Piston, Connecting Rod and Bearings Clean/Inspect

CAUTION: Bodily injury may occur if the carburetor cleaning fluid is exposed to the skin or if the fumes are inhaled.

IMPORTANT: It is essential that the piston ring grooves are cleaned of carbon to the bare metal. Do not scrape the piston skirt.





1. Clean the sludge and carbon from the following parts:
^ The piston
^ The piston pin, clean the varnish from the piston pin by soaking it in carburetor cleaning solution.
^ The connecting rod





2. Clean the piston ring grooves with a suitable ring groove cleaning tool.
3. Clean the oil lubrication holes and slots.





4. Inspect the connecting rod for bending or twisting.
4.1. Install the connecting rod cap.,
4.2. Place the connecting rod assembly on a checking fixture and check for bending or twisting.
4.3. Do not attempt to straighten the connecting rod. The connecting rods should not be bent more than 0.18 mm (0.007 inch) for every 76.2 mm (3.00 inch) of rod length. The connecting rods should not be twisted more than 0.038 mm (0.0015 inch) for every 25.4 mm (1.00 inch) of rod length. Replace it, if it is bent or twisted.
4.4. Check new connecting rods before using them.





5. Inspect the connecting rod bearings for craters or pockets. Flattened sections on the connecting rod bearing halves also indicate fatigue.





6. Inspect the connecting rod bearings for excessive scoring or discoloration.
7. Inspect the connecting rod bearings for dirt or debris imbedded into the connecting rod bearing halves.





8. Inspect the connecting rod bearings for improper seating indicated by bright, polished sections of the bearings.
9. Inspect the inside of the connecting rod bearing and the outside diameter of the connecting rod bearing journal for wear indicating high spots.





10. Inspect the connecting rod bearing bore for taper and out-of-round.
11. Inspect the connecting rod piston pin bore for taper and out-of-round.
12. Inspect the connecting rod bores for nicks or gouges.





13. Inspect the piston for the following conditions. Damage to the top of the piston (1)
^ Scoring of the skirt (2)
^ Scoring of the piston pin bore (3)
^ Broken ring groove lands

14. Inspect the piston pin for scoring or galling caused by improper installation.

IMPORTANT: Measurements of components should be taken with the components at normal room temperature.





15. Measure the piston diameter. When measuring piston for size or taper, measurement must be made as shown with the piston pin removed. If worn or damaged, replace with a standard or oversize piston.





16. Inspect the piston ring-to-piston ring groove side clearance.
16.1. Insert the edge of the piston ring into the piston ring groove.
16.2. Roll the piston ring completely around the piston.
16.3. If binding is caused by a distorted piston ring groove, minor ring groove imperfections may be removed with a fine file.
16.4. If binding is caused by a distorted piston ring, replace the piston ring.





17. Measure the piston ring side clearance with a feeler gauge.
18. If side clearance is too small, try another piston ring set.
19. If proper piston ring-to-piston ring groove clearance cannot be achieved, replace the piston.





20. Measure the piston pin diameter with an outside micrometer.





21. Measure the piston pin bore with an inside micrometer.
22. To determine the piston pin-to-piston pin bore clearance subtract the piston pin diameter from the piston pin bore diameter.
23. Replace the piston and piston pin if not within specifications.





24. Place the piston into the cylinder at the bottom of the ring travel.

IMPORTANT: Measure the piston ring in the cylinder in which it will be used.

25. Place a piston ring on top of the piston.
26. Back off the piston.
27. Measure the ring gap. If the gap is below specification, increase the gap by carefully filing off excess material.