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Author Topic: Final Drive leaking - open to suggestions for next effort  (Read 2176 times)

Offline goldbrixx

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Final Drive leaking - open to suggestions for next effort
« on: September 14, 2025, 01:05:26 PM »
Bike: 81 R65 Showing just under 40k on the clock. Bought bike last Nov, 2024.

I noticed that there was a wet spot next to the rear tire. Tracking the drip, it came from the final drive. Makes sense as the rear brakes were not doing much to stop the bike.

It seemed to me that the arm actuator for the rear brake pedal was weeping. Is this likely? I would not think this would have access to the internal drive oil. Admittedly, I have not pulled the arm out to determine. I have the final drive on the bench and am looking for suggestions on whether a gasket refresh is in order.

Is it possible to overfill the FD with too much oil? Is there a certain measured amount of oil to put in the FD?  TIA.

Offline dogshome

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Re: Final Drive leaking - open to suggestions for next effort
« Reply #1 on: September 14, 2025, 01:43:20 PM »
Hiya, yes it can and yes it can. It depends which FD you have. Some have a bushing that passes all the way through and can't leak. My '83 had that type. The other ones I've seen have a conventional short bush either end of the brake shaft, with the centre exposed to oil. There are a number of O rings on the shaft. Pull the shaft out and shine a torch in the hole. Looking at the oil there, I think you have the conventional type. They also fitted a 4 O ring shaft as opposed to the 3 ring earlier type.

You can get slightly fatter O'rings which are a very tight fit. I put their size on here somewhere! (9x2.5 instead of standard 10x2).

It's very likely your main seal is weak, if not obviously leaking. I can't tell from the pic, but there is a lot of oil there. Since it's out on the bench, replace anyhow. This is very easy to and does not require dissasembling the FD. Also posted on here somewhere and you need a coke can or similar. Standard metric seal, also in one of my posts.

You can overfill, also leak oil from the driveshft into the FD. The driveshaft only needs a splash, but later units had an extra seal. Some special tools are needed to do that. The leak in that case is tiny, they just put a seal on a tight steels to steel joint. I can't remember the amount that goes in the driveshaft, but it is recommended to put less in. All it does is splash on the sliding joint and UJs. So if you have the earlier one without seal, it's going to take a very, very long time to leak.

The level is set using the bike on the centre stand and the little bolt taken out at the back of the FD. It's not uncommon to find these stripped out. Very little torque required. They can be helicoiled, but the diff has to be split. No special tools just for that though. With it on the bench, take all 5 plugs out and check their threads.

A dragging brake can cause heat and the oil to find it way out throught the breather at the top. Your splines look 100%  8)

« Last Edit: September 14, 2025, 01:54:23 PM by dogshome »
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Offline goldbrixx

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Re: Final Drive leaking - open to suggestions for next effort
« Reply #2 on: September 14, 2025, 02:30:59 PM »
Thanks for all the useful info, dogshome! Will get to work on this later this evening.

One part of this that may become clear after I get started, is this statement:
"This is very easy (replacing the main seal) to do and does not require disassembling the FD. Also posted on here somewhere and you need a coke can or similar. Standard metric seal, also in one of my posts."

What's the coke can for?

Offline goldbrixx

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Re: Final Drive leaking - open to suggestions for next effort
« Reply #3 on: September 14, 2025, 02:48:16 PM »
Giving my previous post some thought. Perhaps the coke can is for the main seal going over the spline.

Offline Barry

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Re: Final Drive leaking - open to suggestions for next effort
« Reply #4 on: September 14, 2025, 02:51:34 PM »
dogshome has covered the issue comprehensively but you might be interested in this comment from a BMW service outfit back in the day.

It sounds a reasonable suggestion  because the final drive is not particularly sensitive to oil level for adequate lubrication.
« Last Edit: September 14, 2025, 02:53:51 PM by Barry »
Barry Cheshire, England 79 R45

Offline goldbrixx

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Re: Final Drive leaking - open to suggestions for next effort
« Reply #5 on: September 14, 2025, 02:55:20 PM »
Thanks Barry & dogshome. I'm on my way to clean up and reseal the FD. Will report back later..