Hi

I am carrying out common mod to my 3.0 coupe - I'm bolting a later E32 head on and the bore differs from the original. My question relates to which of the newer gaskets to use (the newer one must be used to match the water courses)...

The original bore is 89.25 and I have the immediately available choice of 89 or 92mm newer gaskets - or does one get different BMW head gaskets for overbores? If not my guess is that the 89 is the way to go as the piston won't ride above the block will it - as a consequence will airflow be affected?

Are there any answers/suggestions out there?

Many thanks

-most vaguely modern combustion chambers have a 'squish' band around the edge of the combustion chamber. This gives a static clearance of 1mm or less between the edge of the piston and the head. However, under dynamic conditions, the piston to head clearance is a small fraction of this, certainly less than the gasket thickness (the con-rod stretches....).

- This means it is normally a really bad idea to fit a head gasket with smaller holes in it than the bore, as you will get interference between the piston and the gasket. 

-you can't really cheat and relieve the squish area; you will rob the engine of both power and efficiency.

-a bigger (too big e.g. 92mm) head gasket will create a pocket for detonation and screw up the c.r.- both bad things

-you really need an 89.25mm (or whatever the new bore is) head gasket.

hope this helps

cheers