How does a builder cut a head?

[IcutMetl]

Say in the case where someone cuts o-ring grooves into the top of the cylinder to do away with head gaskets, or any other reason why you'd want to raise the dome height to modify compression on a stock head...how do they go about fixturing & machining it? 3-axis CNC milling? Turned on a lathe? Form tool?

I understand it from a machining standpoint, but not from one of an engine builder.

[86 Quad R]

generally its done with a lathe and can be done easily with most aftermarket heads because the plug is straight however a jig of sorts has to be made to machine the stock heads due to the fact that the plug is at an angle.

[IcutMetl]

Ahh...that's what I figured. A lathe would be the easiest way for most shops to do it. So it is just fixtured thru the spark plug hole- that makes sense. Is the chamber a constant radius or some sort of special form?