How to Determine Pushrod Lengths

Pushrods lead image

Most competition pushrods are manufactured from chrome molybdenum alloy steel, known as “4130.” Severe-duty pushrods (those employed in Top Fuel and Funny Car racing categories) are usually made from tool steel.

Though the strength of competition pushrods is more important than their weight, nothing contributes to the discussion more than a few lucid pointers on how to determine pushrod lengths.

When an engine is modified, pushrod length will likely change. This can be caused by  alterations to:

• The cylinder block or heads

• The camshaft part of the rocker arm assembly

• The valve stem height

• The cylinder head gasket thickness (Rather than calculating the compressed thickness of the cylinder head gaskets, it’s better practice to install head gaskets before checking pushrod length, as pistons in some engines protrude beyond the deck at their Top Dead Center position and, hence, collide with the cylinder head if the gasket is not installed).

Correct pushrod length is dictated by rocker arm geometry. This means the rocker arm tip must operate at the center of the valve tip, across its widest diameter. If it operates off center—say, closer to the intake manifold—the pushrod is too short. Closer to the exhaust manifold indicates it’s too long. Rockers that are not centrally located on the valve stem tip will cause binding and premature wear to the valves and valve guides. 

Pushrod length 1

Measuring ball/ball pushrods: In former times, pushrod lengths were measured by .140in gauge diameter. This required finding two washers, each with a 0.140in hole, and placing one washer on each end of the pushrod before measuring the distance between the washers.

Today, because oil holes and chamfers on pushrod ends are dimensionally consistent, you simply measure them tip-to-tip. Pushrod lengths of small-block Chevrolet engines vary from 7.750 in. to 8.100 in. Professional engine builders will attempt to find a pushrod within 0.050in of exact length for high performance street engines and within 0.025in for high-revving race engines. If measuring a ball/cup type pushrod, fit a 5/16 in. ball in the cup to configure its exact length.

When checking pushrod lengths of a V8 engine, check both the intake and exhaust located at all four corners. If you have an adjustable pushrod checker, use it in conjunction with light checker springs to avoid damaging the tool’s threads. With the rocker arm removed, start by coloring the tips of the first two valves with a black ink marking pen.

Pushrod length 2

Make sure the valve being tested is closed (lifter positioned on the camshaft’s base circle). You’ll know it’s on the base circle when the opposing lifter is on full lift dwell, which means it remains there for a few degrees of rotation.

Conventional pushrod checkers are usually available in four different lengths, so install the one closest in length and fit the rocker arm to obtain zero lash, which means no clearance between it and the valve tip. This is easily accomplished by spinning the checking pushrod between your finger and thumb until you feel resistance; then set the rocker locknut at that position.

Pushrod length 3

 With hydraulic lifters, ensure the lifter plunger with its pushrod cup is not moving away from its retainer clip—that it’s not being pushed downward as a result of excessive spring pressure.

Rotate the engine at least two cycles, making the rocker arms open and close the valve. With the lifter on the cam’s base circle and tension off the valve spring, undo the locknut, remove the rocker arm, and inspect the witness mark on the valve tip. Aim for a witness mark no wider than 0.050 in. When centrally located, remove the pushrod checking tool, count the number of turns and add their value to the tool’s original length. Each full turn should measure 0.050 in. If using hydraulic lifters, add 0.025 in to account for valve lifter preload.

Lastly, when purchasing custom-length pushrods, think ahead and purchase a few extra.

Erson Cams