39. Piston Selection

1. Measure the piston diameter. Remove the piston pin to measure the piston for size and taper.
2. Replace the piston with a standard or an oversized piston if the piston is damaged.

Important: Consider both the piston condition and the cylinder bore condition when fitting the pistons. Production and service pistons have the same nominal weight. Production and service pistons can be intermixed without affecting the engine balance. If necessary, fit used pistons to any cylinder of the engine. Ensure that the pistons are in good condition.

3. Measure the cylinder bore. If the cylinder bore is worn beyond specifications, rebore the cylinder bore. Use the following procedure to hone the cylinder bore to the correct size:
3.1. Select a piston.
3.2. Hone the cylinder bore to obtain the recommended clearance.
3.3. Scrub the cylinder bore and the piston with soap and water. Remove all foreign material. Dry the cylinder bore. Rub clean engine oil into the cylinder bore.





4. Use the following procedure to measure piston-to-cylinder bore clearance:
4.1. Measure the cylinder bore with a telescoping gauge. If the cylinder bore is damaged, rebore or hone the cylinder bore to size.
4.2. Subtract the piston diameter from the cylinder bore diameter to determine the piston-to-bore clearance.
4.3. Compare the piston-to-bore clearance with the specifications to determine if the clearance is within the acceptable range.
5. If the used piston is not within the acceptable clearance range, check the service piston sizes to determine if a new piston can be selected. Service pistons are available in standard and 0.508 mm (0.020 inch) oversize.
6. If the cylinder bore needs reconditioning, measure the new piston diameter and hone the cylinder bore to the correct clearance.
7. Select a new piston. Mark the piston to identify the corresponding cylinder.





8. Select a set of new piston rings.
9. Place each ring, one at a time, into the corresponding cylinder.
10. Place the piston for that cylinder, upside down (top of the piston pushing on the ring). Push the ring down to the bottom of the ring travel.
11. Remove the piston.





12. Measure the ring gap.
13. If the ring gap is not within an acceptable specification range, increase the ring gap. Do this by filing any excess material from the ends of the ring.





14. Measure the piston ring side clearance (compression rings).

Notice: Do not cut the ring groove with a file or a rough edged tool. Good performance and durability depends on the straightness and smoothness of the ring groove.

15. Use the following procedure to measure piston compression ring side clearance:
15.1. Roll the piston ring around the groove.
15.2. Measure the side clearance.
15.3. If the ring is too tight, check for nicks or burrs.
Use emery cloth to remove burrs.
15.4. Determine if the ring is within specifications. If necessary, replace the ring.