So I re-read through the Engine section of Mike Macartney's book and came across this very useful bit of info: "The flat top pistons for the 3.0S, 3.0Si, 3.0Ci, 3.0CSi, 528, 630i and 530i with US specification 8.3:1 compression have a deck height of 42.6mm, while the 2002 E12 piston has a deck height of 42.1mm. using the taller flat top pistons from these six cylinder engines will raise the compression ratio on a E12 engine by 0.5mm and equate to virtually the same volume as the 1.5mm "grand piano" shaped raised portion on the original 2002 piston."
Some quick cylinder volume calculations:
88.97 "tall flat top" is +3.11 cc piston volume
89.22 "tall flat top" is +3.13 cc piston volume
89.47 "tall flat top" is +3.14 cc piston volume
89.72 "tall flat top" is +3.16 cc piston volume
So I go out to the garage to have a look at the used flat top pistons I grabbed from a local guy and surprise... there's a "5" stamped in the top of one of them.
I'd like to verify it. Is there a way to tell the difference between the 4cyl and 6cyl flat tops? Trying to measure from the center of a hole to the piston deck within .5mm tolerance is pretty impossible with digital calipers.
Each piston is Mahle and has "89V15 (then an unfamiliar symbol)" cast on the inside. Does that mean anything to anyone?