Procedures

Matching of pistons





Important
Pistons of different makes must not be used in the same engine. The name of the manufacturer is cast inside the piston.

Spare pistons are stocked in both standard and oversize diameters. Where the latter are used, the cylinder bore must be honed or rebored to obtained the correct piston clearance.

When matching pistons in the cylinder bores, use a 1/2" wide feeler gauge. The cylinder bore should be lightly oiled during measuring and the piston, without rings, should be placed in its corresponding bore. Hook a spring balance to the feeler gauge and insert the gauge between the piston and the bore at right angles to the gudgeon pin. When the pulling force is 8-12 N (1.8-2.7 lbf), the mean value of the clearance coincides with the thickness of the feeler gauge. The test should be performed at several different levels. For piston clearance information, see Pistons

Example cylinder classification A piston type A.





When the piston clearances are measured in the engine by means of a 0.05 mm feeler gauge, no measurable force will be recorded on the spring balance when withdrawing the feeler gauge from the No. 2 and No. 3 cylinders. This means that the piston clearance in these cylinders is greater than 0.05 mm and the maximum bore in them is therefore judged to be 90.020 mm. Since the pistons may have bedded-in to a certain extent, giving a further +0.003 mm, it may be assumed that the bore is 90.023 mm. Assuming a standard class B piston, the theoretical piston clearance will be 0.034-0.052 mm and this lies within the clearance limits which for class A piston/class A cylinder is 0.031-0.069 mm, and for class B piston/class B cylinder is also 0.031-0.069 mm. In No. 1 and No. 4 cylinders, the pistons cannot be fitted into the cylinder bores with a 0.05 mm or 0.04 mm feeler gauge inserted. With a 0.03 mm feeler gauge inserted the force necessary to withdraw it will be 20 N (4.6 lbf) and with a 0.02 mm feeler gauge it will be 6 N (1.4 lbf). We can therefore assume a piston clearance of 0.024 mm in the No. 1 and No. 4 cylinders and the pistons in these cylinders will therefore not need to be replaced.