Determining your Engine Overhaul Needs - Measuring Pistons and Sleeves
(20 January 2017)
What you are looking for with the piston and sleeves is any visual damage such as scoring, out of roundness
and greater width on the sleeve at the highest point of piston travel (not including the ridge the forms above
the high point of travel). Scoring will indicate the need for replacement regardless of the measurements. The other
conditions are measured as follows.
Out of roundness is measureable by placing the inside micrometer at the top and bottom of the cylinder and then
repeated at right angles to the original measurement. With these numbers, you can see how conical the cylinder
is and how oblong it has become. Repeat the process using your caliper on the piston. There will usually be a factory
specification for what is acceptable. Some pistons are cam ground meaning they are supposed to be slightly out
of round but how much determines servicability.
How conical the bore of the sleeve is, contributes to whether it can be reused. There is more wear at the top
than at the bottom. If the differences exceed the manuals specification, the piston and sleeve will need replacement.
Based on the measurements you did for out of roundness, you would have already have written these down.
If the differences at the top and bottom are acceptable, you will still need to determine if there is too much
distance between the piston and the sleeve. This will be most noticable at the top of the stroke. Most manuals
suggest using a long feeler gauge of a certain size placed between the piston and sleeve with the piston inserted
in the sleeve. The amount of drag on the feeler gauge when pulled out, determines its servicability. Fortunately,
some manuals provide what it should "feel" like to pull it out but many require that you connected a
spring scale and read the pounds of pull as the feeler gauge is removed. Do this process without the rings. Usually
this measurement is taken 90 degrees from the piston pin hole.
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