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The crankshaft is the part that changes the reciprocating motion of the piston into rotating motion, which is transmitted to the rear wheel when the clutch is engaged. Crankshaft trouble, such as excessive play or runout, will multiply the stress caused by the intermittent force on the piston, and will result in not only rapid crankshaft bearing wear, but also noise, power loss, vibration, and shortened engine life. A defective crankshaft should always be detected at an early stage and then repaired immediately.

The following explanation concerns the most common crankshaft problems, the method for measuring warp, play, and runout, and the method for correcting flywheel misalignment. Since the crankshaft assembly requires a hydraulic press and special tools to attain the precise tolerances that are required, a defective crankshaft should be either rebuilt by a properly equipped shop or replaced as an assembly.

Connecting Rod Bending, Twisting

Set the crankshaft in a flywheel alignment jig or on V blocks on a surface plate. Select an arbor of the same diameter as the piston pin and of optional length, and insert it through the small end of the connecting rod.

Using a height gauge or dial gauge, measure the difference in the height of the rod above the surface plate over a 100mm (4 in.) length to determine the amount the connecting rod is bent.

Using the arrangement shown in the illustration, measure the amount that the arbor varies from being parallel with the crankshaft over a 100mm (4 in.) length of the arbor to determine the amount the connecting rod is twisted.


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