Idler Gear Shaft

[jwdieter]

I currently have my atomic four torn down for an overhaul. The idler gear seems to have taken a beating when the oil system was contaminated with metal. The bushing was torn up quite a bit with metal from a spun rod bearing. At the same time the idler gear shaft is somewhat scored but not in terrible shape. Should I just replace the bushing on the gear or is it worth replacing the shaft? I am concerned that trying to remove the shaft may damage the block.

Thanks

[Don Moyer]

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.

Don

[Zack]

Idler Gear Shaft

I have removed the 2 set screws. I was planning on have the machine shop remove the shaft before cleaning the block. Now I am wondering if it is necessary to remove it for cleaning. It does not appear to be worn excessively. What is you opion on removing it.

Thanks,

Zack

[Don Moyer]

Zack,

If the shaft is not noticeably scoured, you're probably safe in leaving it installed. If there is a lot of play between the shaft and the gear bushing (.004" or more), you could have your machine shop install a new bushing in the gear for a better fit on the shaft.

Don

[jwdieter]

Idler Gear

Don,
I went ahead and replaced the idler gear shaft and gear bushing. The clearance was machined to .004 inch. Now I am reassembling my engine and when I tighten the idler gear nut all the way down there is a bit of drag on the idler gear. The bushing appears to be close to flush on both sides of the gear and is not sticking out past the machined flat area on the gear. I can still spin the gear by hand but it does not freewheel as it does if I back the nut off slightly. Is this a problem or should I file the gear/bushing until it freewheels with the nut fully tightened? I could also back the nut off slightly. Also, the center hole in the washer that fits on before the nut is larger in diameter than the threaded section of the shaft and so it does not stay centered. Is this right?

Thanks,

P.S.- Your parts dept is great. Everything I have ordered has shown up promptly.

[Don Moyer]

Hello,

I would definitely not loosen the nut to free the idler gear.

Although the bushing appears to be flush on each side of the gear, our experience is that the bushing is the culprit. I recommend taking a good flat file and working over each end of the hub of the gear until you can tighten the retaining nut securely and have the gear stay free. It should only take a thousandth or so of "adjusting".

The holes in the "thrust washers" are a bit over-sized with respect to the threaded part of the idler gear.

Thanks for the kudos regarding our parts folks. I'll pass along your note. We're always delighted to hear that our services work out for our good customers.

Regards,

Don