Erinyes

I've found some info on this topic, but not specifically what I need to know, and am hoping someone can clue me in before I have to tell the machinist how much to take off.

1965 425, A heads. Heads are currently at the machine shop for light valve trimming due to stem height issues, and 3 angle cuts (assuming there are no other problems existing). The engine still had the factory steel shim gaskets.

Since today's gaskets are thicker (FelPro gaskets are .039 compressed compared to .017 for the factory steel gaskets), I'm considering having the heads milled to maintain or even gain a little compression. From what I've been able to research, every .006" milled from the head results in app. 1cc decrease in compression chamber volume.

I'm contemplating milling .060" from the heads. This would give me an effective decrease of .038" given the thicker gasket, and a compression ratio of 9.2 to 1, according to Summit's calculator and if I'm measuring the dish volume correctly (more on that below). It would also decrease the lifter preload, wouldn't it? The pushrods and rockers are stock original, the heads are moving closer to the block, and there will be a fresh 3-angle valve job. The original preload was at or over the maximum .060, resulting in a valve or two never fully seating.

Would milling the heads .060" (resulting in total movement of .038" closer to the block) move the lifter preload back within tolerance? Move it too far in the right direction? Or am I completely backwards, and the preload would move in the wrong direction?

I had originally wanted to go with roller rockers, but decided the expense wasn't worth the gain. Would this setup require adjustable rockers? It does seem like it would take the guesswork out of the equation...

Besides changing the intake manifold matching, would milling that much from the heads cause any other problems? From what I've read, the intake side is exactly 90* from the valve side, so intake manifold compensation is done by removing .001" from the intake side for every .001" from the valve side. I would have the shop mill .038" from the intake side of the heads to compensate for the manifold fit and use silicone for the front and back rails with a steel turkey pan gasket.

Also, does anyone know what the factory spec is for the low compression (rated at 9.0 to 1) dish?

The piston dish depth appears to be .220" (crappy tools, and I can't find the factory spec for the dish), the dish diameter appears to be 2.865" (ditto on crappy tools), and the deck height appears to be .020" (ditto again on tools, although I think I can make a cylinder strap to get better measurements with if need be). This gives me a dish volume of 23.24cc (actual volume measurements using liquid isn't really an option, since I'm not disassembling the block).

TIA

Oldsmaniac

My 2 cents is if you want compression get it with pistons and not milling heads. A clean-up mill which is minimal is what I do to my heads. The fel-pro gaskets will lower compression a little but for a street engine it doesnt matter.

cutlassefi

Milling the heads past the gasket thickness difference will make everything worse. Why not just buy the steel shim gaskets? M&J has them in stock.

442rocketdave

Dont mill them 060 then your at the limit and like Oldsmaniac says just a clean up mill and Cutlassefi is right just get the originals and you wont get your geometry off on your rockers.

Erinyes

Sounds like a plan! Thanks, all.