Craftsman 143.991200 Specifications download pdf (Page 73)

PISTON

CYLINDER

PISTON RING

PISTON MEASUREMENTS ARE TAKEN AT

BOTTOM OF SKIRT 90

FROM WRIST PIN HOLE

INDICATES

.010" (.254 mm)

OVERSIZE PISTON

PISTONS, RINGS, AND CONNECTING

RODS

Piston

The piston should be checked for wear by measuring

near the bottom of the skirt 90 degrees from the piston

pin hole with a micrometer. Check the ring side

clearance using a feeler gauge with a new ring. Clean

all carbon from the piston top and the ring grooves

before measuring. Visually inspect the piston skirt area

for scoring or scratches from dirt ingestion. If scoring

or deep scratches are evident, replace the piston.

If the cylinder bore needs re-sizing, an oversize piston

will be necessary. Oversize pistons are identified by

the imprinted decimal oversize value imprinted on the

top of the piston (diag. 27).

Piston Rings

After the cylinder bore diameter has been checked

and is acceptable to rebuild, the ring end gap should

be checked using new rings. Place a new

compression ring squarely in the center of the ring

travel area. Use the piston upside down to push the

ring down (diag. 28) and measure the gap with a feeler

gauge. The ring end gap must be within the

specification to have adequate oil control (diag. 29).

This procedure will assure correct piston ring end gap

measurement. Ring side clearance should also be

checked with a feeler gauge when using new rings

with an old piston (diag. 30).

Replace the rings in sets and install the piston, rings,

and rod assembly in the cylinder bore with the ring

end gaps staggered. When installing new rings in a

used cylinder, the cylinder wall should be de-glazed

using a commercially available de-glazing tool or

hone.

Use a ring expander to remove and replace the rings.

Do not spread the rings too wide or breakage will result.

Piston Ring Orientation

If the top compression ring has an inside chamfer,

this chamfer must face UP. The second compression

ring will have either an inside chamfer or an outside

notch. The rule to follow is an inside chamfer always

faces up. An outside notch (diag. 31) will face down

or towards the skirt of the piston.

The oil control ring can be installed with either side

up. The expander (if equipped) end gap and the ring

end gap should be staggered (diag. 31).

NOTE: Always stagger the ring end gaps on re-

assembly.

2ND COMPRESSION RING

1ST COMPRESSION RING

3RD OIL

CONTROL RING

EMISSION RINGS

3RD OIL

CONTROL RING

1ST COMPRESSION RING

2ND COMPRESSION RING

CHAMFER

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Craftsman 143.991200 Specifications Page 73

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