HI all,
Im almost finished with the heads cam install on my new ls6 to be installed and im not sure what length push rods Im gonna need. Its a ls6 crate motor with the MTI R1 cam ,Dart 205 heads(unshaved) and stock head gaskets. I know these heads are kinda new so not alot of people will be able to help.I sold my stock pushrods with the heads and cam out of this engine so i have nothing to even start from.Any ideas????
Is there a vender that might offer some insight as to the proper length I need? Any help will be greatly appreciated.thanks,Normand

 

I'm not sure why you would need anything shorter than the stock length (7.40") pushrods if you're using heads that were not decked (shaved) with stock size gaskets.

I understand it all depends on the final geometry of the valvetrain; however, I believe there are several members with AFR 205 heads decked to .59cc that are still using stock length pushrods.

If you're doing the H/C yourself, I believe there are a few vendors that sell a "Pushrod Length Checker" so you can get the exact measurement required. Check Thunder Racing site: http://www.thunderracing.com/catalog...vid=7&pcid=106

 

theres a possibility that the base circle on the cam is smaller necessitating a longer pushrod so im still not sure

 

You are probably going to have to measure it. Get a PR length checker as suggested above. Put it on the intake valve of the #1 cylinder. Rotate the cam to it's heel for that valve. Tighten the rocker to zero lash. Then tighten with a torque wrench to 22 ft/lbs. Count the number of turns you have to twist the wrench. You should have 1/2 to 3/4 turns. That comes out to ~.050" to .080" of preload. If it less or more loosen it, adjust the PR length checker longer if you had less or shorter if you had more.
The only thing this doesn't take into account is rocker wipe. If you want to check your wipe, after you find the right length pushrod remove the rocker again. Install some light 'checker' springs on the intake valve so you don't squish the lifter. Then paint something on the valve tip like Sharpie. Then remount the rocker, turn the engine over a few times, by hand, (spark plugs removed...), remove the rocker and check your wipe. It should be centered and a thin line. If it is towards the intake side of the head you will need to shim the stands appropriately. Once you have the pattern centered, then you need to repeat the procedure above until you get 1/2 to 3/4 turns. Then read your PR length on the checker and that will be your intake valve PR length. Then do it all over again for the exhaust valve.
It's not easy to do, but it is the only way you will know what pr length is ideal for your combo. You could also forget about the wipe pattern if it's close enough. Hopefully it will be.

 

Could I have a definition for "zero lash"?? What exactly would I be looking for?

 

with darts heads you will probably need the stock length 7.4's. It never hurts to check to make sure though...

 

Zero lash is the point when tightening the rocker onto the pushrod where there is no longer any play and the rocker is about to start pushing the pushrod down into the lifter. You can typically test this as the pushrod will no longer be easily turned between your fingers, as if you could get your fingers in there, as you start to tighten the rocker.

 

One more question. When zero lash is reached do the lifters need to sit for a few minutes before any adjustments are made? Do the lifters in these cars follow the rules of old school hydraulic lifters in this case? Thanks guys...

 

Think of Zero Lash as the pushrods are snug, but the rockers aren't torqued yet. It's the point where no more play is in the pushrod, rocker cup, and lifter cup, yet there is no real pressure on the pushrod so they don't have to sit to bleed. In fact, you don't want this to occur. It's just the point at which the pushrod is snug in the cups, but hasn't yet started to compress the lifter. Then you continue to tighten the rocker bolt until seated. The difference is your pre-load. These are 'old-school hydraulic lifters. They are rollers, not flat tappets though.