“What I Learned Today” With Jeff Smith — Perfecting Pushrod Length
By JEFF SMITH JANUARY 18, 2022
If you are a faithful EngineLabs follower, you’ve no doubt read multiple references to checking and verifying pushrod length. A friend recently purchased a used big-block Chevy engine and complained that the engine seemed down on power and that the valvetrain was extremely noisy. After we verified the engine’s timing and carburetor settings, we pulled the valve covers to look at the valvetrain.
The engine was equipped with a set of Edelbrock aftermarket aluminum heads along with a hydraulic-roller cam and an older set of Crane aluminum roller rocker arms. We pulled one rocker and noticed that the roller rocker tip position witness mark was very close to the outer edge of the valve tip. We set up a dial indicator and measured the maximum valve lift, which came out at a rather mediocre 0.420-inch. Comparing that with the cam card that came with the engine, the card claimed 0.510-inch of maximum valve lift.
We then pulled out a set of Comp adjustable pushrods and with the cam lobe on its base circle, we adjusted the pushrod length until the roller tip contact point was on the inboard third side of the valve tip. We found this by painting the valve tip with a black Sharpie and then making a witness mark with the rocker arm. Some experimenting with different lengths created the proper pushrod length for the intake side at 0.200-inch shorter than what the engine was currently using.
This Comp Cams drawing shows how the rocker tip begins on the inboard side of the valve tip then travels across the valve as lift is created until it reaches max lift. It then travels back across the valve tip to the inboard side again. Using the numbers, valve travel across the valve tip is 1-2-3-2-1.
It’s important to note not to use the adjustable pushrod when running the valvetrain through a lift curve against the normal valve spring load. If you attempt this, it will likely bend the adjustable pushrod. If you want to attempt this to look at rocker tip travel across the valve tip, replace the valve spring with a checking spring that will make it much easier for the pushrod.
If the pushrod is too long, the rocker tip will contact the valve close to the outboard side of the valve tip. If the pushrod is too short, the witness mark will be barely touching the inboard side of the valve tip. Big-block Chevys use a shorter length pushrod for the intake compared to the exhaust and in our case, both needed to be substantially shorter.
With the cam lobe on its base circle, paint the valve stem tip with a Sharpie and then create a witness mark with the rocker tip. Use an adjustable pushrod to create a witness mark that is roughly 1/3rd in from the inboard side as shown here.
We didn’t record the difference in valve lift on the exhaust side, but on the intake side, the 0.200-inch shorter pushrod length created a more ideal travel for the rocker across the valve tip, which improved the rocker ratio and resulted in very close to 0.502-inch maximum valve lift compared to the 0.510-inch spec. So by creating the proper pushrod length, we regained over 0.080-inch of valve lift! Granted, this is perhaps an excessive case, but the engine ran much better as a result of establishing the proper pushrod length.
By JEFF SMITH JANUARY 18, 2022
If you are a faithful EngineLabs follower, you’ve no doubt read multiple references to checking and verifying pushrod length. A friend recently purchased a used big-block Chevy engine and complained that the engine seemed down on power and that the valvetrain was extremely noisy. After we verified the engine’s timing and carburetor settings, we pulled the valve covers to look at the valvetrain.
The engine was equipped with a set of Edelbrock aftermarket aluminum heads along with a hydraulic-roller cam and an older set of Crane aluminum roller rocker arms. We pulled one rocker and noticed that the roller rocker tip position witness mark was very close to the outer edge of the valve tip. We set up a dial indicator and measured the maximum valve lift, which came out at a rather mediocre 0.420-inch. Comparing that with the cam card that came with the engine, the card claimed 0.510-inch of maximum valve lift.
We then pulled out a set of Comp adjustable pushrods and with the cam lobe on its base circle, we adjusted the pushrod length until the roller tip contact point was on the inboard third side of the valve tip. We found this by painting the valve tip with a black Sharpie and then making a witness mark with the rocker arm. Some experimenting with different lengths created the proper pushrod length for the intake side at 0.200-inch shorter than what the engine was currently using.
This Comp Cams drawing shows how the rocker tip begins on the inboard side of the valve tip then travels across the valve as lift is created until it reaches max lift. It then travels back across the valve tip to the inboard side again. Using the numbers, valve travel across the valve tip is 1-2-3-2-1.
It’s important to note not to use the adjustable pushrod when running the valvetrain through a lift curve against the normal valve spring load. If you attempt this, it will likely bend the adjustable pushrod. If you want to attempt this to look at rocker tip travel across the valve tip, replace the valve spring with a checking spring that will make it much easier for the pushrod.
If the pushrod is too long, the rocker tip will contact the valve close to the outboard side of the valve tip. If the pushrod is too short, the witness mark will be barely touching the inboard side of the valve tip. Big-block Chevys use a shorter length pushrod for the intake compared to the exhaust and in our case, both needed to be substantially shorter.
With the cam lobe on its base circle, paint the valve stem tip with a Sharpie and then create a witness mark with the rocker tip. Use an adjustable pushrod to create a witness mark that is roughly 1/3rd in from the inboard side as shown here.
We didn’t record the difference in valve lift on the exhaust side, but on the intake side, the 0.200-inch shorter pushrod length created a more ideal travel for the rocker across the valve tip, which improved the rocker ratio and resulted in very close to 0.502-inch maximum valve lift compared to the 0.510-inch spec. So by creating the proper pushrod length, we regained over 0.080-inch of valve lift! Granted, this is perhaps an excessive case, but the engine ran much better as a result of establishing the proper pushrod length.
