BRP Johnson 4HP Service Manual page 98

POWERHEAD
CYLINDER / PISTON / PISTON RING
Piston ring to groove clearance
Before checking, the piston grooves must be clean, dry and
free of carbon.
Fit the piston ring into the piston groove, and measure the
clearance between the ring and the ring groove using the
thickness gauge.

Thickness gauge
Piston ring to groove clearance
Standard:
1st and 2nd 0.03 – 0.07 mm (0.001 – 0.003 in)
Service limit:
1st and 2nd 0.12 mm (0.005 in)
If the measurement exceeds the service limits, replace the
piston and/or the piston ring.
Piston ring groove width
Standard:
1st and 2nd 1.22 – 1.24 mm (0.048 – 0.049 in)
Oil 2.51 – 2.53 mm (0.099 – 0.100 in)
Piston ring thickness
Standard:
1st and 2nd 1.17 – 1.19 mm (0.046 – 0.047 in)
6-24
Piston ring
Chrome plating
1st ring
2nd ring