Yamaha MM700A Manual page 113

ENGINE
Piston and Ring Inspection
1. Carefully check the piston, for cracks and replace it if
cracks are found. Check the piston skirt for brown varnish
buildup. If there is more than a slight amount indicating
worn or sticking rings, replace them. Clean the piston
carefully before installing new rings.
2. Check the piston skirt for galling and abrasion (Figure
galling is present, smooth the affected area with No. 400
emery paper and oil or a fine oilstone. If galling is exccs-
3. Check the condition of the piston crown. Normal car-
bon buildup can be removed using a wire wheel mounted
on a drill press. If the piston is damaged, it is important to
pinpoint the cause so the failure does not repeat after en-
gine assembly. Note the following when checking dam-
aged pistons:
a. If the piston damage is confined to the area above
the piston pin bore, the engine is probably overhcat-
ing. Seizure or galling conditions confined to the
area below the piston pin bore are usually caused by
a lack of lubrication, rather than overheating.
b. If the piston has seized and appears very dry (appar-
ent lack of lubrication on the piston), a lean fuel
mixture probably caused the overheating. Over-
heating can result from incorrect jetting, air leaks or
over-advanced ignition timing.
piston crown. This condition is discussed in Chap-
ter Two.
d. If the piston damage is confined to the exhaust port
area on the front of the piston, look for a lean fuel
mixture or over-advanced ignition timing.
e. If the piston has a melted pocket starting in the
crown or if there is a hole in the piston crown, the
engine is running too lean. Incorrect jetting, an air
leak or over-advanced ignition timing may cause
this. A spark plug that is too hot can also cause this
type of piston damage.
color indicates proper lubrication (no signs of dry-
ness or excessive heat), the damage may result from
a condition referred to as cold seizure. This condi-
tion typically results from running the engine too
hard without first properly warming it up. A lean
fuel mixture can also cause skirt seizure.
4. Check the piston ring locating pins in the piston (Fig-
ure 23). The pins should be tight and the piston should
show no signs of cracking around the pins. If a locating
pin is loose, replace the piston. A loose pin will fall out
and cause excessive engine damage.
5. Inspect the piston pin retaining clip grooves (Figure
22) for damage that could allow a clip to come out and
cause severe engine damage. Replace the piston if either
groove shows signs of wear or damage.
NOTE
Maintaining proper piston ring end gap
helps ensure peak engine performance. Ex-
cessive ring end gap reduces engine perjor-
may cause overheating.
Insufficient ring end gap causes the ring
ends to butt together and break the ring,
causing excessive engine damage.
6. Measure piston ring end gap as follows.
a. Position a ring in the top of the cylinder, then push it
into thc cylinder with the crown of a piston until the
ring is 20 mm (0.8 in.) below the top of the cylinder.
Using the piston to move the ring ensures that the
ring is square in the cylinder bore.
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