Piston and Rings
Piston and Rings Components and Details
A
B
D
E
C
F
A
Piston
B
Top Compression
Ring
C
Middle Compression
Ring
D
Rails
E
Expander
F
Oil Control Ring
(3 Piece)
Inspection
NOTE: Rings must be installed correctly. Ring
installation instructions are usually included with
new ring sets. Follow instructions carefully. Use
a piston ring expander to install rings. Install
bottom (oil control) ring first and top
compression ring last.
Scuffing and scoring of pistons and cylinder walls occurs
when internal engine temperatures approach welding
point of piston. Temperatures high enough to do this are
created by friction, which is usually attributed to improper
lubrication and/or overheating of engine.
Normally, very little wear takes place in piston boss-
piston pin area. If original piston and connecting rod can
be reused after new rings are installed, original pin can
also be reused but new piston pin retainers are required.
Piston pin is included as part of piston assembly – if pin
boss in piston or pin are worn or damaged, a new piston
assembly is required.
Ring failure is usually indicated by excessive oil
consumption and blue exhaust smoke. When rings fail,
oil is allowed to enter combustion chamber where it is
burned along with fuel. High oil consumption can also
occur when piston ring end gap is incorrect because
ring cannot properly conform to cylinder wall under this
condition. Oil control is also lost when ring gaps are not
staggered during installation.
When cylinder temperatures get too high, lacquer and
varnish collect on pistons causing rings to stick, which
results in rapid wear. A worn ring usually takes on a
shiny or bright appearance.
Scratches on rings and pistons are caused by abrasive
material such as carbon, dirt, or pieces of hard metal.
Detonation damage occurs when a portion of fuel charge
ignites spontaneously from heat and pressure shortly
after ignition. This creates 2 flame fronts, which meet
and explode to create extreme hammering pressures on
a specific area of piston. Detonation generally occurs
from using low octane fuels.
Preignition or ignition of fuel charge before timed spark
can cause damage similar to detonation. Preignition
damage is often more severe than detonation damage.
Preignition is caused by a hot spot in combustion
chamber from sources such as glowing carbon deposits,
blocked cooling fins, an improperly seated valve, or
wrong spark plug(s).
Replacement pistons are available in STD bore size, and
0.25 mm (0.010 in.), and 0.50 mm (0.020 in.) oversize.
Replacement pistons include new piston ring sets and
new piston pins.
Replacement ring sets are also available separately
for STD, 0.25 mm (0.010 in.), and 0.50 mm (0.020 in.)
oversize pistons. Always use new piston rings when
installing pistons. Never use old rings.
Some important points to remember when servicing
piston rings:
1. Cylinder bore must be de-glazed before service ring
sets are used.
2. If cylinder bore does not need re-boring and if old
piston is within wear limits and free of score or scuff
marks, old piston may be reused.
3. Remove old rings and clean up grooves. Never
reuse old rings.
4. Before installing new rings on piston, place top 2
rings, each in turn, in its running area in cylinder
bore and check end gap.
80 mm bore engines: Top compression ring end gap
clearance is 0.100/0.279 mm (0.0039/0.0110 in.) for
a new bore, or 0.490 mm (0.0192 in.) for a used
bore. Middle compression ring end cap is
1.400/1.679 mm (0.0551/0.0661 in.) for a new bore,
or 1.941 mm (0.0764 in.) for a used bore.
83 mm bore engines: Top compression ring end gap
clearance is 0.189/0.277 mm (0.0074/0.0109 in.) for
a new bore, or 0.531 mm (0.0209 in.) for a used
bore. Middle compression ring end cap is
1.519/1.797 mm (0.0598/0.0708 in.) for a new bore,
or 2.051 mm (0.0808 in.) for a used bore.
5. After installing new compression (top and middle)
rings on piston, make sure top compression ring-to-
groove side clearance and is 0.030/0.070 mm
(0.0010/0.0026 in.) and middle compression ring-to-
groove side clearance 0.030/0.070 mm
(0.0010/0.0026 in.). If side clearance is greater than
specified, a new piston must be used.
Disassembly/Inspection and Service
50
24 690 37 Rev. B
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