I've got a set of 64cc chambered heads that I want to put on a 305. Now I've milled heads from 64cc to 58cc in the past and it's worked, but the guy at the machine shop told me today that if I mill the heads from 64cc to 58cc the intake manifold won't line up properly and will end up creating a minor obstruction where I will lose power, especially at higher RPM's. Has anyone heard of this before?

 

I have heard of similar situations, milling could possilby affect the mating surface, additionally I have heard of problems with rocker arm geometry also with extreme milling. can change the distance of the pushrod to the rocker arm tip or some thing.

 

I don't see how it could be any worse, gasket match-wise, than the stock GM configuration.

If you need, you can have the intake milled a bit to compensate. But I think gasket squish, when properly torqued, will make up for it. Just use the squeeze tube silicon for the china walls and all should be fine.

My heads have been milled 3 times and I have NO sealing problems at all.

 

i'd guess he's right, but you can gasket match the ports, and you'll be making more power by milling the heads, or you should be. you do run into sealing issues when you mill .025 or so off the head because it changes the fit between the intake and head surface.

 

Technically speaking, the intake should be machined to match the head height. However, given that the end seals ("China walls") on the most of the SBC engines are so forgiving, you should be able to get decent port alignment and a good seal without a lot of intake work. I'd suggest raising the ports on the heads instead of lowering the ports on the intake to get your alignment.

The question I have is how much are you planning to remove? I'm guessing that 0.035" should bring you up to about 9.5:1 static compression. If you want more than that, you may be pushing the edge a bit. 0.050" would take you to about 9.75:1, but you may run into other problems.

I'd start to get a little more concerned about distributor drive gear alignment and binding the oil pump drive shaft after taking off much more than 0.030". And of course, piston to valve clearance will be reduced by however much you are removing, so if you're pushing the edge now, it isn't going to get any better. Also, If your push rods are customs for a smaller base circle cam, you may have to re-evaluate the geometry there. I only mention it since I've been down that road before. Milling 0.060" made more problems than the gain was worth, and I ended up with two anchors and another pair of heads with designed smaller chambers (although, the originals were 76cc smoggers that I had ported - more wasted effort). Not to mention that the head castings can start to get a little thin after taking half of the design thickness away.

 

Well I'm shooting for 9.0-9.5:1 compression on my 305 with a flat top piston. I thought that I could just gasket match the intake with the head and it should seal properly. I've done this exact same thing in the past and didn't have any sealing problems but the performance issue that the machinist brought up kind of concerned me. It's not like I'm running some radical setup; the cam only has .480" lift (w/1.5) and it's a standard aftermarket SBC intake.