How To Check For Proper Pushrod Length - Push It - CHP Step By Step
Checking Pushrod Length
From the November, 2009 issue of Chevy High Performance
By Henry De Los Santos
Photography by Henry De Los Santos
As complicated as engines can be, they are remarkably resilient. When it comes to valvetrain stability, it's critical to get the tolerances correct for the utmost power and longevity. This means having the proper pushrod length and making sure the rocker arm is centered on the valve tip itself.
Simply put, rocker arms are designed to make contact at the top of the valve tip, with minimal movement. Think of the valve tip in three stages: top, middle, and bottom. The top, facing the intake side of the center, is where it starts from the base circle of the cam. From there, the rocker will move to the center, where the cam is at midlift. The final sweep is at the bottom, where the rocker is seeing maximum lift. This is the ideal geometry for a rocker arm.
If the rocker is not centered, however, you're opening up the door for a host of problems. Simply using the wrong pushrod length will cause the rocker arm to touch the valve tip either too far behind or forward, and that can cause the rocker to run across the entire valve tip and possibly bend the valve stem. This, in turn, can start a volatile chain reaction resulting in unnecessary engine friction to worn-out valveguides and possibly breaking the valves from side load stress. And if you didn't already realize it, more aggressive camshafts will only aggravate the problems.
We used a Comp Cams Hi-Tech...
We used a Comp Cams Hi-Tech pushrod length checker that's adjustable from 5.800 to 6.800 inches. It's an invaluable tool and can be had for as little as $17. Should you need something with a wider range, Comp also offers a four-piece kit that can measure between 5.800 and 9.800 inches for $66.
The fact is, rocker arm geometry shouldn't be taken lightly. You need to realize that every engine build varies. Even off-the-shelf combinations can differ based on the block deck height, head stud boss height, varying brands of lifters, and cam base circle size. If there's a problem from the beginning, be sure to address it. Every once in a while we still hear of people clearancing rocker arms because it's hitting the retainer. If it's hitting the retainer, you're having other issues and should size up the pushrod appropriately. All said and done, it doesn't cost much to do and it's easy enough to check.
What We Did
Checked for proper pushrod length
Accuracy is key
$17 for a Comp Cams adjustable pushrod
We first made sure the lifter...
We first made sure the lifter was on the base circle of the camshaft and started with a 6.200-length pushrod. Obviously this is way too short, resulting in a rocker that isn't touching the valve tip; the underside of the rocker body resting on the retainer; and, up top, the polylock pressed against the rocker itself.
Stepping up to a 6.400-inch...
Stepping up to a 6.400-inch pushrod length allowed the rocker arm's roller tip to touch the valve and even gained additional clearance on the polylock. However, the underside of the rocker is still touching the retainer slightly.
Next, we lengthened the adjustable...
Next, we lengthened the adjustable pushrod by two turns, adding 0.100, giving us a 6.500-inch pushrod. This time around everything looked promising, with the rocker's roller tip being centered on the valve tip and showing ample clearance for the polylock and retainer.
Now that we had a general...
Now that we had a general idea of where we needed to be, we forged ahead to see how a 6.600-inch length would look. Surprisingly this length looked pretty good. It was centered on the valve tip and also had plenty of clearance. To test it, we went ahead and rotated the engine by hand and found that the rocker would push too far forward during a full rotation.
Since we wanted to see the...
Since we wanted to see the effects of a longer pushrod, we tried a 6.700-inch length. You can see that in its resting state it pushes the rocker too far forward and over the center of the valve tip.
To see the effects of an overly...
To see the effects of an overly long pushrod, we then extended the adjustable pushrod to 6.800 inches and gave the engine a full rotation. While the rocker arm produced a full sweep across the valve tip and didn't fall off either edge, this type of stress can induce side load on the valve stem, eventually causing the valve to move side to side. This could lead to prematurely wearing out the valveguide and to breaking the valve itself.
If you're having a hard time...
If you're having a hard time centering the rocker onto the valve tip, make sure the guideplates aren't causing the hang-up. This generally isn't a problem, but if you can't resolve it, a viable alternative is to use adjustable guideplates.
Now that you have a better...
Now that you have a better idea of how the rocker should be in a resting state, you'll want to check the wear pattern of the roller tip in relation to the valve tip. Simply use a marker of your choice (a Sharpie or machinist's dye will work) and cover the entire valve tip-this will eventually wear off after several rotations. Install the rocker, zeroing out the lash or setting it to the recommended preload with a hydraulic lifter.
After rotating the crankshaft...
After rotating the crankshaft by hand, remove the rocker arm and examine where the ink has been worn. Ideally the wear pattern will be centered on the valve tip, similar to this image. If the line is a little too high, the pushrod is too short, whereas if the line is on the lower side, the pushrod is a tad too long. Pretty straightforward, isn't it?