Never assume that a “stock” length pushrod is correct for any given build. A variety of alterations to the stock assembly can affect the required pushrod length, such as reducing block deck height, resurfacing the cylinder head deck, changing camshaft lift, changing rocker arm ratio, installing lifters that feature a different height, etc. Always measure to verify the pushrod length required for any given build. For example, by resurfacing the block decks, deck height might be reduced to a point at which a stock-length pushrod is too long for the application, resulting in the valves not closing fully.
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Pushrods are available in three-piece and one-piece designs. A three-piece design features a tube with hardened lower and upper tips. A one-piece design features a hardened one-piece centerless-ground machined pushrod. Wall thickness of the tube varies depending on application, with most common performance thickness of .080, .083, .135, and .165 inch. Street and many race applications call for .080- to .083-inch wall, with thicker wall pushrods required for more extreme applications. Pushrod diameters vary, again depending on the application. Typical diameters include 5/16 and 3/8 inch, with larger diameters such as 11/16, 7/16, with extra-beefy 1/2 and 9/16 inch often required for race engines that feature very high valvespring rates. Smaller diameter pushrods are also available for applications requiring 3/16- and 1/4-inch diameters. Common tube materials, depending on application, include the use of 4130/4135 chrome-moly, as well as high-tensile tool steel. The larger the diameter and the thicker the wall, the less subject the pushrod is to deflection during high-RPM conditions. Bear in mind that larger diameters and thicker walls often translate into increased valvetrain weight. Space considerations determine diameter, with regard to pushrod passages in the cylinder head. Bigger and heavier isn’t always better.
Although checking new pushrods for runout probably isn’t necessary, it doesn’t hurt just to make sure that none have been damaged during handling. Used pushrods should always be checked for runout. Pushrods can be checked easily for runout (bend) by placing the pushrod in a horizontal cradle (a dedicated pushrod checker stand is best). A dial indicator that is mounted to the stand is adjusted so that the indicator plunger contacts the pushrod at its center. The dial indicator is then adjusted so that the plunger is compressed against the pushrod to achieve a preload of about .050 inch. The indicator gauge is then adjusted to read zero on the gauge. The pushrod is then slowly rotated, noting the amount of runout. Generally speaking, a maximum allowable runout is about .001 to .0015 inch, with 0 to a maximum of .0005 inch preferred.
Never assume pushrod length based only on published specifications. Because of variations in a specific build such as block and head deck height rocker arm ratio, valve length, the size of your camshaft’s base circle, thickness of the cylinder head gasket, etc., it’s critical to take the time to determine the precise pushrod length(s) required. You should also take the time to measure for pushrod length at all intake and exhaust locations, or at least at the front and rear cylinders, in case your block decks are not perfectly parallel and equidistant from the crankshaft centerline.
Some engine designs, where offset rocker arms are employed, may require varying pushrod lengths. For a recent 632-ci race engine, using a Jesel rocker system on Dart Big Chief II cylinder heads, the exhaust pushrods were 1/2 inch in diameter with a length of 11.124 inches, while intake pushrods were 7/16-inch diameter with lengths of both 10.953 and 10.805 inches, depending on location.
Hardened tips are needed at each end of the pushrod to withstand the frictional loads at the lifter and rocker arm. Note the taper at each upper and lower tip on this racing pushrod, to provide added clearance at the lifter and rocker, where the pushrod diameter is larger than the lifter cup and rocker arm cup as shown on this 1/2-inch diameter pushrod.
Performance aftermarket pushrods are often laser-etched by the manufacturer for easy size reference. This pushrod is identified as having a wall thickness of .080 inch and a length of 9.000 inches.
If necessary, use a micrometer to measure pushrod outer diameter.
This pushrod has been laser-etched to indicate diameter, wall thickness, and length. Not all pushrod manufacturers provide this easy-reference identification.
Checking a pushrod for straightness can be achieved by slowly rolling the pushrod on a plate of clean glass.
A dedicated pushrod concentricity checker makes verifying pushrod straightness easy and precise. The pushrod is positioned on the stand’s cradles. The dial indicator plunger is positioned at the center of the pushrod. The gauge is zeroed, and the pushrod is slowly rotated to measure for any runout.
An alternative method for checking pushrod runout is to slowly roll a clean pushrod on a perfectly flat surface, such as a glass panel or a precision steel or granite platform. The pushrod should roll smoothly with no daylight visible between the flat surface and the center of the pushrod (any gap between the center of the pushrod and the flat surface can be measured with a feeler gauge). If runout is excessive, the pushrod must be replaced. Don’t attempt to straighten it.
When building a stock engine to original specifications, a stock-length pushrod will likely suffice. However, given variables such as block deck height resurfacing, cylinder head deck resurfacing, valve length, a camshaft upgrade that features a different height base circle, use of a thicker or thinner cylinder head gasket, etc., it’s always wise to measure to determine the correct pushrod length for a given application. Off-the-shelf pushrods are available in a wide range of lengths to suit any application, usually in increments of .050 inch. In addition, custom-length pushrods can be ordered from a number of pushrod manufacturers easily, including Trend, Comp Cams, Lunati, Crane, and others. Whereas the use of hydraulic lifters and/or adjustable rocker arms allow some leeway regarding length, pushrod length is especially critical when using solid lifters and/or nonadjustable rockers.
Always measure each pushrod location for length. Don’t assume that the measured length for number-1 cylinder intake location suffices for all other locations. If the block deck has not been squared and equalized on both decks, pushrods may need to be shorter at one end and longer at the opposite end of the block. Also, some engine designs that feature offset rocker arms may require specific pushrod lengths for intake and exhaust locations.
Measuring for Pushrod Length
A checking pushrod provides an easy method of determining correct length. This is a temporary pushrod that is adjustable for length.
With the engine long-block assembled (crankshaft, camshaft, timing set, lifters, cylinder head gaskets, and heads), install a checking pushrod into the first location to be checked (for instance, number-1 intake location). Choose a checking pushrod that provides a length range that suits your engine. As an example, let’s say that your engine “normally” requires a pushrod length of 7.500 inches. Choose a checking pushrod that provides a range of 7.000 to 8.000 inches.
Remove the number-1 intake valvespring and replace it with a light checking spring. This makes it easier to rotate the engine and doesn’t fight you as you check your measurement.
Rotate the crankshaft to place the camshaft lobe of the location to be checked on its base circle. In other words, the lifter must be on the base circle and not on the lobe ramp. This places the lifter as far down as it can be in its bore.
Wipe the valvestem tip clean to remove any oil. Using a marker pen, paint the valvestem tip. This provides a witness mark as the rocker tip travels across the valvestem tip.
Measuring for pushrod length is made easier with the use of light checking valvesprings. Use of these light springs maintains rocker arm geometry while checking, but doesn’t cause unwanted resistance while trying to measure for required pushrod length.
Some checking pushrods, such as this example from Comp Cams, feature a row of alignment marks. When the adjustment is fully collapsed as shown here and the two sections butt against each other, the length is as indicated on the body (for example, 7.000 inches).
As the body is lengthened by rotating the sections, each full rotation indicates a change in length of .050 inch. In this example, a 7.000-inch minimum length checking pushrod has been rotated by one full revolution, with the index marks aligned. This now indicates a length of 7.050 inches. This “micrometer” design makes it easier to determine the adjusted length, especially if a long caliper is not available for measuring.
Some checking pushrod designs are multipiece, allowing adjustments in body length and tip-to-body lengths. If the checking pushrod does not feature etched increment marks, it’s best to place matchmarks on the body sections prior to removal from the engine. Then very carefully remove the checking pushrod to avoid accidental rotation, which alters length.
Adjustable pushrod checkers must be handled very carefully when removing from the engine following on-engine adjustment. Any rotation of the checking pushrod alters the measurement. It’s a good idea to perform the adjustment and on-bench measurement several times to verify that the correct length has been achieved.
Checking pushrods are available in a variety of lengths and adjustment ranges. If you build a variety of engine types, it’s best to keep a wide range of checking pushrods in your inventory to cover all potential builds.
With camshaft and lifter installed, a light valvespring and rocker arm is put into place and an adjustable checking pushrod is installed. The cam has been rotated so that the lifter is on the lobe’s base circle. The checking pushrod is carefully adjusted to accommodate the application. If solid lifters are being used, the checking pushrod is adjusted to achieve zero lash at the rocker-to-valve. If hydraulic lifters are used, a slight preload can be added to the measured length of the checking pushrod to suit the requirements of the specific lifters. This added length for lifter preload is in the range of .035 to .050 inch, depending on the recommendation of the lifter manufacturer.
A long caliper is ideal for measuring pushrod length. Obtain a caliper capable of handling a range up to about 12 inches to cover all potential applications.
If the pushrod style requires an upper cup design, place a ball bearing into the checking pushrod’s cup, but do this after the checking pushrod has been adjusted and removed from the engine. Otherwise, if you measure the checking pushrod by placing the caliper’s anvil into the cup, you will have an incorrect measurement. Before measuring the checking pushrod with the ball in place, first measure the ball diameter. The ball diameter is subtracted from the overall measurement to obtain the correct length. The ball simply provides an easy way to make the measurement in terms of caliper contact.
Certain valvetrain designs feature adjustable ball tips on the rocker arm pushrod ends and as a result require pushrods with upper cups. Although the rocker arm ball nestles into the cup during on-engine pushrod measurement, it is extremely difficult to measure the checking pushrod’s cup with a caliper.
With the upper cup clean, insert the ball.
With the ball in place, allow the caliper to contact both the ball and the lower tip.
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This pushrod, with a ball in the upper cup, measures an overall length of 7.6995 inches. Subtracting the ball diameter of .3125 inch, the ideal pushrod length is 7.3870 inches. Depending on the length offered by a pushrod maker, you could then order a length of 7.385 inches, with fine-tuning via valve-lash adjustment.
If the actual pushrod or pushrod checker features an oil hole, measuring with a caliper can be imprecise due to the caliper contacting the hole instead of the full radius. Using a gauged pushrod checker where you know that each full rotation of adjustment represents a distance of .050 inch avoids this variable. If the checking pushrod features no oil hole, caliper measuring can directly achieve the proper results.
When checking for pushrod length, mark the valvestem tip with a black marker before installing the rocker arm. With the checking pushrod in place, the crank is rotated, causing the rocker arm to move through its entire sweep. With the rocker arm removed, check the witness mark on the valvestem tip. Ideally the rub mark should be centered on the tip. If the rub mark is biased toward the center of the engine, the pushrod is too short. If the rub mark is biased toward the exhaust side of the cylinder head, the pushrod is too long.
During pushrod length checking, verify that the rocker arm tip is relatively centered fore/aft on the valvestem tip.
Adjust the length of the checking pushrod to the theoretical length (again, if your engine normally calls for a 7.500-inch length, adjust the pushrod to that length).
Install the checking pushrod, making sure that the lower tip centers properly into the lifter.
Install the rocker arm. If the rocker is nonadjustable, tighten the rocker arm bolt or nut to final torque value. If the rocker arm is adjustable, install at approximately the center of its adjustment.
Pushrod length is affected by the type of lifters being used. I delve into that aspect in Chapter 9. Rotate the crankshaft a full two revolutions (360 degrees).
Remove the rocker arm and inspect the witness rub mark on the valvestem tip. Ideally, the rub mark should be at the center of the valvestem tip. If the rub mark is off-center and closer to the intake side of the head, the pushrod is too short. If the rub mark is biased toward the exhaust side of the head, the pushrod is too long.
When the checking pushrod has been adjusted for proper length, carefully remove the pushrod, being extremely careful to avoid changing its length (if the checking pushrod does not feature index marks, place a matchmark that aligns the two sections as a reference). Using a long caliper, carefully measure the pushrod. If you have hydraulic lifters, add to this length an appropriate amount to compensate for lifter preload, which may be in the range of .035 to .100 inch, depending on the specific style of lifter (check with the maker of the lifters for recommended preload).
If your pushrod design features a radius tip at each end, measure overall length. If your pushrod design features a cup at the upper end (for rockers that feature an adjustable ball), your measurement needs to include the distance from the lower tip to the inside of the cup. Do not attempt to measure the cup’s seat in relation to the upper edge of the cup. Instead, use a 5/16-inch ball to insert into the cup. This provides a contact point for the caliper. After you measure the checking pushrod with the ball in place, subtract the ball diameter to obtain the actual length. For instance, if the overall measurement from the lower tip to the outside of the ball is 7.8125 inches and the ball measures .3125 inch in diameter, the final pushrod length is 7.500 inches.
When measuring a checking pushrod, most feature a fully radiused tip at each end, with no oil holes. If your style of desired pushrod does feature an oil passage, the missing end of the radius (due to the oil hole diameter and amount of chamfer at the oil hole) can lead to an imprecise measurement when using a caliper to measure length.
The theoretical length assumes that the pushrod has no oil hole in the end of it. Therefore, the radius at either end is complete, which lengthens the pushrod approximately .017 inch in the case of a 5/16-inch pushrod with .100-inch-diameter oil holes, minimally chamfered.
The actual length is what you measure if you had a caliper large enough to measure over the oil holes at each end of the pushrod. Unfortunately, this measurement is affected not only by the diameter of the oil holes but also by the entrance chamfer for each oil hole.
The gauge length is the most difficult to measure, as it requires a special-length checking gauge. This is because the oil holes and their chamfers are eliminated from the measurement. The only problem is that not all companies use the same gauge diameter. Leading makers such as Comp and Trend use a .140-inch gauge (which adds .017 inch at each end if measured with a caliper). If using a gauge-type pushrod checker that features no oil holes, the variable of the oil hole and its chamfer are eliminated, allowing you to use a caliper or to simply go by the number of full revolutions of the adjustable checker (with each full revolution equaling a distance of .050 inch).
Gauge-type adjustable pushrod checkers (such as those offered by Trend, Comp, and others) are marked with a standard length stamped in them. This is the length of the adjustable pushrod when the two halves are screwed together completely. As mentioned previously, extending the adjustable pushrod one full rotation lengthens the overall length by .050 inch. For example, a pushrod stamped 7.800 inches and screwed apart one rotation has a length of 7.850 inches (7.800 + .050). Therefore, you order the part number from the catalog that matches this gauge length because they are listed by gauge length.
The length of the required pushrod also depends on the type of lifter being used. If solid lifters are to be used, the rocker arms feature a lash adjustment. Therefore, with the lifter on the cam base circle, adjust the checking pushrod to the point where you achieve zero lash. Adjustment at the rocker arm then allows you to set the required valve lash.
If hydraulic lifters are to be used, you need to consider the plunge travel inside the lifter. If you are using a light checking valvespring, you can carefully adjust the checking pushrod until it just makes contact with the lifter, without compressing the lifter. An alternative is to use a spare lifter of exactly the same make, style, and part number, and tack-weld the plunger, effectively turning it into a solid lifter. This eliminates the chance of accidentally compressing the lifter during adjustment of the checking pushrod. After the checking pushrod has been adjusted to zero lash and measured for length, add the required amount to anticipate lifter preload (again, this compensation may be .03 to .050 inch or so, depending on the lifter specifications). Bear in mind that some hydraulic lifters are designed with very short travel distance (a sort of hybrid between a solid and hydraulic design). Check with the lifter maker for recommended lifter preload to consider when determining pushrod length.
After you’ve determined the correct pushrod length for your engine and have purchased the new pushrods, always check to verify proper length. Test fit the pushrods, following the same procedure outlined here for determining length, as installed to the engine. Make sure that travel is correct, where you achieve a fully open valve condition without coil bind and a fully closed condition when the lifter is on the cam’s base circle.
Using a marker, paint the valve-stem tips, install the rockers, and rotate the crank two full revolutions, then remove the rockers and examine the witness rub marks. Again, if the rub mark is too far toward the intake side, the pushrod is too short. If the rub mark is too far toward the exhaust side, the pushrod is too long. If the rub mark is centered, the pushrod length is okay.
If you find that you have an incorrect length with the new pushrods, it means that you’ve likely made a mistake during your initial length check. Unless the new pushrods were custom ordered with a unique length, you should be able to return and reorder. Never blindly assume that the new pushrods are correct. Always double-check.
Written by Mike Mavrigian and republished with permission of CarTech Inc
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