The piston rings on a Z1 or KZ900 are 65.94mm standard size in a 66.01mm bore. Even a Clymers manual has all the ring specs, if you need them.
903cc spec:
Cylinder ID
standard 66.01mm wear limit 66.10mm
piston/cylinder bore clearance
.06 ~ .08mm
piston ring/groove clearance TOP/2nd
standard .06mm wear limit .18mm
oil ring
standard .03mm wear limit .15mm
ring thickness
*top/2nd
standard 1.45mm wear limit 1.36mm
*oil ring
standard 2.48 wear limit 2.4mm
ring groove width
*top/2nd
standard 1.51mm wear limit 1.6mm
*oil ring
standard 2.51mm wear limit 2.6mm
installed ring gap top/2nd
standard .3mm wear limit .7mm
piston ring free gap
*top/2nd
standard 9mm wear limit 6mm
*oil
standard 8mm wear limit 5mm
