Hey all,

I just wanted to get someone to look over this before I dropped $100 on new pushrods, because the numbers seemed a little off to me...

I used a CompCams Pushrod length checker to determine the proper length pushrod I would need for my roller rocker set-up. I installed the valvesprings, inserted the pushrod length checker (set at the same length as the factory 7.200" pushrods), set the roller rocker arm in the correct location and tightened the lock nut to 1/2 turn past the point where there was no play in the pushrod. I blackened the tip of the valve with a marker and turned the engine over a few times to make a witness mark. The mark was pretty far outboard on the valve-stem, so I repeated the process, continuing to shorten the length checker each time until I had the mark centered up on the valve. However, this happened at a checker length of 6.925"? Doesn't this seem a bit short to anyone else?

My combo:
Factory 1987 Roller-cam 305 Short-Block
Corvette L98 Aluminum heads
Comp Cams 26918 beehive springs
Harland Sharp 1.6:1 Aluminum Roller Rockers
Comp Cams XR258 HR-12 cam
Comp Cams pushrod guide plates

Thanks for the help guys, I really appreciate it.

-Levi

 

Try doing it to make the narrowest mark possible, without worrying where on the tip of the valve it is.

 

"Try doing it to make the narrowest mark possible, without worrying where on the tip of the valve it is."

Exactly. What is the width of the "witness" make with the 7.2" push rod compared to the 6.925" push rod? I would say as long as the witness mark is within the center 1/2 of the valve stem diameter that would be just fine. Pictures are worth a thousand words.

 

I didn't notice a difference between the width of either of the marks, they appeared to be virtually the same. I set it to the center of the valve-stem as the comp-cams tech help, and other websites that I have read said to do.

Setting the witness mark to occur as close as possible to the center of the valve stem should reduce valve guide wear and ensure proper valvetrain geometry. I am uncertain as to the benefits of the narrowest witness mark possible, other than it corresponds with the highest possible valve lift? Wouldn't an offset mark also induce stress on the valve guides due to the moment it creates? Why does Comp not talk about the narrowest witness mark possible?

Thanks,

-Levi

 

Do some research on mid-lift valve geometry. For what you are doing a 7" pushrod will probably be just fine. However if you are after performance than do the research. Also seeing as you have guide plates make sure you have the hardened pushrods.

 

Setting it to the narrowest width has the least side-loading, lowest wear, and highest lift. Not really sure why Comp doesn't do it that way, but it's probably because they already have enough trouble describing the process to center the mark. In an ideal world where all the parts agree with each other the narrowest sweep would be across the center of the tip, but change the valve length, deck the block, mill the heads, use a different brand of rockers, etc., and it can be off considerably.

 

When I did my engine, at zero lift, I wiggled the rocker back and forth to make a mark across the valve tip then rotated the engine to max lift and did the same thing. I kept adjusting the pushrod until both marks were centered on the valve tip then I checked the measurement.

Depending of the pushrods, they're generally sold in .050" increments.

Using a hydraulic lifter will throw out your measurements. You either need to use a solid lifter or modify the hydraulic lifter so that it doesn't collapse while testing the length. Lightweight checking valve springs should be used. They're just strong enough to hold the valves closed and light enough to keep the pushrod from compressing the hydraulic lifter. With heavy spring pressures, it's easy to damage the checking pushrod when the spring is compressed. My triple springs probably have more seat pressure than yours have when open. There's no way a checking pushrod can open my valves.

If you used normal valve springs and a hydraulic lifter, your readings are off.

 

i read the first three posts and and not any further but milling the heads down can change the length and people not using the shims on the the spring cuts cause from what i heard thats a whole .100-.200 difference so try measuring it on another slot.

 

Cutting the spring pockets or otherwise changing the spring height won't change it at all.

 

it does if someone doesnt use the shims and has the valves cut instead of the pockets leveled or shimmed ive seen a pair that had different length valves and spring to accommodate the difference weird yes i know but it has happened before

 

The proximal cause then is the valve, not the spring pocket.