PRE-RELEASE
Matching of pistons
| Matching of pistons |
| Important | ||
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Pistons of different makes must not be used in the same engine. The name of the manufacturer is cast inside the piston. |
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Pistons are stocked as a spare part in standard and oversize dimensions. When oversized pistons are used, the cylinder bore must be rebored to obtain the correct piston clearance.
When matching pistons to cylinder bores, use a 1/2” feeler gauge. When measuring the cylinder bore it should be lightly oiled and the piston, without rings, placed into the bore where it is to work. Attach the feeler gauge to a spring balance and insert it between the piston and the cylinder bore at right-angles to the gudgeon pin. At a tractive force of 8-12 N (1.8-2.7 lbf), the mean clearance equals the thickness of the feeler gauge. Repeat the test with the piston at several different depths.
For piston clearances, see
Example cylinder classification A piston type A
When the pistons are measured in the engine with a 0.05 mm feeler gauge, no measurable force will be recorded on the spring balance in cylinders 2 and 3. This means that the piston clearance in cylinders 2 and 3 is greater than 0.05 mm and these cylinders are judged to have a maximum tolerance, 90.012 mm, and to be slightly bedded-in, which should result in +0.003 mm = 90.015 mm.Assuming a class AB piston, the theoretical piston clearance will be 0.026-0.035 mm.
In cylinders 1 and 4, the pistons cannot be fitted together with a 0.05 mm or 0.04 mm feeler gauge.
With a 0.03 mm feeler gauge, the force necessary to withdraw it is 20 N (4.6 lbf) and with a 0.02 mm feeler gauge, it is 6 N (1.4 lbf).
The piston clearance in cylinders 1 and 4 is assumed to be 0.024 mm and the pistons in these cylinders will not need replacing.
