Eccentricity and/or insufficient interference fit


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Posted by Gordon Maney on Saturday, November 01, 2003 at 8:23AM :

In Reply to: Pilot Bushing in Cast Crank posted by LP Taylor on Friday, October 31, 2003 at 10:18AM :

You wrote: “...it does not have the clean-up cut that provides the smooth surface and perfect diameter for a slip fit of the bushing.”

The finished, machined bore, must also be concentric with the crankshaft. That is crucial to the longevity and service of these parts.

You also wrote: “Has anyone dealt with this problem? (other than removing the crank and machining the bore properly) “

Your parenthetical statement manages to answer your own question, and I think you already hold the central belief that it answers your own question.

Any eccentricity in an installed pilot bushing or bearing will translate into premature wear and damage to the transmission. If the nose of the main drive gear (input shaft) orbits due to eccentricity, then the front case bearing will be stressed due to the motion created at that end of the shaft. Also, the nose of the main shaft (output shaft) has a pilot that runs in roller or needle bearings in the rear end of the main drive gear. That bearing will be stressed and subject to premature wear. Motion at that end will cause problems with the rear case bearing and also to some degree the fit of the main shaft’s gears with respect to the counter gear (cluster gear).

If the bore is oversize, that adds to the possibility of the pilot bushing at some point in its service life starting to spin in the bore. A loose pilot bushing can get loose on the inside or the outside. Most get loose on the inside, but it is not uncommon to find them the other way. The proper interference fit is critical.

I admire Paul’s self reliance, but I fear for his long term success, and the possibility of catastrophe. It would still not be hard to remove that transmission and at least measure that bore with an inside micrometer, or telescoping gauge and outside micrometer, and have a bushing’s OD reduced on a lathe to a diameter consistent with the optimum interference fit and also concentric to the centerline of the bore.

The first symptom you will see as a result of an eccentric pilot or loose pilot (which will ultimately be the fate of the eccentric pilot) is the trans won’t want to stay in direct. And, if you run it long enough, you will probably see, upon disassembly of the trans, damage to a great many things. By that time, it is likely too late to have the trans be practical to repair.

Sometimes there is no easy way to do things. There is only the right way.



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