PTV clearance on engine rebuild

[Scott Davis]

So based on my calculations I thought I would be close to .09 clearance with my current setup. I installed .04 cometic gaskets for improved quench and I've tested two cylinders and got about .06 PTV clearance with checker springs on the intake side. I've heard that due to using weaker check springs you can add .015 to your measured PTV. At .075 PTV I would be pretty comfortable but I'm not sure if that is a stretch/wishfull thinking.

On the exhaust side which is typically the riskier valve I have plenty clearance at .120. This is probably cause the cam is cut with an Early EO.

So would you guys run this at .06 to .075?

 

[Jac Mac]

If its a hydraulic cam I would add another 0.050" to that 0.090"unless your only using minimal preload.

 

[Scott Davis]

Yes its a hydraulic cam on a ls1. The preload I was planing was on the low side of the range of whats recommended for the lifters at @.04-.045. With that preload and stock rocker ratio of 1.7, if the lifters get pumped up or valve float it could easily eat up that PTV. I guess I should probably go with thicker gaskets. The usual minimum recommended I've read is .080 but probably not for an engine that lives at high RPM (which this will not). The .09 was an estimate I think I'm closer to .07

 

[Jac Mac]

Thicker gaskets are not really the answer since you should aim for around 0.040" minimum piston to head clearance as well. You could reduce the lifter pre-load to around 0.010" which would give you 0.100" PTV on your numbers, if you need/want more then fly/cut piston notch's or choose a slightly smaller cam. Remember alloy parts expand when hot and 0.010" lifter preload when cold might not be enough when hot. PTV will also increase a small amount when hot.

 

[Scott Davis]

Thanks for the help. Spoke also to Advanced Induction who did the CNC work on my heads. I decided to stick with the current setup.

@ .070" PTV
.04" cometic gasket
.04" Preload
7.503" 11/32 .120 series 5 Manton pushrods

Hopefully that leaves .002" if the the lifter is fully pumped up. But relying on upgraded springs and pushrods to keep valvetrain stable so hopefully that's not really an issue just a (very) small safety window.