I haven't measured the M20 crown height lately but the stroke difference between an M21 and an M20 is 18mm (.708")...So the difference in the crown height between the two pistons would be half of that figure (9mm/.354")..Using Michiels dimension for the M21 you should be able to work out the final figure for an M20 by adding the two figures together...
I'd get that figure confirmed absolutely at some point but it will give you a pretty accurate figure to work with...Ian
They call it the 'compression height', distance from center of the pin to top of the piston.
Surprisingly the difference is 0.375" / 9.53 mm instead of the 9 mm as you indeed would expect.... So it looks the top of M20 piston gets 0.5 mm higher up in the bore than the M21 piston.
