A piston must be in good condition if it is to be reinstalled in the engine. This means that it must have no cracks, scores or scratches. It must be the correct size and have the correct clearance in the cylinder.
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
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
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
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| Fig 7.7 |
Using a micrometer, measure the piston skirt across the thrust faces as shown in Figure 7.7. In most cases, the measurement is taken at a point which is approximately one-third of the skirt height. This is the nominal diameter of the piston and should be the greatest dimension.
Other measurements can be taken as a check and to make sure that the piston has not collapsed. A collapsed piston will have a reduced diameter at the lower end of the skirt.
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| Fig 7.8 |
The engine manufacturer’s service manual should be consulted for piston dimensions and clearances. It will also have information on how to measure the piston. Some pistons are cam-ground, some have a tapered skirt and some are slightly barrel-shaped. All pistons are not measured in the same way.
Figure 7.8 is an example of piston measurement. It shows the places on a piston where measurements would be taken. These are as follows:
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
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| Fig 7.9 |
The fit of the piston in the cylinder can be checked with a feeler-gauge strip. The following is a typical procedure.
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.
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.
A refinement of this procedure, which is more exact, requires the use of a spring scale. The force required to pull the feeler strip from between the piston and cylinder is measured (Figure 7.9). The following is an example: a strip of feeler 12 mm wide and 0.04 mm thick should be able to be withdrawn from between the piston and the cylinder wall with a pull of 20 to 40 newtons. If the feeler strip pulls out too easily, the fit is too loose, if it is too hard to pull, the fit is too tight.
Fitting piston rings
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.
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.
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| Fig 7.1 |
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
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| Fig 7.11 |
gap clearance should be 0.03 mm for each 10 mm of cylinder diameter, measured in the unworn part of the bore.
· The ring must not be measured at the top of the bore because its gap will close as it is moved towards the bottom of the cylinder. Without a gap, the ring will break or cause scuffing of the cylinder walls.
Continued
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