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Hello Everyone,

I recently discovered looseness in my final drive. I began disassembly to check bearings.

While taking the final drive off I noticed the outer pivot pin was slightly tighter than finger tight. The pin came out easily for two threads then stopped. Moving the pin in and out to work it loose only produced bits of aluminum dropping out of the threads. Once removed both pivot pin and swing arm threads were destroyed.

Rather than buy new or from E-bay, I decided to bush the swing arm and make a new pivot pin. Of course having a mill and a lathe helps. In my opinion the repair turned out great, five buck for the bolt and had 6061 aluminum for the bushing.

Now on to the final drive.
 

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Very nice repair. I am not very knowledgeable about such things but do you think that your sleeve will be able to be tightened to the specified 118 Ft Lbs without breaking loose? I see some type of loctite product was probably used from the red stains and I can't tell if it was pressed in or threaded in. I am assuming pressed.
 

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The threaded bushing is long enough to be flush with the outside of the swing arm, when tightened it will be tight to the bottom of the bolt head. No stress there. The flange is located on the inside of the swing arm and has approx. .44 sq in of shear area. The shear strength of 6061 is approx. 35000 psi, this means to pull out the flanged bushing would require 15000 lbs after the pivot pin was installed.

The pivot pin and bushing need to endure radial loads and relatively small axial loads.
 

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The threaded bushing is long enough to be flush with the outside of the swing arm, when tightened it will be tight to the bottom of the bolt head. No stress there. The flange is located on the inside of the swing arm and has approx. .44 sq in of shear area. The shear strength of 6061 is approx. 35000 psi, this means to pull out the flanged bushing would require 15000 lbs after the pivot pin was installed.

The pivot pin and bushing need to endure radial loads and relatively small axial loads.
I was actually talking about the rotational stress for the bushing breaking loose and spinning in the swing arm as you tighten the bolt to 118 Ft Lb. I do see that a locking product was used but not sure what force would break that loose. It is a big bolt at a high torque value.

Depending on what Ebay sells swing arms for, you may be able to drum up some business in repairing these.
 

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Hello Everyone,

I recently discovered looseness in my final drive. I began disassembly to check bearings.

Now on to the final drive.
Nice work! However, as you mentioned, a lathe and a mill needed :grin: Lucky you!
A shop would charge you and arm and leg to do this work. It may be cheaper to just buy a used arm.
What I don't understand: why in the world didn't BMW come up with a better solution not using aluminium threads in that area (and others)? A big conundrum to me.
 

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Nice work! However, as you mentioned, a lathe and a mill needed :grin: Lucky you!
A shop would charge you and arm and leg to do this work. It may be cheaper to just buy a used arm.
What I don't understand: why in the world didn't BMW come up with a better solution not using aluminium threads in that area (and others)? A big conundrum to me.
Why? Aluminum is just fine as long as people follow directions and know how to use a torque wrench.
 
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Why? Aluminum is just fine as long as people follow directions and know how to use a torque wrench.
Aluminum is fine but it does degrade with multiple assembly and disassembly for things like the frame top cage over the fuel tank to replace the air filter. After 20 years, there has to be some deterioration on these things as I have had to repair several holes in the aluminum frame with timeserts in order to be able to reassemble things once again firmly. Aluminum does have its limits for re-usability in some of these applications. Add loctite where it isn't supposed to go and you just buggered another hole. Now, the effort Kirk had to go through with his recent swing arm and pivot pins was just ridiculous and no loctite was used. I think someone shrunk the pins with liquid nitrogen and used an impact wrench just to make sure no one would ever be able to remove them again. Probably a Harley owner who had to replace a LT clutch for a $.039 cent O-ring :) OK, jesting aside. The holes do deteriorate with repeated use.
 

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Aluminum is fine but it does degrade with multiple assembly and disassembly for things like the frame top cage over the fuel tank to replace the air filter. After 20 years, there has to be some deterioration on these things as I have had to repair several holes in the aluminum frame with timeserts in order to be able to reassemble things once again firmly. Aluminum does have its limits for re-usability in some of these applications. Add loctite where it isn't supposed to go and you just buggered another hole. Now, the effort Kirk had to go through with his recent swing arm and pivot pins was just ridiculous and no loctite was used. I think someone shrunk the pins with liquid nitrogen and used an impact wrench just to make sure no one would ever be able to remove them again. Probably a Harley owner who had to replace a LT clutch for a $.039 cent O-ring :) OK, jesting aside. The holes do deteriorate with repeated use.
Most deterioration is caused by stress from misaligned fasteners, corrosion or overtorqueing the fastener. I have had my seat frame off probably a dozen times with no visible deterioration of the holes in the frame. The frame is often hard to align with the holes and it is tempting to try to use the bolt to pry things into alignment as the bolt is inserted and that is a recipe for trouble. I get the frame tweaked into position with pry bars or straps or whatever is needed so that I can thread the bolts in by hand at least a few turns. I have never had an aluminum thread go bad that way. I have changed the oil in my Sonata probably 30 times now and it has a steel drain bolt in a cast aluminum oil pan and the threads look like new.

Yes, thread locker can cause issues. I have had as much issue with rust in steel threads as I have had with aluminum threads. I will take aluminum threads all day long as long as they are designed properly (you need a fair bit more depth if you are using steel fasteners).
 
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