Final Drive Strut Bolt Nut

[jwurbel]

I am changing out the final drive fluid and I noticed from a previous post sometime ago that he said you needed to replace the above every time you do this. Is that correct?

 

Thanks

[Twisties]

You are supposed to. I think the nut has a self lock design. However when I ordered some from our local shop they looked at me funny. I don't think they ever replace them. I don't think most folks replace them and I've never heard of one coming off.

 

I think the nuts cost me like $0.86 ea. so if you have time to order one in you might as well. On the other hand if you are going to need to wait on the nut, have to pay shipping, or such, I would just go ahead without it.

[Grumpy-ol-Fart]

Twisty is right, the nut is self locking, actually called a "prevailing torque" fastener. There are two styles, one has a captured nylon interference ring, and the other is staked. The staking can be identified by the presence of small triangular shaped, or sometimes round, indentations on the face of the nut. These indentations are made when the nuts are staked in a hydraulic press. The little observable indentations in the face have an accompanying deformation in the threads just inside of them. When they are installed, the resulting interference frictionally locks the nuts in place. These upsets are really very small and removing the nut the first time essentially machines away the interference. The nuts are structurally sound, they are just no longer self locking. Use Loctite 272 when you reinstall them and everything will be golden.

[Firefight911]

Use Loctite 272 when you reinstall them and everything will be golden.

 

242 is more than adequate, IMO.

 

Never replaced mine. Just clean and dry threads with 242.

[jwurbel]

Lord, I love this forum.

 

Thanks to all who responded.

 

Jim

[Bruce H]

I guessed from BMW's stand point it was easier to use that type of nut while assembling the bike than putting on a drop of loc tight...

[marcopolo]

I have mine replaced every time I have the FD fluid changed. I figure BMW must have had a valid reason to call for its replacement, why would I not? Cost is clearly not a factor.

[JayW]

I have mine replaced every time I have the FD fluid changed. I figure BMW must have had a valid reason to call for its replacement, why would I not? Cost is clearly not a factor.

 

That's what I have done too. I just have to remember to pick one up when I am at the dealer for some other reason.

 

Jay

[OmahaAl]

Twisty is right, the nut is self locking, actually called a "prevailing torque" fastener. There are two styles, one has a captured nylon interference ring, and the other is staked.

 

OK, I could be totally wrong here, but I thought they used a third, different style.

 

I thought the final drive nut and the one on the top shock mount were slightly oblong. When you tighten the nut down, the round bolt deforms the nuts "oblongness" to round. That deformation of the nut is what provides the capture tension.

 

I also thought that deformation was why BMW wanted them replaced after each use.

[smiller]

I'm with Tim and Phil, just use some Locktite and be done with it...

[OmahaAl]

So I looked these up on the internet (so it must be true).

 

They are called Distorted Thread Lock Nuts or Oval Lock Nuts. The top of the nut is pinched in an oval. As the nut is put on the bolt, the oval shaped threads at the top of the nut are forced back to round to provide the locking force.

 

From what I could tell, it looks like they are common on auto suspensions. They aren't considered reusable because they lose approx 60% of their locking force after being installed and removed one time.

 

The nut on the R12RT gets torqued to 42 N-M. The breakaway torque is for Blue Loctite is only 17 N-M and Red is only 25 N-M (from the Loctite site).

 

For a buck, I'm thinking I'll stick with the new nut unless it's an emergency.

 

[Twisties]

So I looked these up on the internet (so it must be true).

 

They are called Distorted Thread Lock Nuts or Oval Lock Nuts. The top of the nut is pinched in an oval. As the nut is put on the bolt, the oval shaped threads at the top of the nut are forced back to round to provide the locking force.

 

From what I could tell, it looks like they are common on auto suspensions. They aren't considered reusable because they lose approx 60% of their locking force after being installed and removed one time.

 

The nut on the R12RT gets torqued to 42 N-M. The breakaway torque is for Blue Loctite is only 17 N-M and Red is only 25 N-M (from the Loctite site).

 

For a buck, I'm thinking I'll stick with the new nut unless it's an emergency.

 

I'm not sure you can compare the breakaway torques like that. Afterall, the nut still get torqued to 42 Nm. You would have to compare the additional breakaway of the distorted lock to loctite breakaway.

 

Loctite will do fine in my opinion.

[T__]

Al, if the torque is 42 nm.. That torque is including the prevailing torque of the lock nut so you have 42 nm total (break away probably somewhat higher due to thread & nut friction after torque)

 

If you use lock-tite the 25 nm breakaway is added to the final torque.. Probably 42 nm would be too high as some of the nut prevailing torque would be gone so even at 35 nm torque that be 35+25 nm or a total of 60nm break away force + the basic tight nut break away friction..

 

 

Twisty

 

[Jim VonBaden]

Al is correct in the type of nut used. My boss, a mechanical engineer, took a look at one of mine and said it would be fine for at least 4-6 uses. I asked my local tech, a friend that works with me on the BMW racing bikes, and he said the dealer he works for never uses new nuts (and doesn't charge for them), and there has never been an issue.

 

I just reuse them, no locktite at all, no issues.

 

Jim

[Don_Eilenberger]

Ditto on the Loctite.. Loctite came out with a new form of their products - in stick form, you just rub it on the bolt. MUCH neater than the dribble-bottle.