Hi there,
I've got a 1986 K100 RT which has just 20,000 miles on the clock. It covered about 13,000 miles in its first 3 years, then around 3,000 in the next 20 years. I bought it in July and have put over 4000 miles on the clock, though most of the time I've had it, it has been stored inactive in a garage. It has recently developed an oil leak, the source of which is not clear. The oil is at the top of the shaft drive (next to where the speedometer impulse transmitter is) down to the final drive drain plug. It only leaks when driven and is depositing oil on the rear tyre and rear brake disc. I've just had the bike serviced (oils changed and valve clearances done) and the final drive drain plug was replaced and fitted with a new seal, however oil is still leaking - though not from the drain or filler plugs (as far as I can tell anyway). The shaft itself does not seem to be weeping oil. Is it possible that oil is leaking out of the rear shock? The rear shock is the original one. I can't think of alternative source for the oil.
Thanks
Philip

how soon does it leak when it is cleaned, could you really thoroughly degrease with strong detergent so the paint surface looks a bit chalky, go for a ride and see where it is darkening with oil from?
Regards,
K-BIKE

Hi Philip,

Might be worth removing the speedo transmitter and make sure the o-ring is in good condition as it is doubtful it could come from anywhere else and double check the oil level in the final drive as it is only meant to up to the base of the bottom thread of the filling plug.

Regards,

Phil

1. shock oil... there is not that much oil in the shock and they only tend to weep, from experience.
2. can you put some on a clean finger and smell it. Diff oils have a nasty distinct smell.
3. 4 holes in a diff:
a. vent hole (top)
b. speedo sensor
c. fill hole
d. drain

vent and speedo may be a problem. These are places I would check first.
I suspect o rings and overfill. Is it possible that the diff was filled through the vent hole?

The differential can also leak at the shaft seal on the wheel side.

Thanks for all the responses.
I cleaned the area and rode the bike for 2 minutes before checking the oil leak site. It does seem to be leaking from the shock, the foam buffer (or what remains of it after 24 years) at the bottom of the shock is soaked in oil and running my finger around the plastic seal (where the piston goes in) showed that oil was weeping out. Estimating the amount of oil that has leaked out is difficlut but I'd say at least 40 ml has been lost. Not sure how much oil is actually inside a shock absorber, but the leak does correlate with a loss of performance. I carried a 15 Kg load yesterday on the back of the bike with the shock set on the middle setting - I got a whack with every small bump in the road! Even today with no load the shock was not absorbing the bumps.
Having just spent £300 pounds on a service I'm inclined to change the shock myself. It looks an easy job. Any advice on a brand of shock and the complexity of the swap welcome.
Many thanks for the advice received.
Regards Philip

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Changing the shock is very simple and should be done in less than one hour.

I fitted a Hagon shock in the end (nitro I think). On the expensive side (just shy of £300 including delivery) but it can be rebuilt and has a huge number of settings. Ordered by phone late Tuesday afternoon and recieved by Thursday lunch time. It was very easy to fit. I needed to use an alignment tool (narrow cone shaped tool) to line up the top eye of the shock with the twin holes in the frame and couldn't have got the bottom eye on the bottom bolt without the help of a mate pulling the rear of the bike up.

Philip

If you overfill you will blow oil out and wet your rear tyre.
At a low mileage I would have thought you have overfilled it.
Always ensure you have the level in the sight glass correct.
Do this before anything else.
Ajays

If it is the shocker and you have it on the centre stand be sure to support the wheel when removing the shock. Important to do this.

You can also ask there
http://brooksuspension.co.uk/shop/bmw-emc-c-27_43_106_628_641.html?osCsid=372116798b85c5f8ac96f46890527b25

the EMC shocks are totally rebuildable. Here in France it takes 3 days and costs 180€... But the shocks are much more expensive here... don't know why but for you english people it's a bargain!!