Expert: Dan Liddy - 4/6/2004

Question
Hi Dan,

I have a 1978 Ford 302. It had a wild gouge in the one deck surface and both decks were machined down 0.026" for clean up. The worst piston is out of its bore 0.018" now. I have a set of 1966 289 heads that have stock valves; no milling done to them. My head gaskets are a compressed thickness of 0.045"

Non-roller cam:
Camshaft lift is Int. 0.496"  Ext. 0.520"
Camshaft duration is Int. 224¡ã  Ext. 234¡ã

Do you know if 0.027" piston-to-head clearance is sufficient? Is piston-to-head clearance based on expected engine max. RPM? (I expect 6500 RPM).

The valve-to-piston clearances average 0.160" (intake only); exhaust not even an issue.

Thanks.

Answer
(hee hee) Hi Dan- - -this is Dan:

Usually a piston to head clearance of about .030" is about right, and you are fudging already by about .003" .  Depending on how bad the gouge is, you can do one of four things.

a) It may be possible to fill the gouge with JB Weld epoxy  ,if it's away from cylinders, and water passages.   

b) use a second steel gasket as a spacer on that bank, (or both ) .

c) or machine (mill) the tops of the pistons on that bank.  That of course is going to upset your balance, and since a V-8 can be dynamically balanced, it will have to be redone.
Before you do this, accurately measure the thickness of the piston crown in the center.  You need a good 1/4 inch for cast pistons.

or d) scrap the block (arrrrggggg !!)  

It happens that I spoke with a Modified stock car racer this weekend and they found that the max stretch in the valve train was bout .060" at 8500 RPM, so you're OK there.

Jeeze- - -that's a bummer, you have my sympathies, 3 weeks ago I had an oil filter gasket fail at high revs at Sebring and destroyed the entire crank assembly. Pistons, rods, crank - - -etc.

Good luck with that.  

Dan Liddy,

Sarrasota, Florida