Piston: Testing and Inspection

PROCEDURE

CAUTION:
^ Pistons of different makes must not be used in the same engine.
^ The name of the manufacturer is cast-in on the inside of 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 obtain the correct piston clearance.

When matching pistons to the cylinder bores, use a 1/2-inch wide feeler gauge. First lightly oil the wall of the cylinder in which the piston will be working and then insert the piston, without rings. Attach the feeler gauge to a spring balance and insert it between the piston and cylinder wall at right angles to the axis of the gudgeon pin. At a tractive force of 8-12 N (1.82.7 lbs.), the mean clearance equals the thickness of the feeler gauge.

Repeat the test with the piston at several different depths.

NOTE: For piston clearances, refer to the "Specifications" section.

EXAMPLE:
Class A Cylinder And Class A Piston:
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 90.012 mm. Since the pistons have bedded-in to a certain extent, giving a further 0.003 mm, it may be assumed that they have a bore of 90.015 mm.

Assuming a standard class AB piston, the theoretical piston clearance will be 0.026-0.035 mm and this coincides fairly well with the "Clearances" in Technical data".

In No. 1 and No. 4 cylinders, the pistons cannot be fitted into the cylinder bores with a 0.05 mm or 0.04 feeler gauge inserted. With a 0.03 mm feeler gauge inserted the force necessary to withdraw it will be 20 N (4.6 lbs.) and with a 0.02 mm feeler gauge it will be 6 N (1.4 lbs.).

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.