What is the difference between high and low compression is it the heads or pistons, i am thinking its the block as comp ratio is stamped on the block.

on the heads - the latest ones ( which are the same as the old ones but ) have had more skimmed of - deisigned to be used with the composite head gasget ( thicker than the tin ones ) as mentioned dont worry about the last row of bolts.

There are lots of diferent pistion configurations that can give you a compresion ratio between 8.3 to 10.5. most common is arround 9.35 ish and 9.75

To get even more complicated you can start combining later heads but use a tin gasget (My Tip use well-seal if you do ) .

Got some good reading if you are going down the V8 rebuild route u2u you email and ill send it through ( when I can find it )

regards

Agriv8

With the 3.5's and 3.9 it is the pistons that change the comp ratio.

Cheers
Dan