Pistons and Connecting Rods
PISTONS AND CONNECTING RODS
Tool Required: J 24086-B Piston Pin Remover/Installer
CAUTION: Use care when handling the piston. Worn piston rings are sharp and may cause bodily injury.
Disassemble
1. Piston rings (87, 88 and 89). Use a suitable tool to expand the rings. Piston rings must not be re-used.
2. Place the piston (86) and connecting rod (83) assembly into J 24086-B and press out the piston pin.
Refer to Figure 32. Be sure the flat side or inside of piston is up when installing piston and rod assembly into fork.
Important: Make sure no load is put on piston (86) during disassembly.
CAUTION: Avoid inhaling fumes or exposure of the skin to carburetor cleaning fluid, as bodily injury may result.
Clean
^ Piston (86), piston pin (85) and connecting rod (83) of:
- Sludge.
- Carbon.
- It is essential that the piston ring grooves are cleaned of carbon to the are metal.
- Varnish from the piston pin (85) by soaking in carburetor cleaning solution.
^ Do not scrape the piston skirt.
Inspect
1. Connecting rod (83) for:
^ Bending or twisting.
a. Install the connecting rod cap (81).
b. Place the connecting rod assembly (83) on a checking fixture and check for bending or twisting.
- Do not attempt to straighten the connecting rod.
- The connecting rods should not be bent more than 0.007 inch for every three inches of rod length.
- The connecting rods should not be twisted more than 0.0015 inch for every inch of rod length. If bent or twisted, replace.
- Check new connecting rods before using them.
^ Outside of the connecting rod bearing and the ID of the connecting rod lower end for wear indicating high spots in the connecting rod lower end.
^ Upper end for scoring.
2. Piston pin (85) for:
^ Scoring.
^ Galling caused by improper installation.
^ Fit in connecting rod and piston.
3. Piston (86) for:
^ Scoring of the skirt.
^ Cracks.
^ Broken ring groove lands.
^ Wear.
Piston to Bore Clearance
Important
^ When fitting pistons, both piston and cylinder bore condition must be considered together. Production and service pistons have the same nominal weight and can be intermixed without affecting engine balance. If necessary, used pistons may be fitted selectively to any cylinder of the engine, providing they are in good condition and the same weight.
^ Do not cut oversize pistons down or engine balance will be affected.
Measure
1. Piston diameter.
When measuring piston for size or taper, measurement must be made as shown with the piston pin removed (Figure 34). If worn or damaged, replace with a standard or oversize piston.
Important Finish hone when selecting piston.
2. Cylinder bore (Figure 33), with a telescoping gage.
If worn beyond specifications, rebore and hone to size.
3. Subtract piston diameter from cylinder bore diameter to determine piston-to-bore clearance.
4. Compare piston-to-bore clearance with specifications to determine if clearance is in acceptable range.
5. If used piston is not acceptable, check service piston sizes and determine if a new piston can be selected.
(Service pistons are available in standard, and several oversizes.)
6. If cylinder bore must be reconditioned, measure new piston diameter then hone cylinder bore to correct clearance.
7. Select new piston and mark piston to identify the cylinder for which it was fitted.
Piston Pin Bore-to-Piston Pin Clearance
Measure
1. Measure the piston pin bore diameter with an inside micrometer.
2. Measure the piston pin diameter with a micrometer.
3. Subtract piston pin bore diameter from piston pin diameter to determine clearance.
4. Compare clearance with specifications to determine if clearance is in acceptable range.
5. If it is not acceptable, replace piston assembly.
Piston Compression Ring End Gap
Important Do not reuse piston rings. Fit rings to match the cylinder they will be used in.
Measure
1. Place piston into the cylinder at the bottom of the ring travel.
2. Place a piston ring on top of the piston.
3. Back off the piston.
4. Measure the ring gap (Figure 35). If the gap is below specification, increase the gap by carefully filing off excess material.
Piston Ring Side Clearance
Important Do not attempt to cut the ring groove, although high spots in the ring groove may be cleaned up by careful use of a point file.
Measure
1. Piston compression ring side clearance.
2. Roll the piston ring around the groove in which it is to be installed and measure the side clearance (Figure 36). If the ring is too thick, try another ring.
Assemble
Important Make sure that the arrow on top of the piston faces towards the front of the engine.
1. Place piston (86) and connecting rod (83) into J 24086-B (Figure 37).
2. Coat the piston pin (85) with clean engine oil.
3. Press the piston pin (85) into place.
Inspect Piston (86) for freedom of movement.
4. Oil control ring assembly (87).
^ Expander.
^ Lower oil control ring.
^ Upper oil control ring.
NOTICE: Use a piston ring expander to install the rings. Avoid expanding the rings more than necessary, which may cause ring damage.
5. Upper and lower compression ring (88 and 89). Manufacturer's mark facing up.
Important: In order to provide an effective compression seal, the ring gaps must be staggered (Figure 39).