"Increasing chamber cc's reduces the compression ratio."

My mistake. I described what I was trying to say wrong. What I meant to say was increasing the cc's to get the desired comp ratio to under 11:1 when using flat top pistons. I'm getting things crossed up between raising the comp. to under 11:1 by raising the cc's using flat top pistons or lowering the cc's using dished pistons.

If you're dealing with an experienced "engine builder", just have him order what he needs.

Math time: A 512 engine has 1049cc swept volume. To get to 10.5:1 compression divide swept volume by 9.5. That's 110cc.
ASSUME: Piston at zero deck, 12cc head gasket, 5cc valve pockets, 84cc head = 101cc. That's going to get you 11.4:1.

You need to lose 9cc from the half of the piston not under the squish area. Let's say you use a 3.5" circle, you need a half circle 0.0115" deep. That's with all my other assumptions.

It's just math.

What I think you and your engine builder are missing is that you don't have to buy the crank, rods and pistons as a kit. If you buy the pistons separately, you can get exactly what you need. Buying a piston with too large a dish and then milling a brand new head to get the compression ratio you need is IMHO pretty dumb.
Hogging out the combustion chambers to 93cc is also dumb, that's quite a bit of aluminum and you don't want to lose the squish pad area. So that makes the amount taken out of half the chamber pretty big.

R.

Icon, Diamond and others stock -12.5cc reverse dome pistons in 1.32" Compression height in various bore sizes (I'll assume you are going 4.375).

With a zero decked block (9.980) and a .040ish head gasket you can get by just fine on pump gas in the 10.8-11.2 range if you respect the ignition timing gods and keep the chambers in the 82-86cc range. Adjust to a CR you are comfortable with by using various head gasket thicknesses in your calculations.

My 400 based engine uses Diamond shelf pistons as mentioned above. Ported Edelbrock Victors yielded about 84cc chambers.

My 2 cents from doing our 511.

Well, there you go.

R.

Never heard mention when milling heads that either the intake manifold or intake side of heads must also be cut. Can be a future pita to deal with if wishing to swap either to another engine. Most are aware, just heads up if not.