So, do you think a normal head gasket made 1mm thicker and giving a comp of 8.2 : 1 would be good enough?

You've gone and made me pull out my calculator again!

Using a bore of 73mm. 1mm of height added to the clearance volume adds 4.185cc. Starting with a clearance volume of 37.5cc for your 9:1 comp ratio. Add the extra clearance volume then re-calculate comp ratio. It comes out to be 8.2:1 and then with a 2mm increase it's down to 7.54:1 etc.

I would have thought a shim with hylomar would be better than a thick headgasket...

I've read somewhere that 1mm equals one ratio unit of compression, so a 1mm shim would take it down to 8:1. I cant remember where I picked that up or how credible the source was...

Hi all
Just a thought.
Here in South Africa we have a gasket making company called "Payon".
They can make, if needed for overskimmed cylinder heads, a thicker head gasket up to a max of 3mm thick.
How much would comp drop if this is done on a standard A12 motor that has a 9:1 comp as standard? Is this not a better and cheaper idea than fitting a spacer plate and sealant that will cause possible failure under boost applications?
I know of a mechanic that uses this method on the older box shape BMW's who's cylinder heads have been overskimmed.
Have'nt had gasket failures yet.