Push rod length question!

[MyHoFoSho]

Ok i am putting some gt40 heads, tfs 1 cam and scorpion 1.6 rr this winter. I had the heads rebuilt and decked .0030" and also with the addtion of the new cam and roller rockers should I measure for new push rods. I know the rockers may or may not have to be shimmed. Is the shimming of the rockers enough to get the proper valve train geometry. Or should I not shim the rockers and measure for new push rods. I am not sure what to really do here, I have read how to set up the rockers and know how to measure for the push rods but not sure if I should just do one or the other or both. Any help or ideas whould be great.

 

[liljoe07]

Shiming does two things, it moves the rocker sweep toward the exhaust side of the valve stem when you add shims, and it also effects the lifter preload.

When setting up pedestal mount rockers, you will have to do both to achieve the correct geometry. If you use a solid lifter to check Push Rod length, all those measurements are done at 0 lash. Once you figure out the correct amount of shims and Push Rod length, you would then add length to the Push Rod measurement, and that gives you the preload. So if you add .040 to your 0 lash Push Rod length, it will give you .040 of preload on the lifter.

 

[90 lx convert]

Whatever you decide, shims or new pushrods always check for proper valvetrain geometry. You're changing alot of parts and decking the heads so it's possible shims alone may not bring it back. Mike

 

[MyHoFoSho]

well now should i check for the shims with the stock push rods in or check it with the push rod length checker. And liljoe you are saying that if it takes .040 of shims to get the rockers at 0 lash i then have to add .040 to the push rod length, i think that is what you are saying?

 

[liljoe07]

I always use a Pushrod checker. Generally speaking since the heads have been decked .030, the pushrods will be long about that much, specially since the rocker pad and head deck are on the same plane. Or should be. Stock geometry can be off quite a bit, so it probably wasnt right to begin with. Been there, done that.

0 lask means no lifter preload. You use the best combination of shims and pushrod length to get the best pattern(thinest sweep) at 0 lash. This is done with a solid lifter and should be done with a light checking spring, as the Pushrod checker is kinda weak and will deflect. Once you have the correct shims and Pushrod length figured out, you add length to the pushrod measurement you get. The amount you add will be the amount of preload thats put to the lifter. Typically Pushrods come in .050 increments, so you just find one thats in range, or call a company like Trend or Smith Brothers and order you a set.

Any time you add or take away, shims from the 0 lash measurement, you add or take away preload from the lifter.