You don't have many options for replacement pistons for the ATE calipers .
The pistons are 36 mm in diameter for the ATE calipers .
If the pistons prove to be too costly to replace, Brembo calipers from an R65 will work and the replacement parts are available and less costly than ATE parts .
Rebuilding the ATE calipers on my '81 R65 is a job I am going to do when the weather cools down in 2 months here in the suburb of hell, I call home .
I've got a set of Brembo calipers off of my parts bike, '84 LS, I'm going to disassemble both sets and see if the Brembo pistons will work in an ATE caliper, I know the dust covers on the pistons are different between the two different calipers, so I don't know if they will work or not .