Boxerworks.com

I'm continuing to work on a '77 R100RS. I have it running now (a video will post to my channel tomorrow). Still some things I'm trying to figure out though. After a ride to show it to some fellow vintage motorcycle enthusiasts, it started leaking gas again. The spring behind the petcock lever looks like someone cut it short. I wonder if anyone has a junk petcock that I can scavenge a spring from (or just a spare spring)? While I was looking into that today, I noticed fluid on the floor by the rear wheel. It appears to be oil from the final drive. The entire drive housing was coated in it. It appears to be coming from that little domed cap all the way at the top. I assume this is some sort of breather? The Clymer manual I have isn't clear on it. It mentions that some have a fill hole at about axle high and some have one at the top. The one I'm working on has an allen head plug that I assume is the fill hole (about axle high) in addition to the domed cap up top. Any theories why it would be burping oil out the top?

If you want to check out the video it should post at noon tomorrow (link to channel is below).

Cheers

https://www.youtube.com/@6-Bike-Garage

The final drives used on airheads come in two styles. I've picked a couple of images off the internet to illustrate.

https://www.bayermotor.de/media/images/ ... 93-1-1.jpg
The earlier models have this type. The filler is a hex plug on the step halfway down the casing and the breather at the to of the casing is pressed in and shouldn't be removed.

https://i.ebayimg.com/images/g/s~sAAeSw ... -l1600.jpg
Late Models (after 1980 I think) have this type. The filler is incorporated with the breather at the top of the casing and there is a level plug (6mm screw) at the rear.

The symptom you are describing is charactersitic of an overfilled final drive. It's not unusual to get a little oil out of the breather but what you are describinf is WAY too much.

On the early type of drive, the level is set by filling the drive to the level of the lowest thread in the filler hole.

On the later drive, remove the level plug and fill till oil starts to run out.

Now to the bad news... an overfilled final drive is sometimes caused by oil migrating from the drive shaft void to the final drive. This is caused by failure of the oil seal in the final drive and/or by overfilling of the drive shaft void. This, in turn, can be caused by oil migrating from the gearbox... which can be caused by failure of the oil seal in the gearbox or an overfilled gearbox.

My advice would be to check the oil levels in all three... final drive, drive shaft void and gearbox. Make sure they are within spec, then ride the bike. Keep a check on the oil levels and watch for a recurrence of the leakage.

You do need to fix this. Oil on the rear wheel is a recurring cause of accidents to airheads.

Rob

Reassembling the fuel tap can be quite challenging to put it mildly and it can seem near impossible without devising some sort of jig to compress the assembly. It's one of those frustrating jobs where if you struggle, then it's best to walk away from it for a while and think of a solution. I can easily envisage that might have encouraged a previous owner to shorten the spring.

6bikegarage wrote: ↑Fri Jan 09, 2026 11:03 pm I'm continuing to work on a '77 R100RS. I have it running now (a video will post to my channel tomorrow). Still some things I'm trying to figure out though. After a ride to show it to some fellow vintage motorcycle enthusiasts, it started leaking gas again. The spring behind the petcock lever looks like someone cut it short. I wonder if anyone has a junk petcock that I can scavenge a spring from (or just a spare spring)?
https://www.youtube.com/@6-Bike-Garage

I you are in the UK, then I have a spare spring and would be happy to send it... but, from you use of 'Gas' and 'Petcock' rather than 'Petrol' and 'Fuel tap', I surmise that you are elsewhere so postage would probably be an issue. Let me know.

FWIW, it's always a good idea to have your location somewhere on your post (signature is ideal)...

Rob

He's been using "jason in Seattle" on his other posts.

viewtopic.php?p=136243#p136243

Kurt

MaxBMW shows part 16121236390 is less than $3. There appears to be a dealer in the Seattle area. I believe Shail's BMW is nearby as well, but appears to be in Vancouver.

Kurt

Rob Frankham wrote: ↑Sat Jan 10, 2026 4:49 am Now to the bad news... an overfilled final drive is sometimes caused by oil migrating from the drive shaft void to the final drive. This is caused by failure of the oil seal in the final drive and/or by overfilling of the drive shaft void. This, in turn, can be caused by oil migrating from the gearbox... which can be caused by failure of the oil seal in the gearbox or an overfilled gearbox.

My advice would be to check the oil levels in all three... final drive, drive shaft void and gearbox. Make sure they are within spec, then ride the bike. Keep a check on the oil levels and watch for a recurrence of the leakage.

Many thanks sir. What you describe makes so much sense. I will check the oil levels and figure out which seal is leaking. I don't know for sure if that is what is happening until I check, but it certainly sounds like a plausible explanation. (FWIW, it is the earlier design with the hex head plug)

Also, thanks for the spring offer, but yeah here in Seattle. (BarryH) I know how difficult that spring can be to compress (I worked on a '72 R75 last year). I said I never wanted to open one again . I think I have the leak stopped for now. I aligned the high (pointed) end of the spring with the lever so it is putting the most pressure right over the passage hole. Also I appreciate the MaxBMW point out. I just hate ordering a $3 part and paying $10 to ship it .

I don't know if I'll end up being the one replacing oil seals between the tranny and final drive, but if I do have to order other parts, it definitely softens the shipping cost blow.

Thanks again all for your input.

jason in Seattle / 6 Bike Garage

6bikegarage wrote: ↑Sat Jan 10, 2026 1:09 pmAlso I appreciate the MaxBMW point out. I just hate ordering a $3 part and paying $10 to ship it .

So no option to drive to your local dealer and see if they have the part? I suspect they probably won't!! But grab a burger and drink...make a day of it.

BTW...MaxBMW includes a bag of M&Ms in their shipments...so there is that.

Kurt

6bikegarage wrote: ↑Sat Jan 10, 2026 1:09 pm
Also, thanks for the spring offer, but yeah here in Seattle. (BarryH) I know how difficult that spring can be to compress (I worked on a '72 R75 last year). I said I never wanted to open one again . I think I have the leak stopped for now. I aligned the high (pointed) end of the spring with the lever so it is putting the most pressure right over the passage hole. Also I appreciate the MaxBMW point out. I just hate ordering a $3 part and paying $10 to ship it .

jason in Seattle / 6 Bike Garage

The spring has a BMW part #; as such, the part can be ordered by any dealer and shipped there at no charge with all their other supply orders. That will take a few weeks though, but you wouldn't pay for shipping. You would have to pick it up from the dealer, which would be a good option if you have dealer close by.

Euro Motoelectrics also has them in stock. I don't know what the shipping costs would be though.

https://www.euromotoelectrics.com/produ ... ing390.htm

That being said, how's the condition of the rest of the petcock parts? It might be better (and perhaps less expensive) to just order a new petcock at $52 each at EME. And, yes compressing that spring to get the cover back on is a ROYAL PITA.

If it was my bike, to do with what I wanted. I think I would strip it down to the frame and take care of all the little things that should really be addressed, but I can only do what I can do. The neighbor who owns it wants to sell it and she is firm that she wants 3k for it. I don't know how these things value, but with all the issues it seems like a big ask. I've been working on it for free to help her out, but I have another guy who wants me to work on his '75 Ducati 860 so I want to get this one moved on from the garage. Unfortunately, I can't bear the thought of it sitting outside and she doesn't have a garage so I'm trying to help her find a place to store it too. For now, with the fuel leak stopped (at least for now), the bigger concern is the oil seal between the shaft and the final. There was a lot of oil in that final!

https://youtube.com/shorts/B___lDS9HOU?feature=share

Cheers all,
jason in Seattle