Geometry Lesson

Changing to a set of only slightly longer 8.050-inch Comp Cams pushrods moved the witness mark more toward the middle, improving the rocker geometry and reducing the side load on the valve stem. This also reduced the amount of travel across the face of the valve-stem tip.

Changing to a set of only slightly longer 8.050-inch Comp Cams pushrods moved the witness

Adjustable Pushrods

Here's where we get down to the how-to details. All you really need to check for proper pushrod length is a black felt-tip marker and a couple handtools. Yank a valve cover and rotate the engine over until the rocker arm of your choice is on the base circle of the lobe. Loosen and remove the rocker arm. Clean the top of the valve-stem tip and then paint the tip with the black marker. Then place the rocker back over the stud and rub the roller tip to create a witness mark on the valve-stem tip. Now remove the rocker arm and look for the mark.

If the pushrod length is correct, the witness mark should be located on the inboard third of the valve-stem tip. If the witness mark is too close to the intake side of the valve-stem tip, the pushrod is too short. If the witness mark is in the middle or toward the exhaust side of the valve-stem tip, then the pushrod is too long.

Everyone talks about lightweight valvetrain components, but one area where a heavier component might be an advantage would be with the pushrods. Most small-blocks run a 5/16-inch pushrod. The larger 3/8-inch rod is much stiffer.

Everyone talks about lightweight valvetrain components, but one area where a heavier compo

Let's assume you check several rockers and all your pushrods are too short. The best way to establish the proper length is to use an adjustable checking pushrod. Crane, Isky, and others offer adjustable pushrods, but the slickest tool is made by Comp Cams. This tool screws apart just like the others, but rather than having to measure the actual length of the pushrod, the Comp Cams tool reads out the length on the side of the pushrod just like a micrometer. Comp sells individual checking pushrods, or you can purchase an entire set intended for those who build many different types of pushrod engines. With this tool, you merely adjust the length of the pushrod until the witness mark is on the inboard third of the valve-stem tip. Then count the number of turns the pushrod is lengthened and you have the proper pushrod length, since each turn is worth 0.050 inch. It's best to wait until you've mocked up the engine for the last time to check for pushrod length.This way, a change to installed height or a cam change won't require a pushrod change.

Setting proper pushrod length isn't going to make an extra 100 hp, or make your engine last forever. But, it is one more step in the process of building a durable, efficient, high-performance engine that will make great power without the usual parts carnage.

Parts List

Component; Company; PN

The simplest way to reset the lash on any four-stroke engine is to work on one cylinder at a time. Bump the engine over until the exhaust valve just starts to open. This places the intake on the base circle. Now set the intake.

The simplest way to reset the lash on any four-stroke engine is to work on one cylinder at

Master pushrod checking kit; Comp; 7705
SB Chevy checking pushrod; Comp; 7703-1
Adjustable pushrod, 5.7-8.7; Crane; 99726

The Variables

These are just some of the variables that will affect pushrod length.

* Block deck height
* Rocker ratio
* Mechanical versus hydraulic cam
* Small-base-circle cams
* Different intake and exhaust base circles
* Valve installed height
* Lash caps
* Variations between manufacturers' rocker designs
* Rocker-stud runout

By Greg Smith

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