8.25" pinion bearing preload varies during turn

I have a "new" (from salvage yard) 8.25" sure-grip on jack stands, and I believe the pinion seal was leaking. Before changing it, I went to measure the torque required to turn the pinion. It varies from about 3.5 in-lb to 7 in-lb going around, and sometimes takes more like 10-11 in-lb to get moving. Is this normal? From what I see online, it should not vary at all. The variation occurs at the same angles each time the pinion goes around, so I'm pretty sure it's the pinion itself, not the axles or anything else. Possibly related: every once in a while (maybe every 20th rotation or so) the pinion catches and needs much more torque to get past the catch. I don't see anything obvious like a metal chip in the gears. This is my first time taking apart a differential, so I'm not sure how to proceed.

Breakaway torque is always higher.
Check the backlash at the sticky spot, and compare it to the backlash at a loose spot.
I suspect there is debris jammed between a couple of teeth at the sticky spot.But if no debris then I suspect the pinion is bent or twisted, or the RG is wanting to shed some teeth.
As a final test, pull the ring-gear assembly out, and recheck the turning torque. A sticky spot now could only be a bearing.
If you decide to run it like this, it for-sure will be noisy. And if it's not debris, I imagine it would be a tooth or two, coming off. So look for cracked teeth on both the RG and the pinion.

OOps I assumed the axles were out. Silly me. If the axles are in, remove them first and recheck the turning torque. If the problem disappears, look for a bent axle tube, or bent axles or polluted axle bearings. Of course if the drums are still on, oh-boy, they shouldanotabin.
So a little detective work for you then.You are doing pretty good so far.

Is it pinion only with no differential or with differential are axles installed are brakes installed?

Differential and axles are installed, but brakes are not.

If it is a non-hunting gear ratio it could take 20 revolutions for those two specific teeth to touch and cause the resistance you feel.

I would pull the axles out of the differential and do another comparative rotation test.

Coronet 500 said:

it could take 20 revolutions for those two specific teeth to touch

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I thought of that, but the pinion has 17 teeth and the ring has 46.

AJ/FormS said:

Breakaway torque is always higher.
Check the backlash at the sticky spot, and compare it to the backlash at a loose spot.
I suspect there is debris jammed between a couple of teeth at the sticky spot.But if no debris then I suspect the pinion is bent or twisted, or the RG is wanting to shed some teeth.
As a final test, pull the ring-gear assembly out, and recheck the turning torque. A sticky spot now could only be a bearing.
If you decide to run it like this, it for-sure will be noisy. And if it's not debris, I imagine it would be a tooth or two, coming off. So look for cracked teeth on both the RG and the pinion.

Click to expand...

All good info, but the tighter spot (not the hard catch) occurs at the same pinion clock position each time, which would be a different spot on the axles and the ring gear each time--that makes me think it must be the pinion itself or its bearings. Tomorrow I'll take it outside in the light and inspect the gears more carefully for contamination and damage. I'll also see if I can nail down where the "catch" happens. Anyone else have any other recommendations?

You can't measure pinion preload with the carrier installed. Must be pinion only, I usually set them up just a couple inch lbs lower than what's called for because a lubricated pinion seal will add a couple inch lbs to the rotational drag. Once you get the pinion right you add the carrier, GM 12 bolts call for 10 inch lbs of carrier preload. Would think the 8.25 would be similar values

tnts72 said:

You can't measure pinion preload with the carrier installed. Must be pinion only, I usually set them up just a couple inch lbs lower than what's called for because a lubricated pinion seal will add a couple inch lbs to the rotational drag. Once you get the pinion right you add the carrier, GM 12 bolts call for 10 inch lbs of carrier preload. Would think the 8.25 would be similar values

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But you can use your method to get back to square one, when changing the seal. However, I mark the pinion and it's nut, with a die-grinder and a tiny little cut-off wheel. That takes the seal drag right out of the equation.