Pushrods are often taken for granted by novice engine builders; they need to meet the loads and performance demands of a particular engine. In many cases, if someone is building, say, a smallblock Ford engine, they tend to simply order the replacement size (diameter and length) listed for that particular engine, without any regard to valvetrain geometry. Many folks assume that “if it’s listed for my engine, it must be the right one.”
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Pushrods transmit the reciprocating motion of the valve lifters to the rocker arms. It may sound simple, but there’s more to the story. Although transferring motion that’s created by a highperformance camshaft profile, pushrods are exposed to extreme shocks and vibrations as the valves are forced to open and slam shut (in high-performance engines, via high or extreme valvespring pressures). The more aggressive the cam and the higher the spring pressure (and the higher the engine speed), the greater the stress experienced by the pushrods.
Check for Runout
Before installing any pushrod (new or used), check it for straightness. This can be done in several ways. You can slowly roll the pushrod across a piece of flat glass and look for a gap between the pushrod and glass surface. If the pushrod doesn’t roll smoothly across the glass (wobbles or bumps), it’s bent and must be discarded.
You can chuck the pushrod in a drill press and spin it. If it wobbles, it’s bent. Be sure to eliminate any variable, such as a drill press with spindle or chuck runout or a pushrod that isn’t chucked squarely. Be careful not to overtighten the chuck, which damages the pushrod wall surface.
You can use a pushrod checker. This is a bench-top stand that positions the pushrod horizontally. As you slowly rotate the pushrod, a dial indicator monitors runout at the pushrod’s center. Pushrods should have zero runout. If bent, it’s scrap.
Even if the valvetrain theoretically requires standard production length pushrods, don’t assume they are correct. Test install the heads that are fully torqued to spec and have exactly the type of head gaskets used in final assembly. Using a pushrod checker, individually measure for optimum pushrod length at every intake and exhaust location. Record the results and, if necessary, order custom-length pushrods. It is important to greatly minimize or eliminate a tolerance range. Will this make a difference in the final result? Maybe or maybe not. The point is to eliminate as many variables (however small) as possible.
Each pushrod length is critical in obtaining optimum valvetrain geometry. The goal is to allow the rocker arm to contact the tip of the valvestem at the center of the valvestem tip during rocker arm operation. Whether you’re rebuilding an engine to OEM stock specs or building a custom race engine, never assume that off-the-shelf pushrods are the correct length.
Stock replacement pushrods may function in a stock engine, but that doesn’t necessarily mean that the center of the valve tip is in the optimum position. If the pushrod is too short, the rocker arm may concentrate its contact at the outboard area of the valvestem. If it’s too long, contact may be concentrated too far inboard on the stem tip. If the rockerto-valve is off center, it can result in side loading the valve, inducing an off-center path of the valvestem within the guide. Although this may not be critical in a low-RPM grocery-getter, it can eventually cause premature guide wear or other problems under high-load, high-RPM use.
Basically, when the cam lobe is at its peak (maximum lift), the rocker arm tip should be centered on the valvestem tip. I’m focusing on optimized highperformance engines here, not bythe-numbers rebuilding a stock lowperformance engine, so you must be aware of the importance of optimizing valvetrain geometry.
A number of engine modifications can alter this geometry. One is running high-lift camshafts, having a smaller base circle, which lengthens the distance from lifter to rocker, which relocates the rocker-tip-to-valve contact path). Another is milling the cylinder heads and/or block decks, which shortens the distance, requiring shorter pushrods. Running longer valvestems is another mod. A number of variables can throw the rocker arm contact path at the valve out of whack.
In order to measure and check for pushrod length, the camshaft and lifters must be installed in the block (solid lifters should be used for checking, even if you intend to run hydraulics). Although you can use your real-world springs, this job is much easier if you use light checking springs (available from any cam and spring manufacturer, such as Summit or Jegs. With the valves and springs installed, install the cylinder head using the exact type and thickness of head gasket that you intend to use during final assembly. Torque the head to spec.
Use a special pushrod to check length. This is an adjustable-length pushrod specifically designed for determining pushrod length. These are available in a variety of lengths and come in two- or three-piece construction with threaded adjustment. Some are laser etched with reference marks (instructions may say that one full turn is equal to .050 inch in length) and the minimum length.
With the checking pushrod fully collapsed (threaded to its shortest length), you can determine final length by counting the number of revolutions as you adjust the pushrod (increasing length). If, for example, the short length is 7.000 inches and you rotated it three full turns to achieve proper length (assuming one full turn equals .050 inch), add .150 inch to the starting length of 7.000 inches. This indicates the need for a 7.150-inchlong pushrod.
Regardless of the style of checking pushrod, you can use a digital caliper to measure the adjusted length with the checking pushrod. This means that you need a long caliper (with a range of zero to 10 inches or so).
Perform pushrod length checking one pushrod location at a time. Ideally, for blueprinting, you perform pushrod length determination at each intake and exhaust location. By performing the measurement check at only one cylinder, you are assuming that the remaining locations require the same length. Using the same length for all intake positions and all exhaust positions is acceptable for most builds, but with blueprinting you are attempting to eliminate all variables. In the case of pushrod length, variables could include camshaft bore position and parallelism to the decks, valve length, rocker arm length and ratio variances, and camshaft lobe variances. If your intent is to perform a blueprint build, each pushrod location should be checked for absolute verification. In most cases, the same-length intake pushrod is ideal for each intake location, and the same-length exhaust pushrod is ideal for all exhaust locations. But by checking each, you’re working with a quantified approach instead of making an assumption.
Clean the valvestem tip to remove any oils. Using a black marker, paint the entire valvestem tip surface. This provides a witness mark during the check (when checking for rocker arm to valve centering).
Rotate the cam so that the lobe is at its base circle (the shortest lobe area). Adjust the checking pushrod to its shortest length, insert the checking pushrod into the lifter, and install the rocker arm. Adjust the checking pushrod (increasing length) until the rocker arm tip contacts the valvestem tip with zero clearance (the valve must remain closed, with no pressure pushing the valve down), then adjust for the valve lash that you intend to run (check your cam card info for valve lash recommendation).
Verify that each end of the checking pushrod is properly seated in the lifter and in the rocker. If the checking pushrod is double-nutted, tighten to lock-in the current length. Double-check for lash and re-adjust if needed. If you plan to use hardened lash caps, be sure that the caps are in place during the checking phase.
Rocker Arm Sweep
Once you have this base measurement of pushrod length (zero lash with valve closed), check for rocker arm sweep across the valvestem tip. Rotate the camshaft as you watch the rocker arm tip contact at the valve, as the valve opens and closes. When the valve is closed, the rocker arm should contact the inboard side of the valvestem tip. As the valve opens at half of maximum lift, the rocker arm should sweep across the center of the valvestem tip, moving slightly outboard. During the last half of the lift cycle, the rocker should again sweep across the valve center.
After rotating the cam through a couple of lift cycles (and carefully watching the rocker-to-valve contact sweep), bring the cam lobe to its base circle (zero lash) and remove the rocker arm. The witness mark on the valve tip should be concentrated at (or very close to) the center of the valve tip. The witness mark should measure about .080 inch or less in width (thickness of the mark/tip travel).
Regardless of which style checking pushrod you have, remove the rocker arm, and carefully remove the checking pushrod without disturbing the current length.
On a clean work surface, measure the pushrod length, using a caliper, and record the length. For safety, re-install and perform a second check to verify your measurement. When using a checking pushrod, measure from the lifter contact to the rocker arm contact. If the style of pushrod for the engine has a ball end at each end, simply measure from ball tip to ball tip.
If the top end of the pushrod is cupped, measure from the lower (lifter) ball end to the base of the top (rocker) cup. Insert a precision steel ball into the pushrod cup, and measure from the lower ball tip of the pushrod to the top of the ball that you inserted into the top cup. To obtain the correct pushrod length, subtract the ball diameter. The ball must be small enough in diameter to properly seat into the base of the pushrod cup (depending on cup diameter, this might require a 1/4- or 5/16-inch ball).
Make flat contact with the caliper on each end of a pushrod that has oil holes. This allows you to measure the actual length. However, this does not include the theoretical length, since flat-anvil calipers don’t capture the full radius of the tip due to the diameter and chamfer of the oil holes.
You measure at the small flat across the oil hole. Pushrod manufacturers use a special gauge to measure the exact length that includes the full radius. Because you probably don’t own this special gauge (and different manufacturers use different gauges), simply tell the pushrod manufacturer that you measured actual length. They can then determine the finished-product length.
The theoretical length assumes that the pushrod has no oil hole and that the tip radius is complete. To achieve the theoretical length you’ll have to add a little when you measure the actual length of an oiled pushrod. For example, in the case of a 5/16-inch-diameter pushrod, you add approximately .017 inch.
If you measure the overall length of a cup-type pushrod, you get a false reading, and the subsequent pushrod will be too long. Remember to keep a record of each pushrod length by location. Once you have obtained the measurements for all pushrod locations, chances are, depending on the style of engine, you need one length for all intake valve locations and a different length for all exhaust valve locations, or one common length for all. On rare occasions, you may need specific lengths for each valve location.
After you’ve determined the pushrod length for your engine, you can purchase off-the-shelf pushrods of the required length or order custom-length pushrods if your length is not available.
Several valvetrain parts manufacturers offer custom-order pushrods (Crane, Comp Cams, Crower, Trend, Manton, Scorpion, etc.). Simply tell them what diameter you need (5/16-, 3/8-, 7/16- inch, etc.), what wall thickness you want (choices are generally .080 or .100 inch but are available up to .188 inch depending on application), and what length. Off-the-shelf pushrods are typically available in length increments of .050 inch (.050 inch shorter than stock or longer than stock). Custom-order pushrods are commonly available in .050 inch increments, but some manufacturers offer even tighter increments. Also tell them what end style you need (ball or cup). Generally, custom pushrods can be delivered in as little as two days to two weeks.
Obtaining the optimum pushrod length helps to eliminate a few variables for valvetrain geometry, and aids in extending the life of the valvetrain and in optimizing valvetrain performance to maximize power output. Incorrect pushrod length can quickly cause a number of problems, including excessive and premature valveguide wear, reduced valve lift, coil spring bind, side loading on the valvestems, poor valve-to-piston clearance, and even rocker-arm-to-retainer clearance issues. Generally, longer pushrods decrease rocker arm tip travel. If you can’t achieve tip travel of .080 inch or less after trying several pushrod lengths, you may need to switch to another brand of rocker arm and start over.
When checking pushrod length with roller-tip rocker arms, inspect roller tip position when the valve is at one half of its net lift. Measure with a dial indicator. Ideally, the centerline of the rocker arm roller tip should coincide with the centerline of the valve at one half of net lift (this provides an equal amount of rocker arm tip travel on each half of the valvestem tip.
Pushrod guideplates limit the deflection of the pushrods during engine operation. If your heads require them, don’t just take them out of the box and slap them onto the heads. Closely examine the edges of the pushrod slots.
Using an abrasive cylinder on a die grinder or similar tool, carefully deburr the edges of the slots to eliminate any sharp edges. Finish by polishing the slots with a fine emery cloth (you can wrap emery cloth onto an appropriatediameter bolt and hand-lap the slots). When using guideplates, make sure that the pushrods have adequate surface hardening. Make sure that you order hardened pushrods. When checking for pushrod clearance (at the guideplates, through the head), don’t get carried away by overclearancing the passage(s). Too much clearance allows more pushrod deflection. Generally speaking, a clearance of .010 inch is a safe minimum. When using guideplates, you must run hardened pushrods to avoid premature wear issues.
Run a pushrod with the largest diameter that the engine accommodates because it reduces deflection and increases valvetrain stability. However, that doesn’t mean that you necessarily should move from a stock 5/16- to a 7/16-inch diameter. Today’s pushrods are comprised of advanced materials and are available in wall thicknesses to enhance stability. I’m simply saying that if a 3/8-inch-diameter pushrod easily fits, it is preferable to a 5/16-inch, etc.
I recently built a 600-hp/604-ft-lb Pontiac 501 with 5/16-inch Trend pushrods and had no valvetrain issues at all. Of course, this engine was only being ramped up to about 5,600 rpm at peak. Don’t be overly concerned about pushrod weight, since the pushrod is on the slower side of the valvetrain. When it comes to weight savings, it’s more important to consider the faster side of the system (rockers and valves).
Pushrods are exposed to forces that try to make them deflect (bend) and that generates unwanted valvetrain harmonics. This is because of the eccentric loading that results from the angular load created as the rocker arm moves through its arc. Materials and design aside, the longer the pushrod, the more beneficial it is to use a larger pushrod diameter (theoretically, a large-diameter tube deflects less than a small-diameter tube). In a recent 632-ci build with extremely high lift and extreme spring pressure, I was forced to go to a combination of 7/16- and 1/2-inch pushrods, since lengths were in the high-10- to low-11- inch-length range.
Pushrods are available in straight (same diameter from end to end) or tapered designs. Tapered pushrods are available with a single taper (tapered at one end) or double taper (tapered at both ends). Tapered ends are often needed for clearance at lifters or rockers, and heavy-wall tapered designs also provide (theoretically) increased strength and decreased harmonics, especially in highlift/ high-RPM applications.
Pushrod Wall Thickness
It’s best to consult with the pushrod manufacturer when ordering custom pushrods. In very general terms, about .080-inch wall thickness should be adequate for most high-performance street engines and even many race applications. However, when building a raceonly engine that experiences high lift and high-RPM use, moving up to .120 inch or thicker is prudent. Again, don’t be concerned with pushrod weight; it’s on the slower end of the valvetrain. Trying to save weight by going to a smaller diameter and/or thinner wall isn’t going to gain anything, and may result in excessive pushrod deflection.
By moving to a single-taper pushrod (thicker area close to the lifter), you reduce deflection even more. This is of more benefit when using roller lifters, high-ratio rocker arms, and multiple valvesprings at high RPM. However, you can’t run a tapered pushrod if you’re using guideplates. With guideplates, you must run only straight pushrods.
Also, it’s common for stock pushrods to be manufactured using 1018 steel tubing. For high-stress applications (high RPM, high spring pressures, etc.), you should always move up to chrome-moly tubing (4130 or 4140). 1018 steel pushrods should not be used with open spring pressures of about 400 psi or greater.
Pushrod End Types
Depending on the specific application (engine make, model, rocher style, etc.), pushrod ends have a ball or radius at the top (for rocker arm engagement). Radius ball ends (top and bottom) can also differ in diameter, so inform the manufacturer (when ordering customs) not only of length, diameter, and wall thickness, but of ball diameter as well. Also, depending on the application, the pushrod may or may not have an oil passage.
In some cases you may wish to restrict the oil being pushed through the pushrods (in applications where excessive oil is being delivered to the valvetrain and minimizing oil delivered to the bearings). One method of restricting oil is to use pushrods with a smaller oil passage (down to about .050 inch).
Clean the Pushrods
Before installing any pushrod (new or used), make sure that it’s clean, and that means both inside and out. During manufacturing and packing, oils, machining contaminants, etc. may be present inside an oil-through pushrod.
Use a dedicated pushrod cleaning brush and solvent (followed by compressed air) to make sure that the oil passage inside the pushrod is clean and not obstructed. These brushes are tapered with a small diameter (similar to rifle-cleaning brushes). They are available from major cam and valvetrain manufacturers. They’re inexpensive and somewhat delicate, so I suggest buying two or more.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks