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 engine specifications:
Standard cylinder bore diameter – 66.000 – 66.019mm (Wear limit – 66.10mm)
Standard piston skirt diameter – 65.93 – 65.95mm (Wear limit – 65.80mm)
Maximum cylinder bore taper – 0.05mm
Maximum piston skirt to cylinder clearance 0.060 – 0.079mm
Piston Ring end gap – 0.2 -0.4mm (Wear limit 0.7mm)
Piston ring clearance in groove:
Top Ring – 0.045 – 0.080mm (Wear Limit 0.18mm)
Second Ring – 0.010 – 0.050mm (Wear Limit 0.15mm)
Oil Ring – 0.010 – 0.050mm (Wear Limit 0.15mm)
Measure pistons for cylinder clearance check at a point 5mm from bottom of skirt at a right angle to piston hole.