Pushrod length is critical
By Mike Mavrigian
Pushrod length is critical for proper valvetrain geometry and valve travel. If too short, rocker arm contact at the valve will be compromised, the lifter will not be able to maintain lifter contact and the valve opening will be restricted. If too long, valve spring bind can occur and the chances of pushrod bending is increased. If pushrod length is not correct for the application, rocker contact at the valve will be off-center (too close to the intake or exhaust side of the valve), resulting in potential walk-off of the rocker tip (in extreme cases) and by exerting off-center force at the valve tip, excessive/premature valve guide wear can easily result. If pushrods are too short and lash is excessive, this can result in valvetrain noise, and can damage lifters, cam lobes, rockers and valve tips. If pushrods are too long, you run the risk of valves not being able to fully close, which not only presents a performance issue but can result in piston-to-valve issues.
Pushrod lengths should never be selected based on assumptions. There are too many potential variables that can affect length, even when dealing with a relatively mild “stock” engine rebuild. Variables include such factors as block deck height resurfacing, cylinder head deck resurfacing, valve length, a camshaft upgrade that features a different height base circle, lifter length, head gasket thickness, etc. Pushrod length should always be carefully determined for any build, from stock to builds that push the performance envelope. When choosing length, you have the option of taking advantage of ready-made off the shelf pushrods (which are commonly available in 0.050″ or finer increments), or ordering custom lengths. A wide range of lengths, materials and designs are readily available from sources such as Elgin, Trend, Comp Cams, Lunati, Manton, Crane and others.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ4/#p=44
While a “stock” rebuild application may accept intake and exhaust pushrod lengths based on length checking only one cylinder location, it’s best to measure each pushrod location for length. Don’t assume that the measured length for number 1 cylinder intake location will perfectly suffice 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.
PUSHROD DYNAMICS
The entire valvetrain is subjected to potential harmonics and valve spring frequencies. In an OHV engine, pushrods play a major role with regard to valve spring dynamics, valve stability and valve bounce. Pushrod stiffness is a critical issue. When pushrods flex under extreme springs loads and high lift conditions, the flexing can interrupt the transition of dynamic force between the lifter and valve, to the point where it can result in erratic and somewhat late camshaft timing.
Pushrod taper (one end or both) provides clearance (where needed) for rocker arm clearance, and where required, for lifter clearance and pushrod-to-head as well. This allows the manufacture to maintain a larger body diameter while accommodating clearance issues. If a pushrod features a single taper, the tapered end mates to the rocker, where the heavier-mass end is directed to the lifter, where impact and dynamic loads are higher.
MEASURING FOR PUSHROD LENGTH
Never assume pushrod length. Determining correct pushrod length is critical to achieve proper valvetrain geometry. An adjustable checking pushrod is required. This is a segmented pushrod (usually in a two-piece format), readily available from most pushrod and/or camshaft manufacturers. Checking pushrods are offered in various minimum-maximum length ranges for given applications.
Choose a checking pushrod that provides a length range that suits your engine. As an example, let’s say that your engine would “normally” require a pushrod length of 7.500″. In this case, select a checking pushrod that provides an adjustable range of 7.000″ to 8.000″.
With the engine long block assembled and without the intake manifold, remove the No. 1 intake valve spring and replace it with a light checking spring. This will make it easier to rotate the engine and won’t fight you as you check your measurement. Carefully rotate the crankshaft in order to place the lifter of the location to be checked on its camshaft’s base circle. This will place the lifter as far down as it can be in its bore.
Clean the valve stem tip to remove any oil. Using a felt-tip marker pen, paint the entire surface of the valve stem tip. This will allow a witness mark to be obtained the rocker tip travels across the valve stem tip.
Adjust the length of the checking pushrod to the theoretical length (again, if your engine normally calls for a 7.500″ length, initially 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 non-adjustable, tighten the rocker arm bolt or nut to final torque value. If the rocker arm is adjustable, set the required clearance. Slowly rotate the crankshaft a full 360 degrees.
With the valve fully closed, remove the rocker arm and inspect the witness rub mark on the valve stem tip.
The witness mark should be at the center of the valve stem tip. If the mark is off-center and closer to the intake side of the head, the pushrod is too short. If the mark is off center towards the exhaust side of the head, the pushrod is too long. Repaint the valve tip, re-adjust the length of the checking pushrod and try again. The goal is to achieve a witness mark that is as close to the center of the valve tip as possible.
Once 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 0.035″ to 0.100″, 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 seat in relation to the upper edge of the cup. Instead, place a 5/16″ ball into the pushrod cup. This will provide a contact point for the caliper. Once you measure the checking pushrod with the ball in place, then 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″ and the ball measures 0.3125″ in diameter, the final pushrod length would be 7.500″.
When measuring a checking pushrod, note that if your style of desired pushrod features 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″ in the case of a 5/16″ pushrod with .100″ diameter oil holes, minimally chamfered.
The actual length is what you would 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 checking pushrod length may be 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. Leading makers such as Comp and Trend use a 0.140″ gauge which adds 0.017″ 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 0.050″).
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 minimum length of the adjustable pushrod when the two halves are screwed completely together. For example, a pushrod stamped 7.500″ and screwed apart one rotation would have a length of 7.755″. Therefore you would order the part number from the catalog that most closely matches this gauge length.
Once 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 conditions without coil bind and a fully-closed condition when the lifter is on the cam’s base circle.
ROCKER STUD VS ROCKER SHAFT SYSTEM
The type of rocker system presents different concerns relative to pushrod length. On a “conventional” system where the rocker arm is mounted on a rocker stud, pushrod length dictates rocker tip-to-valve-tip sweep location. Valve lash is adjusted at the rocker stud’s nut.
On a shaft system, where the rocker arms pivot on a rocker stand, valve lash is adjysted at the rocker’s pushrod cup. While the correct length pushrod remains a critical element, the length of the pushrod does not dictate how the rocker tip sweeps at the valve tip. Rather, the height of the rocker stand provides the adjustment of rocker tip sweep (the stand may be shimmed to raise stand height when needed).
On a rocker stud system, if the pushrod is too long (moving the valve tip sweep pattern towards the exhaust side), more of the rocker stud is exposed, which can lead to deflection of the stud. Over time, the potential for stud failure increases.
PUSHROD CONSTRUCTION
Commonly, pushrods are available in three-piece and one-piece designs. A three-piece design features a tube with added hardened 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 0.080″, 0.083″, 0.135″ and 0.165″. Street and many race applications will call for 0.080-0.083″ 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″, with larger diameters such as 11/16″, 7/16″, with extra-beefy 1/2″ and 9/16″ often required for race engines that feature very high valve spring rates. Bear in mind that large diameter pushrods may interfere with pushrod passages in the cylinder head. In some cases, the passages will require slight enlargement to clear.
PUSHROD INSPECTION
Especially when dealing with pushrods that have previously been used, always check for pushrod straightness/runout. This check can be performed with a pushrod checking stand that features a dial indicator. The indicator plunger contacts the pushrod at its length center. The dial indicator is then adjusted so that the plunger is compressed against the pushrod to achieve a preload of about 0.050″. 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 0.001-0.0015″, with zero to a maximum of 0.0005″ preferred.
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.
GUIDEPLATES
Pushrod guide plates (where applicable) serve to guide/center the pushrods to avoid influencing the rockers from moving off of the center relative to the valve tips. Note: If using guide plates, the pushrods must be hardened steel to avoid pushrod wear.
For applications where the valves are offset, as in big block Chevy applications, stock-style one-piece guide plates may be suitable for stock builds, but when more precise aftermarket roller rockers are employed, one piece guide plates may or may not allow proper centering of the rocker tips to the valves. To alleviate this issue, Dart offers two-piece adjustable guide plates. A screw and locking nut at the hinge point allows easy adjustment. Slightly loosen the pivot just enough to allow forced movement. Install rocker studs and rockers. If a rocker tip is not centered at its valve, use a brass drift and hammer to gently tap the guide plate’s pivot point to center the intake rocker to its valve. Tighten the intake rocker stud. Do the same at the exhaust location. After torquing both rocker studs, re-check to verify that both rockers align to their valve tips. Then tighten the guide plate locking nut.
VALVE LASH CAPS
Valve lash caps (hardened steel caps that fit onto valve stem tips) may be used in certain instances to protect softer-than-steel titanium valves from wear. Note that some titanium valves may already feature pressed-in steel tips. If the valves do feature steel tips, do not add a steel lash cap, as the potential for cap-to-tip movement may result in a sheering effect, where the pressed-in steel tip may loosen and separate. Valve lash caps are available to suit a variety of valve stem diameters, as well as various thicknesses to accommodate valve length where valves may be too short to obtain desired rocker arm geometry. Use of lash caps on small-diameter valve stems can be beneficial, as the caps provide increased surface area for rocker contact.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ4/#p=44