Chris,
Here is what you are looking for. This applies to the KX500 motor built after 1988:
Piston ring end gap 0.2mm to 0.4mm (service limit is 0.7mm)
Piston groove width 1.24mm to 1.26mm (service limit is 1.34mm)
Piston ring thickness 1.17mm to 1.19mm (service limit is 1.10mm)
Piston ring-to-groove clearance 0.05mm to 0.09mm (service limit is 0.19mm)
I hope this helps,
Al