How to measure caliper piston sizes?
 

I'm trying to find out what calipers I have here, they were supposedly from a 993 C4 rear. The pistons however seem to measure about 31/32 and 36/37 mm across.

From previous posts the 993 C4 rear was supposed to have 2x30 and 2x28 pistons... These are definitely not that, unless I am way off measurements..

More likely they are the 993TT rear calipers which used the 2x30 and 2x36 pistons?

How do I measure the pistons to be sure?

Here is a pic of a typical piston, you measure the diameter at the widest part;
http://www.prospeedmktggrp.com/Brake...new_piston.jpg
Here is a closeup of the new style seal and piston installed in a caliper;
http://www.prospeedmktggrp.com/Brake...on_example.jpg

In the rear the 993 tt uses 28/28 pistons the 993RS which is very rare here in North America uses 30/36, all other 993 C2/4 use 30/28 all other 964 C2/4/RSA use 30/28, all 964 T/TL use 30/34, all 964 RS use 30/34