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From Don:
Yes, the idler gear shaft is indeed difficult to remove and replace. The first booby trap that you'll encounter is that there are 2 (not one) quarter/twenty Allen-headed set screws in the same hole which will be holding the shaft in place from below.
Most machine shops seem to be able to safely pull and replace the shaft if it needs machining, but please resist the temptation to simply pound it out from the crankcase side of the block. To do so not only damages the inside end of the shaft, but risks cracking the block. We recommend removing the shaft to be re-machined any time that it is noticeably scoured. Otherwise, the bushing (even a new one) will quickly wear excessively.
In terms of a specification for the brass bushing to shaft clearance, our machine shop tells me that a standard ASTM clearance for that type of bearing would be .002. However, Universal recommended a wider clearance (up to .004"), so that a generous amount of oil would flow past the bushing and work its way out to the outer edge of the idler gear to be "flung" over to the small catch basin in the rear face of the accessory drive. You can read more about this system of providing oil for the accessory drive in a Tech Tip on our website called "Accessory drive repair and modification".
As a summarizing note, we have seen many idler gears and shafts come into our shops with quite badly worn shafts and bushings, and while we know that loose idler gears can create quite a "clicking" sound at low RPM, we have never heard of a gear or shaft actually failing in service.
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