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Figure 3C-17. Checking Connecting Rod for Crankpin Bearing Wear

Figure 3C-18. Measuring Cylinder

Inside and outside micrometers used for cylinder -piston fitting should be checked together to be sure they are adjusted to read exactly the same. By subtracting piston measurement from bore measurement, amount of piston-cylinder clearance is obtained.

Bore measurement of a used and worn cylinder should be taken in ring path, starting about 1/2 in. from the top of cylinder, measuring front to rear then side to side. Repeat procedure at the center and at the bottom of the ring travel (see Fig. 3C-18). This process will determine if cylinder is out-of-round or "egged" and will also show any cylinder taper.

Piston measurement should be taken at extreme bottom of skirt, measured front to rear, 90° from center line of piston pin (see Fig. 3C-19).

If cylinders are not scored and are worn less than .002 in., it is not usual practice to refinish oversize. If the total piston clearance is .002 to .006 in., a new standard piston, or piston of the same oversize to which the cylinder was last refinished, should be fitted to reduce clearance and effect reasonably quiet operation.

If cylinders show more than .002 in. wear, they should be refinished to the next oversize step and fitted with new corresponding pistons and rings.

Exact final size of the cylinder bore is determined by size of the piston to be used in that cylinder. Measure piston diameter accurately as described previously, then add desired piston clearance in cylinder. This will equal the exact final size to which cylinder bore should be refinished. Example: The .020 inch oversize piston to be used measures 3.0190 inches, adding .0025 inch (desired clearance) equals 3.0215 inches (finish-honed size). When cyl-inders require reboring to beyond .070 inch oversize to clean up, cylinder oversize limit has been exceeded and the cylinder must be replaced.

Pistons are available in the following oversizes: .005, .010, .020, .030, .040, .050, .060, and .070 in. Oversize pistons have their size stamped on head; for example: 10, 20, etc.

Cylinders can be refinished oversize either with a hone, or a boring bar followed by a finishing hone. In general practice only cylinders not scored and not badly worn are refinished using only a hone. Cylinders badly worn or deeply scored are first re-bored to nearly the required oversize and then are finish-honed to exact size. When cylinders must be rebored to beyond .070 in. oversize to clean up, their oversize limit has been exceeded and the cylinders must be replaced with new ones.

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