Originally Posted by boomers T/A
How much is safe to mill a D port head?
I've milled many of them .065" with no problems at all. Then you have to mill the intake and intake side of head to match. What I'd usually do it knock .010" off the intake side of the head, and .055" off the intake manifold, so the ports line up. This trues up the intake side of head which is a good idea anyway. Or, you could take all .065" off the intake.
But then you have an intake that's matched to those heads and wont' line up on any other heads, unless you stack gaskets to make up for what you milled off it.
I milled a pair of 1971 round port casting 191/197 heads .065" and milled the GM aluminum intake and separate crossover .055", and the intake side of the heads .010"- it all lined right up on the engine, perfect port match. Ran the top end on a 400 w/RA IV cam. Traded the top end to a guy, he put the heads on a 455 + .060" and it ran like a raped ape, it was a rip asz engine. So it works. It won't raise compression a lot but it raises it enough, if you have one of those abysmally low 8:1 or 8.5 engines.
We also did it again on a 400 w/7K3 heads, where a previous owner had mistakenly installed new pistons with 16cc valve reliefs, with the outer edge chamfered (the pistons w/8 valve reliefs per piston). Those pistons are a good idea on a 62 cc heads, but not on a 7K3 head, the compression was like only 7.7 CR or less. He had a 96cc chamber w/10cc gasket w/16cc dish on a stock 400, the total chamber volume was like 122 cc. So we milled those heads .065" and also milled the intake side .010" and cast iron intake .055" to match it up. That bumped the CR up to 8.7:1 which was better but still a little low, but that's the best we could do with it, without teardown and putting in flat tops. That engine will run on anything even 86 octane.
Straight from Nunzi, .065" safe for street, 100" for race only. i.e. you can mill them a full .100" to raise compression on a towed car that's race only. With nearly 60 years of experience, I'd take his word for it. It worked for me.
Milling them .065" takes about 16 cc's out of the chamber volume, and bumps the compression up around a point or so, give or take. It's a good fix to raise CR on a low CR engine. But it's last in place after getting the right piston in the engine to set the CR correctly, to begin with. Reason- once you hit them with a heavy mill cut like that, you have custom parts that won't line up without the corresponding intake also cut. It reduces resale value on the heads. It's generally something done on the later 1973-up smog heads, as their chambers are huge to begin with.