302 Piston to valve clearance

CobraEd · #1 (**permalink**) 11-14-2004, 01:06 PM

Any Guru engine builders out there who can advise me on this. I don't want to yank the heads to check this.

I have a Crane cam with 288 total duration, 228 @ .050 and .488 lift with a lobe separation of 108. I want to go to 1.7 rockers from my current 1.6 roller rockers. This is an old 302 with dished pistons, not a newer 5.0 with flattops. So, . . . if I go from .488 lift to .518 with the 1.7 rockers, am I gonna make it???

The heads are Edelbrock aluminum with 1.90 intakes.

thanks
Ed

blykins · #2 (**permalink**) 11-14-2004, 01:37 PM

Did you measure the piston to valve clearance when you assembled the engine?

Big-Foot · #3 (**permalink**) 11-15-2004, 11:33 AM

Ed,

I could say that the odds are in your favor that you will have sufficient clearance at low RPM. However, as things wear and get to shaking at higher RPM, you may end up with just a touch of valve float and potentially some piston rock since your engine is not exactly fresh as a daisy. The primary thing you need to be concerned with is the piston chasing down the exhaust valve as it is moving to the top of the exhaust stroke. You want .100" clearance during this process to take care of any incidental valve float as well as to compensate for growth of the piston (yep they grow a fair amount and no two are the same) and rod stretch in the higher RPMs.
The only way you will know - for sure - is to pop one of the heads and do a "clay" test on one of the pistons. Yeah, I am fully aware of how much work that entails - so pick the easy side to do it...

If you knew of someone else who had the identical configuration and they had done the clay test, you could probably use their data.

Otherwise - Unless you are willing to pop one of the heads, I would say that the gamble is just not worth it..