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If new rings and/or piston does not seem to restore the normal amount of
power and the usual starting and running still suffers, then bring the motor to
a machine shop for a thorough inspection using precision measuring tools.
This detailed inspection follows below.
Engine tolerances -
Measure piston OD as shown in illustration III-18.
Measure cylinder bore from front to rear near the top, in the middle and
near the bottom. Check again from side to side in the same manner.
Calculate piston to cylinder clearance using the largest cylinder measurement
and the piston measurement.
Piston to cylinder clearance
= (cylinder bore - piston size)
Piston to cylinder clearance should be between .03mm and .07mm
Check the engine compression while the bike is new, and again after it is
broken in and write it down. To check compression, remove spark plug and
screw in hose snugly for compression gage. Hold throttle wide open, and
cycle kickstarter several times. Record highest pressure reached. If testing at
a later date reveals a much lower compression, suspect a badly worn or stuck
ring, a leaking combustion chamber o-ring, cracked or damaged
reeds, clogged air filter, or leaking crankshaft seals. Rings can be
worn but still record a compression test near normal. A leakdown test
is a much better indicator of ring seal and motor condition. This test
measures leakage of rings as a percentage. A special tool is required
for this test and a source of compressed air. If bike was overheating
badly and discharging coolant, look for a leak from the combustion
chamber or cylinder into coolant passages.
Cylinder ID - Iron cylinder: 40.0 - 40.015mm new,
40.030mm max
Aluminum cylinder: 40.180 - 40.220mm new,
40.230mm max
Piston to cylinder clearance: .030mm min, .070mm max
Ring Gap: .45mm max
Reassembly
Clean all parts thoroughly before assembly!
Piston ring
- Remove old ring by spreading ends of ring with
fingers and lifting off piston. Run end of old piston ring along piston ring
groove to clean it being careful not to scratch piston skirt (if re-using old
piston). Install new ring by spreading with fingers the least amount possi-
ble, and easing ring over top of piston and into ring groove. Align gap in
ring with pin in piston groove. Works model single ring does not have a top
or bottom side.
Rod bearing
- Lube inside of rod bore with premix oil. Lightly coat rod
bearing with premix oil and insert into rod bore.
Piston
- Install one circlip into piston with opening to top of piston. Lightly
coat sides of piston, pin bores and ring with premix oil. Note arrow on top
of piston, it must point to front of motorcycle. Lower piston over connect-
ing rod and line up holes in piston with rod bearing. Insert piston pin
through piston and bearing and snug up to circlip. Install 2nd circlip into
piston with open end up. Make sure both circlips are fully seated into their
grooves.
DO NOT re-use circlips.
They loose tension with each installa-
tion and could become loose and severely damage the motor.
Measure piston diameter at points indicated.
Note position of circlip when installing.
Squeeze ring(s) on both sides of piston when inserting into
cylinder. Make sure ring(s) are centered on alignment pin(s).
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Grind away
Modify pliers to install hose clamps
side view of plier jaws
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