Took the motor apart on the '69 K1, started to tear down the '75 K6. My buddy and I ordered a new set of rings, gaskets, and clutch fiber discs. On the '69 K1, the piston has 0.50 stamped into it. If I'm not mistaken, this should mean it was bored to 0.50mm, right?

1969 CT90K1
1975 CT90K6

San Bernardino, CA, USA