b2 piston release clearence
 

Can anybody tell me how to check the b2 band release clearence.
I am going to fit a revised b2 piston in my 1982 380 SL and i have read that the clearence was changed to reduce delayed engagement complaints.
There are six different sized pushrods available at the merc dealer so what i want to know is how to measure and determinewhich one that i need.
Many Thanks,
Lester.