Restoring a 1973 BMW R75/5 Motorcycle

April 29, 2007

Found what I’m good at

Filed under: /5, airhead, BMW motorcycle, motorcycle restoration, R75/5 — Penforhire @ 4:32 pm

You may recall the bottom of my gas cap from an earlier post.


The cork seal is so filthy and hard I thought it was some other material. Joe warned me the press-fit retaining pin is a pain to remove. What is it with BMW and press fits? It is giving me a fit!

First step is to drill a hole.


Yeah, once again I know it is supposed to be centered. Do you have any idea how awkward it is to clamp that gas cap under a drill press? I sort of jammed it between a fence and a miter-clamp. I’d tell you not to try this at home but if I still have all my fingers odds are you will too. I was smart in that I started with a small bit, 1/16″ if I recall correctly, and ran low RPM. I got dumb when I worked up to a 5/32″ bit. It got jammed in the hole and my first reaction was to let go of the drill press handle. Well, that lifted the workpiece above the fence and it spun freely. Good thing it was roughly balanced. I managed to turn off the drill before it either flew off or broke the bit. Actually I doubt it would have gone any better had I used my OTHER hand, which was holding the clamp in place. It would have been a perfect moment for a foot switch, something I don’t have.

Anyway, I managed to get a big enough hole to then embed a sheet metal screw in it. While that did not let me pull it out intact (my hope) it did pop the top off the retaining pin. Here are the guts.


There is a messed up rubber bumper, surrounded by a spring, and the cork sealing disk is trapped under the pin I drilled out by a washer. You’ll notice there is still some metal jammed in the hole. Yep, back to the drill press until —


Now I did not know there was any sort of rubber bumper under the seal. I could feel the spring tension and I just figured the spring forced the seal. So I never ordered a replacement bumper. This one is way too ratty to go back in there. The BMW gods may frown upon me now but I thought about it and whipped out a metric o-ring kit.


Voila! A tower of snugly fitted o-rings. What’s the worst that can happen? I know… fire, flood, and disaster. Here’s the finished restored gas cap.


No snooty concours d’elegance judge will ever be able to detect the non-BMW workmanship. Well, he may spot the second washer I used because I didn’t like the stock size overlap on the sealing disk. Not much metal is keeping the disk from flying off the press-fit pin. And no way did I want to do this again.

Since I’ve got the gas tank back from painting I went to gather all the pieces. I need to let the paint dry for another month or two but I figured I could get a sneak preview of how it was all going to look. Unfortunately (I use that word a lot don’t I?) one of the screw-on emblems developed a chip while on the shelf.


I didn’t drop anything on it so it cracked spontaneously. Sigh. That will be a $44-ish part from Bob’s. Now I figured, what-the-heck, I might as well use the broken emblem to see if a polishing wheel would restore luster to other emblem. No go. It seems to just highlight the corrosion in the “leaded” sections and does not improve the colored sections. Might just be buying TWO of these before I’m done.

On to another old friend, Mr. bevel drive.


The drive shaft connects to the left side here and the teeth in the center engage the rear wheel’s hub. There are some diagnostic checks to do on this drive. If it requires teardown then the manuals discourage you from doing it. Haynes flat-out says take it to a dealer and has no instructions. Clymers has a half-dozen dense pages calling for special tools and warning you not to attempt it without those tools. That puts me deep in the do-not-attempt category. Here’s the problem.


This is a close up of the ends of the drive teeth. Ignore any fuzz you see. That’s just lint from rags getting caught on the worn metal burrs. The teeth are starting to wear badly. None of the manuals tell me how much wear is too much but I don’t feel good about this much. What say you? The rest of the mechanism feels tight. There is no slop in the internal 90-degree transfer of power from the drive shaft to this ring.

Here is the magnetic drain plug on the bottom on the pumpkin.


I had a little brain fade and forgot to take the picture while a pile of fine black sludge was still on it. You’ll just have to take my word for it but there was a nasty glob of magnetic black goo stuck to it. Seems the magnet is a smart idea.

The rear brake cam, a lever that spreads the shoes to press against the linings, is also a part of this pumpkin. Here’s one of the cam edges.


Doesn’t look too bad. There is a little wear but nothing I’d call damage and the other came edge is even better. The whole lever didn’t move freely enough. I could feel some drag and that is not something you want when the drum brakes are barely adequate when properly working. Here’s the other end of the cam, where it exits the pumpkin.


Before you loosen the pinch clamp to remove the lever make sure you mark the position. It needs to be reassembled in the same position unless the brakes were incorrectly adjusted originally (we’ll see). Note the scratch I made at the pinch gap on the splined shaft.

Here’s what you get when you take it apart.


There is a felt seal under the lever. Of course mine is hard as a rock and ratty as it can get without actually disintegrating.

Here’s a close up of the shaft.


Note the polished section near the center. I imagine this was due to abrasive wear over the years but who knows? Doesn’t matter. There is plenty of clearance and the reason for sticky motion seemed to be old hardened grease on all surfaces. I cleaned it all up and pumped it full of a synthetic grease. The manual says to use any moly disulfide grease. Curious to me because MoS2 is an extreme pressure additive and I wouldn’t think that necessary for this brake-cam shaft.

Who uses felt seals these days? Nobody. That’s who. And to prove it I was unable to find felt washers at three automotive parts stores and three hardware stores (even Cannings in La Habra, which is legendary for their small fastener selection). Sigh. I found a piece of felt that was about the right thickness and cut it to size.


Felt is a more complicated fabric than you might imagine. It is the oldest form of fabric and is not made by weaving. There are more variations of it than you can shake a stick at. So the BMW gods are surely unhappy that I used a random piece of felt. After the o-ring incident I figured my fate was sealed and futher transgressions today won’t make a difference.

The day was going okay so I thought I’d touchie-feelie another dangerous part of the machine. Here’s the rear end of the transmission.


I started to clean the road splooge off it. The whole thing is caked with many miles of it. Come to think of it, I don’t recall my dad ever washing his bikes. That probably explains why it gets to an eighth inch or more in places.

Here’s the front end of the tranny.


The only functional part is a long spline engagement to the engine in the center. Those splines are sharp but I don’t see wear on them. They are evenly sharp for their entire length. You can see me starting to clean the splooge off the right quadrant.

Here’s the same front end after a bit more elbow grease and Engine Brite.


Yep. I think I found my calling. I seem to be good at removing grime. I just need to find restoration candidates that only need cleaning. Then I’d be set. You’ll notice I haven’t said a word about the guts of the tranny. Yet.



  1. Eric,

    You are on a roll, even with a few bootleg parts. I think you should make this blog into a book when finished, educational and entertaining.


    Comment by Bryan — May 1, 2007 @ 3:56 pm

  2. Eric,
    A word from a weaver:
    felt has long been made from wool, woven into a “plain” weave (over, under, over, under (nothing fancy). Then it is washed in warm water, the more warm, the more the shrink, the denser the fabric. Then it is felted. Felting was (in the old days) done with needles. Your store-bought piece may or not be wool, and may not be woven, but I would be surprised. You want me to make you some?


    Comment by Mary — May 3, 2007 @ 11:15 am

  3. Unless it leaks that part is NEVER coming off the bike again! But thanks for the offer.

    Comment by penforhire — May 3, 2007 @ 12:22 pm

  4. Hi
    Nice to see an good thourough blog as this. I stumbled over youre site in search for information for replacing final drive on the rear bevel (BMW R 90/6). And are now stucked! How did you replaced the final drive, cant seems to find any good manuals. I live in Norway so it far between experise. I would appreaciate any tip that will lead to an solution.
    Best regards
    Espen Schuren

    Comment by Espen Schuren — November 13, 2016 @ 9:53 am

  5. Hi Espen! I recall cleaning up the final drive but not replacing it. I sympathize about the repair manuals. It seemed they often lacked the view I needed or some critical description of a method. Maybe one of my other readers can give some suggestions.

    Comment by Penforhire — November 20, 2016 @ 11:47 am

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