head to the piston.
Use the clay-checking method or a degree wheel and a dial indicator or to measure piston-to-valve clearance. When you do, don’t rely on just one measurement at just one cylinder location. Tolerance stackup could result in different piston-to-valve clearances from the front of the block to the rear, so take the time to measure your valve clearance at each cylinder.
In either case, assemble the block with the crank and its main bearings for at least one rod and piston. I recommend doing this for number-1 cylinder. Then install the camshaft and solid lifters. If you’re using hydraulic lifters, the solids need to be the same length at the corresponding cylinder location, timing gear setup, a head gasket , and a cylinder head with valves.
For the clay checking method, it’s best to use the actual springs that you plan to use in the final assembly because the light checking springs may not be strong enough to fully compress the clay, which results in a false (too shallow) depression. Install the correct pushrods (the length that you plan to use) and rocker arms on the corresponding valve location. Make sure that the cam timing is adjusted where you plan to run the bumpstick, since this affects valve clearance.
For this method, lower the piston below deck (about an inch or so) before installing the cylinder head. Apply a light coat of assembly oil to the cylinder wall to prevent the clay from sticking to the wall. The piston dome should be clean and dry so the clay can stick to the piston. Apply a coating of oil to the intake and exhaust valves and the quench area of the combustion chamber so the clay does not stick to the valves or head.
Apply a thick slab of modeling clay (available at any craft store) to the piston valve pockets. Overlap a bit of clay beyond the valve pocket edges.
Install a head gasket (preferably one already crushed). If you don’t have a used head gasket or the same type you plan for final installation, you can measure your new gasket and then check with the gasket manufacturer to determine the compressed thickness. The difference between the new and compressed figures can be subtracted from the total valve clearance.
Install the cylinder head and hand tighten at least four head bolts. Do not torque to final spec. Install the pushrods and rocker arms and zero the valve lash. Using a socket wrench on the crank snout, rotate the crank 360 degrees at least two times in the engine’s direction of rotation. Remember, the crank needs to rotate twice in order to rotate the cam once.
Carefully remove the cylinder head. You should see clear valve impressions in the clay.
Using a sharp razor blade, carefully cut through the center of the exhaustvalve pocket area; be careful not to gouge the piston. Carefully remove the outer half of the clay. Now you can see a crosssection of the clay in the valve pocket.
Using a depth mic or the end of a dialcaliper rule, measure the thinnest section of clay in the pocket (closest to the pocket “eyebrow”).
Repeat this procedure at the intake valve. Record your findings.
Granted, the clay method is definitely old-school, but it gets you close enough. In order to obtain higher accuracy, it’s best to measure with a dial indicator.
If you’re using aluminum connecting rods, you likely need to allow an additional .030 inch or so for clearance, since aluminum rods tend to expand more when hot.
For the dial-indicator measurement method, install light checking springs on the intake and exhaust valves instead of using the actual valvesprings. This makes it easier to move the valves rather than fighting spring pressure.
With the degree wheel mounted to the crank, rotate the crank in the direction of common operation until your degree wheel indicates that number-1 piston is at 10 degrees ATDC on its intake stroke. Why? Because this is the position where valve-to-piston clearance is likely the tightest.
Without disturbing the piston location, mount the dial indicator onto the cylinder head. The indicator base must be mounted solidly. If you have an aluminum head and using a magnetic base, you may be able to solidly mount a steel pushrod guideplate at the adjacent cylinder to provide a flat contact point.
Position the dial indicator plunger onto the number-1 intake valve or its retainer. The dial indicator plunger needs to be parallel with the valvestem. Don’t mount it at an angle.
Preload the dial indicator at about .050 inch, then zero the gauge. Carefully push down the rocker arm tip until the valve stops as it touches the piston. Note the amount of movement on the gauge and record this distance.
In order to measure exhaust valve clearance, smoothly rotate the crankshaft in the same direction as before until the degree wheel indicates 10 degrees BTDC on the exhaust stroke. This is the area at which the exhaust valve is likely closest to the piston.
As before, place the dial indicator on the exhaust valve, preload the plunger, zero the gauge, push down on the exhaust rocker tip until the valve hits the piston, and record the number.
As a general rule, you should have at least .080-inch valve-to-piston clearance at intake valves, and about .100 inch at the exhaust valves. The exhaust valves soak up more heat and expand more than the intakes.
Piston Compression Distance
Piston compression distance (CD) is the distance from the centerline of the piston pin bore to the flat deck of the piston. The height or this measurement is critical for establishing compression ratio and valve and head clearance. CD is one of the factors that determines where the piston deck is located relative to the block deck when the piston is at TDC. The centerline of the crankshaft main bore is a fixed position. The distance from the main bore centerline to the block deck is the reference for where the piston deck is at TDC. To determine this, with the crank rod journal at TDC, add together the following dimensions:
Block Deck Height = 1/2 stroke + rod length + piston CD + deck clearance
The result of this combined measurement is then compared to the distance from the main bore centerline to the block deck.
As an example, let’s say that your block deck height (main bore center to bock deck) is 10.2 inches. Let’s say that your crank stroke is 4.500 inch and the rod length is 6.700 inches.
One-half of stroke (since you’re only concerned with the max distance that the crank pushes the rod up at TDC) in this case is 2.250 inches. Adding this 2.250 inches to the 6.700-inch rod length results in 8.950 inches. Subtracting this 8.950 inches from the block deck height of 10.2 inches gives a dimension of 1.250 inches. This 1.250 inches is the available piston CD in order to place the piston deck flush with the block deck. If you want the piston deck to be below the block deck by, say, .015 inch, subtract this from the initial CD, which means that you want a piston CD of 1.235 inches. Remember that you still have to consider the crushed cylinder-head-gasket thickness (which gives us more volume above the piston) and required valve clearance.
Compression ratio represents the volume of the cylinder when the piston is at BDC compared to the volume when the piston reaches TDC. These factors are all affected by cylinder displacement: bore diameter, crank stroke, deck clearance, head gasket volume, and combustion chamber volume.
When you consider the piston’s dome and dish/valve reliefs, you subtract piston dome volume and add dish or valve pocket volume.
See Chapter 5, page 46, for the formula and examples to find the compression ratio.
If the piston top quench area (not including a dome protrusion) is flush with the block deck at TDC, it is called zero deck and doesn’t need to be included in the formula. If the top of the piston is below deck (negative deck) or above deck (positive deck), then this volume must be considered. If the piston is below deck, this volume is added to the formula (since it adds volume). If the piston quench is above deck, subtract this volume.
See Chapter 5, page 48, for the formula and examples to find the deck height volume.
The bore of the head gasket is larger in diameter than the cylinder bore. Even if the cylinder head gasket bore is not perfectly round, measure the diameter to obtain an average diameter.
See Chapter 5, page 48, for the formula and examples to find the head gasket volume.
Piston Dome Volume
Quality piston manufacturers provide dome volume figures for their pistons. You can rely on those figures or you can measure for yourself. Measure piston dome volume with the piston installed, with at least the top compression ring installed (to seal the bore). Rotate the crank to bring the piston down the bore near BDC. Then coat the cylinder wall with a thick lithium grease (to aid in sealing) at a point of about 1.50 inches below block deck.
Rotate the crank to raise the piston to a point exactly 1.00 inch below block deck (use a depth micrometer to measure this). Wipe off any excess grease that might be on the top of the piston.
Smear lithium grease onto the block deck around the bore and place a piece of stout, flat clear plexiglass or acrylic, with a small chamfered hole drilled in the plate (about 1/4 inch in diameter) onto the block. Press the plate gently to obtain a good grease seal.
With the block deck level, you use a use a burette to add fluid (blue windshield washer solvent works well) until the cavity is filled, with no air bubbles trapped under the clear plate. Fill the burette to its top index mark. As you release fluid, monitor how much fluid it takes to fill the cavity (you may need to refill the burette several times).
Before adding fluid, place a clean piece of paper under the bore and watch for leaks. If fluid passes through the bore past the top ring, you gain a false reading.
See Chapter 5, page 47, for the formula and examples to find the piston dome volume.
Combustion Chamber Volume
You use the burette here also. With valves installed, place the head upside down on a workbench. Install a spark plug. Apply white lithium grease around the chamber and position your clear plate. Using the burette, add fluid and record how many cubic inches it takes to fill the chamber (make sure that no air bubbles are trapped.
Once you have recorded your data (combustion chamber volume, piston dome volume, head gasket volume, deck height volume, and cylinder swept volume, you’re ready to do a bit of math to determine compression ratio.
See Chapter 5, page 47, for the formula and examples to find the combustion chamber volume.
Due to aluminum heads’ faster heat dissipation, you are able to run a higher compression ratio than with cast-iron heads. Considering today’s fuels, you should be able to get away with as much as 11.4:1 or so compression ratio on high-test pump gas (92 octane or higher) before pre-ignition/detonation problems occur. Any higher requires high-octane race gas or alcohol.
Whether you’re using pistons that already have valve reliefs or flat-top pistons that require reliefs due to increased cam duration and/or oversized valves, you can perform your own check for valve diameter clearance.
The first step is to wipe the piston dome clean with a solvent and “paint” the dome with a broad, black marker.
Mount a degree wheel to the crank and adjust it to indicate TDC for the cylinder at hand.
Install a bare cylinder head (with no valves) to the block, using a crushed head gasket of the same type planned for assembly. Grab a junk valve that has the same stem diameter as the valves you intend to use and cut the head off the old valve. You can also use a piece of steel rod as long as it’s the same diameter.
On a lathe, grind the cut end of the valve to a point. You must use a lathe in order to achieve a centered point. Don’t try grinding the point by hand.
Apply a light smear of lube onto the stem to allow it to glide through the valve seal.
Rotate the crank to 10 degrees BTDC, where the valve is closest to the piston. Insert the pointed valvestem, which now serves as a center punch, into the exhaust valve location until it touches the piston. Lightly strike the punch with a hammer to create a witness mark. Remove the punch and rotate the crank to 10 degrees ATDC (this is where the intake valve is closest to the piston). Insert the punch into the intake valve location and tap a mark onto the piston.
Remove the cylinder head. You see both witness marks on the piston dome (one for intake and one for exhaust). These marks indicate where the center of each valve is in relation to the piston.
Next, carefully measure the diameters of the intake valve and the exhaust valve. Transfer these valve head diameters to the pistons, using the witness marks as your centers. You can use a sharp compass scribe to lightly scratch the valve head diameters.
In order to provide enough valve head clearance around the perimeter of the valve, add .080 inch to the measured diameter to compensate for piston rock and valve float. If your pistons already have valve notches, you are able to see if your valves clear the reliefs with about .080-inch clearance.
After you know where the outer diameter of each valve is in relation to the piston, determine valve-to-piston depth. Again, you’re back to clay checking or measuring valve clearance with a dial indicator.
If valve reliefs need to be cut (or existing reliefs cut larger and/or deeper), this must be done on a vertical mill. An old-school method (which I don’t recommend) is to take a junk valve, with the same valve head diameter and stem diameter as your good valves, and cut sawtooth notches onto the valve head. You then install the valve to the head, install the head to the block (with gasket), and turn the valve as you would turn a cutting tool with a hand drill. This is a crude and potentially disastrous method, so don’t do it. Instead, pay a skilled engine machinist to handle this for you.
Before cutting the valve reliefs, the dome area must be measured for material thickness to make sure that there’s enough material to handle the depth of the cut and still leave enough material to handle the combustion pressure. Generally speaking, the absolute minimum for a naturally aspirated engine is .150 inch of material thickness in the piston. For any forced-induction or nitrous application, around .200 inch is the bare minimum and .250 inch is better.
After the reliefs have been properly machined, re-install the head with valves and recheck valve-to-piston clearance. Once you’ve achieved the desired dimensional results, soften the sharp edges created by the pocket cutting: Remove the pistons from their rods. Using a hand-held die grinder or a Dremel with abrasive “tootsie-roll” sanding roll or drum, gently radius all sharp edges. This reduces the chance of having hot spots in the combustion process that can result in detonation. After softening these edges, thoroughly wash and rinse the pistons to remove all debris.
You want the valve relief to be at the same angle as the valve. A protractor or a valve angle gauge (from Goodson Tools for example) can be used to measure the angle of the installed valve. This is the angle between the cylinder head deck and the face of the installed valve. The same angle needs to be created at the valve pocket in the piston.
Ordering Custom Pistons
When an off-the-shelf piston isn’t available to suit your needs, it’s time to turn to custom sources such as JE, Diamond, Mahle, Ross, Probe, CP, Wiseco, and others. They offer CNC machining, which can provide about any dimensional piston configuration.
Custom pistons may be needed because of the combination of diameter, compression distance, valve clearance, compression ratio, skirt clearance, ring groove dimensions, height locations, etc. Or maybe you’re restoring a vintage engine and you simply can’t locate the pistons you need (discontinued or you’ve made changes so that a stock replacement doesn’t work). Thanks to piston design and the increased use of CNC machining, today you can basically obtain any piston configuration. “That isn’t available” no longer applies.
The cost of a custom piston was once astronomical and only within the budget of well-funded pro teams. Thanks to CNC machining, the cost has come down to the “still not cheap, but finally affordable” level, making custom pistons a viable option for just about any builder. Also, the turnaround time for custom orders has been reduced from months to weeks.
Ordering a set of custom slugs to work with your new heads and your bore/stroke combination requires you to provide the piston manufacturer with some detailed information. This includes data to achieve the correct sizing, compression ratio, and clearance.
Today’s custom forged aluminum pistons are CNC machined from precision-forged 2618 or 4032 aluminum alloy slugs. Due to the precision and repeatability of CNC processing, piston dimensions (within a set or for future replacements that reference the original order) should be exact matches in every detail, including weight (usually +/- 1 to 2g).
When you place an order for a set of custom pistons, the manufacturer asks you to complete a custom piston data sheet, which can be requested from the piston manufacturer or downloaded from their website. The piston manufacturer’s engineers examine the data sheet and may contact you to discuss any questionable areas, to confirm that the order is completed correctly.
It’s important to provide the correct information For instance, if you say that your block deck height is 9.240 inches, when in fact it is 9.230 inches, your new pistons will be .010 inch taller than required, which can result in inadequate piston-to-valve clearance. It is your responsibility to provide accurate dimensional information on the data sheet.
When filling out your data sheet, avoid using nominal published dimensions for things such as block deck height, crank stroke, rod length, rod pin bore diameter, etc. Since actual dimensions can often vary (if even by a tiny bit) from published specs, having pistons made using theoretical measurements can create potential disasters on assembly day.
Eliminate potential variables by measuring, checking, and verifying each component. This can’t be stressed enough. For instance, let’s say that a certain OEM block is listed as featuring a 9.8-inch deck height: Never assume that your block meets that spec. Actually measure the distance from the main bore centerline to the block deck (on each bank, and front-to-rear), since your block might not match factory spec. Plus, your block may have been previously resurfaced for cleanup or it may have been cut for block squaring to a specific dimension for a previous build.
The piston manufacturer needs actual dimensions for your specific application. You want to make the piston to fit your application, rather than have something that doesn’t work properly because no one measured the actual stack up of the parts beforehand. In other words, take the time to measure block deck height, check the block decks for squareness, measure crank stroke, measure connecting rod length, and check for piston skirt and pin boss clearance issues. Measure combustion chamber depth and volume, and measure valve-open locations, spark plug depth, etc.
If you provide the wrong information and the piston is made incorrectly, don’t blame them if you have a fitment problem on assembly day. Most manufacturers work with an engine builder to correct a problem; They have tractability built into their product via an engraved job number, so errors can be traced all the way back to the point of order, if necessary।
When specifying your bore diameter, don’t assume that you can have any piston diameter that you want. Yes, the piston manufacturer can produce whatever diameter you request, to fit almost any bore size, but if you can’t obtain the proper ring set for your new bore size, you can’t use the pistons. Always select your rings before you order the pistons to ensure you can get the custom rings to fit the pistons. The piston manufacturer may be able to provide matching ring sets for many popular custom requirements. You can always have custom pistons made, but you can’t have custom ring sizes made unless you have enormous amounts of money.
Once you know that you have the correct rings, you can then specify your finished bore size. Some piston manufacturers can supply the needed rings, in which case they machine the ring grooves to match the style and radial depth of the rings on your new pistons. But remember, only certain specific ring diameters are available. Regardless of where you obtain your rings, make sure that the correct rings are actually available. This dictates your choices for desired piston size and finished bore diameter.
Valve Pocket Depth
Custom piston manufacturers have layouts for most popular OEM and aftermarket cylinder heads, but every engine combination is different so valve pocket depths vary. Valve-pocket depth directly relates to an engine’s valve timing events, and actual valve position on the seat. The depth of the valve pockets absolutely dictates top ring placement and the total effective volume of the piston.
Flat-top and dished pistons are not quite as sensitive to ratio calculations as domed pistons. Domed pistons for some engines can be a real challenge to get perfect the first time.
In order to get as close as possible to the correct pocket depths, determine lifter rise at 10-degrees BTDC overlap on the exhaust side. For the intake side, you need to know lifter rise at 10-degrees ATDC overlap. This usually requires pre-assembly mock-up of the valvetrain.
Additional information you need includes the rocker arm ratio, valve angles (for both intake and exhaust), and valve free drop (how far below or above the leading edge of the valve in relationship to the deck of the head). The piston manufacturer can then calculate more accurately the required valve pocket depth for a specific combination. If you’re in doubt, call the manufacturer. They can help/advise you based on their experience on what you may need and how to obtain the required data so they can process your order. Any good custom piston manufacturer is willing to work hand-in-hand with you to achieve your goals.
Cylinder Bore Length
The piston manufacturer may also ask for the cylinder bore length. This refers to the actual length of the cylinder, from bottom to top. This length is easily obtained by using a simple tape measure. Determining the cylinder bore length is important in order to make sure the piston remains properly supported at BDC.
The manufacturer uses stroke, rod length, and cylinder length to determine if the piston can be made with the proper ovality and skirt taper to operate properly, at the top and the bottom of the cylinder.
It also tells them if the cylinder length is too short to provide proper support for a specific combination. This can pose a problem on some LS1 combinations with custom stroker configurations, where the entire piston skirt might fall too far out of the bottom of the cylinder. Combinations with too long a stroke and too short a rod/ cylinder length are the cause of this problem. This results in annoying piston slap, which, in a very short time, leads to damaged skirts and severely deformed cylinders.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks