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Reshimming the gearbox


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I have a 912 ULS gearbox that is being uncooperative. It was dismounted and I'm servicing and reshimming it. Had to swap the clutch with a spare because it failed its check.

I currently have 2 of the 1mm shims installed, and I only get 384 inch pounds on the friction in free rotation test. While true that this is technically a pass, I am trying to get closer to the upper range (~600 inch lbs). I tried putting in another 0.5mm shim, but by that point I can't even compress the gearbox enough to insert the keepers. Tried a .1mm shim and it didn't change the results of the test.

I wiped down the faces of the dog gear so they're clean. The belleville washers are brand new.

On the ends of the dogs and the lands, there is a little bit of wear. You can feel it with your finger, and see it, but the manual says mild to moderate wear is allowable. No deep pitting, but you can see where the movement occurs while idling.

Curious to see what everyone else's experience is after reshimming and if it's normal to sometimes not see an upper friction value no matter what you try.

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  • 4 months later...

I just finished a rebuild and put new Bellville washers in. I too did the check and added shims to get it farther up on the torque range.  I shimmed to the max as you are seeing and still being able to insert retainers. I did not get to the top end of the friction torque range. I assumed it was variations in the washers. I will be in your area tomorrow afternoon if you would like me to stop and chat about what I found. Rod


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We already had that call back then.

I forgot to follow up. After the engine ran for a bit and things got oil bathed and anti-sieze flushed out, the friction in free rotation rose closer to the high limit. Everything's good and the inspection of the mag plug at the oil change had a little fuzzy (which I assumed was break in), but the one after that was almost completely clean.

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