I have read many threads on TGO and many other chevy forums about checking valvetrain geometry and pushrod length. I've even followed comp and lunati's procedure to make I'm doing things right. So I've been using one of comp's high tech 6.800" adjustable pushrods.

I made a solid lifter at the same height as my hydraulics and also used lightweight check springs. With the center of the roller tip centered on the valve, i came out with a length of 7.288". My question is, do I add the 0.030" preload now? I have the stock hydraulic lifters and my rockers and cam call for a .030 preload.

I've called CompCams tech line today and the guy I spoke with said NOT to add preload.....but that is contrary to what I've read everywhere else, and it doesn't quite make sense to me when I think about it. Personally, I was going to add the .030 preload and come out with 7.318", then err on the bigger side and get the comp pushrods @ 7.325". Well let me know what you guys think!

 

You don't add preload for the hydraulic lifter and I don't add any valve lash for my solid lifters when checking the pushrod length.

Most pushrods are available in .025" lengths. Since your testing has found 7.288 to be a good pattern, you'll need to decide if 7.275 or 7.300 will make a better pattern on the valve tip.

7.800" is the standard SBC pushrod length. 7.300" is the standard length for SBC with a retro fit roller cam. You'll probably find 7.300" easier than 7.275".

 

I see. What I'm trying to understand is the dynamics of how the lifter and pushrod will work once the engine is running. Here is what I am visualizing mentally: The adjustable PR was used to measure the proper sweep pattern with a lifter that was solid at the maximum height of its travel. Therefore, this ideal sweep pattern is at the max height of the lifter, so if I took the SAME length pushrod and pushed it down into the lifter .030 for the preload, my valve sweep pattern would change slightly because the PR now sits lower than it used to in reference to the rocker fulcrum.

I might be visualizing it wrong, but I guess the most important question is, what happens to the hydraulic lifter when the engine is running? Does it ever pump up so much that it touches the retainer clip at the max height, or does it generally sit around the preload distance during normal operation? Thanks a ton!

 

bump just in-case anyone else wants to add something about PR length and preload.

 

7.300 will do just fine. Theory's sometimes a helpful tool, but experience is even better.

 

ok cool, thanks guys! looks like I'll be ordering those and installing this weekend, hope to get some pics and a vid up for the first start

 

I got lucky when I measured my pushrods. Big heads and long valves required longer pushrods. After measuring the lengths, it was completely by accident but both the intake and exhaust pushrods are .400" longer than stock. Unlike a SBC, BBC use different lengths for the intake and exhaust. A tall deck block uses .400" longer pushrods than a short deck block.

So my short deck block uses standard length tall deck pushrods. Because they are a common size, they were easy to find. If they were an oddball size, I'd have to special order them.

 

is the pushrod from a 400 longer that a 350 pushrod

 

All SBC pushrods are the same length.