I'm pretty sure that is the difference on the ProMAXX 63 and 65 CC heads. The 65 have a little more room for the positive height pistons. I think the Edelbrock do the same, hence the 340 version? [/quote]

So the 65cc heads are an open chamber design to clear the piston. The only pics i could find of them looked like closed chambers but maybe that was a generic pic?