1. See Figure 3-59. Check piston pin. Pin must slide without
binding.
a. Insert a lightly oiled good piston pin into the piston pin bore to feel for the proper interference fit. The pin should slide in and out without binding, but also without pivoting or rocking.
b. Measure pin and pin bore diameters to determine running clearance. Replace piston and/or pin if clearance exceeds 0.0008 in (0.02 mm).
NOTE
Pistons with superficial wear marks, minor scratching or mild scoring are acceptable for use.
2. Carefully inspect the pistons for damage or excessive wear. Discard if any of the following conditions are found:
a. Cracked, worn or bent ring lands.
b. Cracks, gouges, deep scratches or heavy scoring.
c. Evidence of burning, etching or melting.
d. Marks or imprints caused by contact with valves.
3. Run your index finger around the edge of the piston crown to feel for dings, nicks or burrs. Lightly file the edge of the crown to remove any defects.
NOTE
Worn ring grooves result in high oil consumption and blow-by of exhaust gases. Blow-by of exhaust gases contaminate the engine oil supply with acids and leaves sludge in the crankcase.
It also reduces engine efficiency by weakening the combustion seal necessary for efficient transfer of energy to the piston.
4. See Figure 3-60. Measure piston ring side clearance.
a. Insert the edge of a new ring into the piston ring groove. Insert a feeler gauge between the upper surface of the ring and the ring land.
b. Since the grooves wear unevenly, repeat this check at several locations around the piston groove circumference.
c. Discard the piston if the side clearance of either compression ring exceeds 0.0045 in (0.11 mm).
d. Discard the piston if the oil control ring side clearance exceeds 0.010 in (0.25 mm).
NOTES
• Check the piston clearance in the cylinder in which the piston will run. The cylinder must be deglazed and have torque plates installed when measuring bore.
• This inspection is very heat-sensitive. Both the piston and cylinder must be at room temperature before proceeding. Do not check piston running clearance immediately after honing or deglazing cylinder. Even holding the piston for too long can cause measurements to vary by as much as 0.0002 in (0.0051 mm).
• See upper frame of Figure 3-61. Piston measurement is taken on the bare aluminum for greatest accuracy. The coating has an oval-shaped opening (1) on each side of the piston for proper micrometer placement.
• See lower frame of Figure 3-61. The oval openings are too small for a standard flat anvil micrometer which would result in measuring errors. Use a blade or ball anvil style micrometer to measure piston.
5. See Figure 3-61. Measure running clearance of pistons as follows:
a. Measure the piston skirt at the bare aluminum openings (1) in the coating. Transfer that measurement to dial bore gauge.
b. Using a grease pencil, mark the top, middle and bottom of the piston ring travel zone in the cylinder bore. Measure at markings in cylinder parallel and perpendicular to crankshaft.
c. Replace piston and/or cylinder if running clearance exceeds 0.003 in (0.076 mm).
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