Estimating mileage

[Dizerens5]

As often happens I'm looking to to experts for a little advice. My 79 shows just over 40,000 miles on the odometer. It came with some paperwork from a PO including the remark that "maybe, maybe" the mileage is genuine although very low for a 30 year old bike.
Certainly though the gearbox has been overhauled the engine shows no sign of having been opened.
I've just noticed that the alternator brushes are just about end-of-life, with about 4 mm left. Can anyone say if that degree of wear corresponds approximately to 40,000 miles? Or does it tell the tale on a much higher mileage?
Maybe I should add that the bike runs very well and that includes the alternator. I have new brushes available though!

[nhmaf]

It's hard to tell - but I could believe that the brushes could be worn down (they definitely need replacing @ 4mm) that far in 40K miles, especially if it was a fair percentage of city/stop&go riding.

How do the footpeg rubbers look, and the "points" on the rear brake pedal surface - those are only very general guides, but if the bike had 140K miles, that rear brake pedal, if not replaced, would likely be worn down pretty smooth....

If it hasn't been replaced for some reason, the rear splines on the final drive-to-rear hub might tell you if the bike is ALOT higher mileage than the other factors indicate.

[Bob_Roller]

I replaced the brushes on my '81 R65 (I'm the original owner) at about 65,000 miles, the brushes were worn to the point that the 'GEN' light would stay on dimly all the time, I'd say pretty close to what you have left on yours .

[Barry]

Have you read Snowbum on the topic ? He says they last approx. 80,000 miles but that there are lots of variables. I guess they would last longer on the bigger slower revving engines than they would on an R65.

http://bmwmotorcycletech.info/altbrushrotor.htm

[Rob Valdez 79 R65]

Just keep an eye on the valve gaps, esp. on the exhaust.  Keep a record, and measure the gap BEFORE adjusting and write down both the before and after gaps in your log.

If your exhaust gap is closing up within 2500 miles, check it again at 1000 miles.  Maybe even less.

I discovered mine going bad on a trip of 450 miles.  
About 2/3s of the way there, I stopped at a rest area and found the engine would not idle.  It seemed to run alright at speeds over 1500 rpm or so, so I continued to my destination.
The next morning I checked the valves, and the left exhaust valve had less than 0 clearance!  I adjusted all 4 valves, and I when I returned home the left ex. was closed up again.
I quickly packaged up my heads and sent them off to Bob Grauer, retired machinist and lover of airheads.  He is in the Bay area, and if you are in North America, he is worth the postage.

[Dizerens5]

Thanks everyone. A lot of wise thoughts there! But it's clear with brushes able to last anything from 40,000 to 80,000 miles, brush wear is no way of estimating mileage! Btw footrest rubbers and other similar parts have been replaced. There's a lot of slop on the gear pedal linkage but curiously this does not seem to affect the use of it.

[Barry]

There's a lot of slop on the gear pedal linkage but curiously this does not seem to affect the use of it.  

They were probably much like that when new. Mine had a fair bit of slop at 6000 miles and even the footrest rubbers were beginning to wear at that mileage so I turned them over.

I'd go for rear wheel splines as a better indicator.