I can pop the O-rings out and measure when I get home. Probably won't be back for another week, though (work).Do you happen to have to piston & oring seal measurements? Actually need reference for a set of M640's but I'm thinking I can subtract the difference..
Having really bad luck with the shimano levers + wandering/no bite point..
FWIW (just another data point from a random average MTBer), I used the same M820 calipers since 2013 but went through 3 sets of M985/M9000 levers. The MC bore on my last RH lever was completely worn out - I put the piston/seal assembly from an old leaky LH lever in it, thinking that would get me by for a week, but it didn't even make it through one ride.It's the Shimano calipers that suck, not the levers.
I replaced them with a pair of Quadiems. So far so good.