Piston: Service and Repair

CRANKSHAFT AND PISTON

CYLINDER AND PISTON





1) The cylinder bore size is stamped on cylinder block's front upper surface.

NOTE:
^ Measurement should be performed at a temperature of 20°C (68°F).
^ Standard sized pistons are classified into two grades, "A" and "B". These grades should be used as a guide line in selecting a standard piston.

Standard diameter:
A: 92.005 - 92.015 mm (3.6222 - 3.6226 inch)
B: 91.995 - 92.005 mm (3.6218 - 3.6222 inch)





2) How to measure the inner diameter of each cylinder:
Measure the inner diameter of each cylinder in both the thrust and piston pin directions at the heights shown in the figure, using a cylinder bore gauge.

NOTE: Measurement should be performed at a temperature of 20°C (68°F).

Taper:
Standard 0.015 mm (0.0006 inch)
Limit 0.050 mm (0.0020 inch)
Out-of-roundness:
Standard 0.010 mm (0.0004 inch)
Limit 0.050 mm (0.0020 inch)
3) When the piston is to be replaced due to general or cylinder wear, determine a suitable sized piston by measuring the piston clearance.





4) How to measure the outer diameter of each piston: Measure the outer diameter of each piston at the height shown in the figure. (Thrust direction)

NOTE: Measurement should be performed at a temperature of 20°C (68°F).

Piston grade point H: 40.0 mm (1.57 inch)
Piston outer diameter: Standard
A: 91.985 - 91.995 mm (3.6214 - 3.6218 inch)
B: 91.975 - 91.985 mm (3.6211 - 3.6214 inch)
0.25 mm (0.0098 inch) oversize 92.225 - 92.235 mm (3.6309 - 3.6313 inch)
0.50 mm (0.0197 inch) oversize 92.475 - 92.485 mm (3.6407 - 3.6411 inch)
5) Calculate the clearance between cylinder and piston.

NOTE: Measurement should be performed at a temperature of 20°C (68°F).

Cylinder to piston clearance at 20°C (68 °F):
Standard 0.010 - 0.030 mm (0.0004 - 0.0012 inch)
Limit 0.050 mm (0.0020 inch)

6) Boring and honing:
(1) If the value of taper, out-of-roundness, or cylinder-to-piston clearance measured exceeds the standard or if there is any damage on the cylinder wall, reboring it to use an oversize piston.

CAUTION: When any of the cylinders needs reboring, all other cylinders must be bored at the same time, and use oversize pistons. Do not perform boring on one cylinder only, nor use an oversize piston for one cylinder only.

(2) If the cylinder inner diameter exceeds the limit after boring and honing, replace the crankcase.

NOTE: Immediately after reboring, the cylinder diameter may differ from its real diameter due to temperature rise. Thus, pay attention to this when measuring the cylinder diameter.

Limit of cylinder enlarging (boring): 0.5 mm (0.020 inch)

PISTON AND PISTON PIN
1) Check the pistons and piston pins for damage, cracks, and wear and the piston ring grooves for wear and damage. Replace if defective.
2) Measure the piston-to-cylinder clearance at each cylinder. If any of the clearances is not within specification, replace the piston or bore the cylinder to use an oversize piston.





3) Make sure that the piston pin can be inserted into the piston pin hole with a thumb at 20°C (68°F). Replace if defective.
Standard clearance between piston pin and hole in piston:
Standard 0.004 - 0.008 mm (0.0002 - 0.0003 inch)
Limit 0.020 mm (0.0008 inch)





4) Check the snap ring installation groove on piston for burr (A). If necessary, remove the burr from groove so that the piston pin can lightly move.
5) Check the piston pin snap ring for distortion, cracks and wear.