The piston is measured with an outside micrometer or vernier calipers. The cylinder is measured with an inside micrometer and the two sets of measurements are compared to find the piston clearance.
Measuring the piston
BB is the nominal piston diameter
AA is the reduced diameter after cam grinding
CC is the diameter at the top of the skirt
DD is the diameter at the bottom of the skirt
EE is the reduced diameter at the lands.
· These measurements enable the piston size, cam grinding, skirt taper and land
relief to be determined.
Checking the piston in its cylinder
Place the piston in the cylinder upside down with the feeler strip, lightly oiled, placed 900 from the piston-pin holes. This is the greatest piston diameter. The feeler strip should extend the full length of the piston. Different thicknesses can be tried to determine the clearance.
Replacement piston rings are supplied as a package kit to suit the particular engine being repaired. They can be obtained in various over sizes to suit oversize cylinder bores.
Piston rings are supplied for a particular bore diameter. Their ends should not be filed to fit them to smaller bores because they will become oval-shaped when installed in the cylinder, if the cylinder has been re-bored or made oversize by honing, it will require oversize rings; if not, standard-sized rings must be used, irrespective of the wear that might have occurred to the upper portion of the cylinder.
Checking rings in the cylinder bore
New piston rings should be checked in the cylinder to make sure that they are correct for the bore size (Figure 7.10).
A quick check can be made with the ring near the top of the bore to make sure that it has a gap. However, before a measurement is actually taken, the ring is pushed down the cylinder with the head of a piston (Figure 7.11). This makes sure that the ring is sitting squarely in the bore. Worn bores will be tapered, and so the ring should be pushed down to the part of the bore that is least worn. This will be below the lower limit of ring travel.
Check the gap between the ends of the ring with feeler gauges. A rule of thumb is that the