TOHATSU MFS 25D Service Manual page 208

Power Unit
34) Inspection of Piston Clearance
1. If the piston clearance is greater than the specified limit,
replace the cylinder block, or replace the piston and piston
rings as a set, or replace all of them.
Piston Clearance (Cylinder inner diameter –
Piston outer diameter):
0.020 to 0.055 mm (0.00079 to 0.00217 in)
Functional Limit:
0.150 mm (0.00591 in)
35) Inspection of Piston Ring Side Clearance
1. Measure the piston ring side clearance. Replace the piston
and piston rings as a set if greater than the specified value.
Piston Ring Side Clearance:
Top Ring a: 0.03 to 0.08 mm (0.0012 to 0.0031 in)
Second Ring b: 0.02 to 0.06 mm (0.0008 to 0.0024 in)
Oil Ring c: 0.03 to 0.13 mm (0.0012 to 0.0051 in)
Functional Limit
Top Ring a: 0.10 mm (0.0039 in)
Second Ring b: 0.09 mm (0.0035 in)
Oil Ring c: 0.15 mm (0.0059 in)
36) Inspection of Piston Rings
Push the piston ring 1 into the ring gauge parallel to the
1.
top edge. Measure the cylinder bore at the top or bottom
sections where there is little wear.
2. If a ring gauge is not available, push the piston ring into
the cylinder with the piston crown in a parallel manner.
Measure the piston ring end gap a. Replace if greater
3.
than the specified value.
Piston Ring End Gap a:
Top Ring: 0.13 to 0.25 mm (0.0051 to 0.0098 in)
Second Ring: 0.35 to 0.50 mm (0.0138 to 0.0197 in)
Oil Ring: 0.20 to 0.70 mm (0.0079 to 0.0276 in)
Functional Limit:
Top ring: 0.50 mm (0.0197 in)
Second Ring: 0.70 mm (0.0276 in)
Oil Ring: 0.90 mm (0.0354 in)
Replace the oil ring when the top ring or second
ring is replaced.
5-68 4st 25/30 2022