Cas I think you need to confirm if this is for an M20 or M21 crank. I don't know, but I presume this has the same crown height as an M21 piston and therefor probably not suitable for an M20.......Surely there is not enough material in that dome to safely machine it flat anyway??? Ron
As Ron notes, the long rod pistons only work with the M21 crank and there wouldn't be enough material in the crown to machine the piston flat, or even nearly flat anyway...The low compression short rod B33 piston would be the one if the crown heights work out...However, 85mm on an M20 crank will only give you 533cc so little real gain for the effort..Ian
My lack of knowledge on this topic is beginning to show, but I'd like to do some more digging on this. Where could I find out the pin to crown length for both the M20 and M21?
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.
