Restoring a 1973 BMW R75/5 Motorcycle

January 21, 2007

Are you *#%ing me?!

Filed under: 1973, airhead, BMW, motorcycle, R75/5, restoration — Penforhire @ 6:43 pm

I don’t think I mentioned Eurotech shipped the wrong size diaphragms with my carb rebuild kit. They were sized for 40 mm instead of 32 mm. You have to imagine me playing with the carbs trying to finish that rebuild and wondering if the rubber parts somehow expanded. I’m stubborn so it took me a few tries, holding the parts together like a square trying to fit a circle, before I gave up. I pulled out my invoice and, yep, says 32 mm just like it should. I must say they were very quick in sending the proper replacements and I guess the extra o-ring-and-gasket sets they sent with the right diaphragms (another complete rebuild kit) takes the sting out of the delay and cost to mail ’em back. I finished carb assembly today but it will be quite a while until we find out if my rebuild is acceptable. I didn’t take a picture because, well, you know what these carbs look like, right?

I talked to Joe today and he had another good idea. A veritable fount he is. He says let the starter wait until I remove the tranny. The casting webbing holding up my socket driver is part of the tranny housing. That won’t be in the way when I drop the tranny. Joe mentioned he has, somewhere, a custom-bent wrench to access the starter as it lies. Probably another BMW special tool, eh?

Okay, the title of today’s post comes from what I said after pulling the headlight out of its housing. Here’s a photo of the interior. I might also have muttered a Charlie Brown-like Waaaugh!


There are way more wires and bits in this headlight bucket than there should be! You could say I was a little intimidated at first glance. Here are most of the interior parts sitting on my bench a few minutes later. Oh, so I’m not fooling you? It was about at least a half hour later.


There are seven bulbs in this photo. Again at first blush, I can’t recall needing seven bulbs in addition to the headlight! See those four blue things on the left side? Those are splices and the physical pieces are stamped as a 3M part. I could be wrong about this but I’m thinking those splices do not belong in there. No way were 3M electrical parts used by the BMW factory. A Bosch splice I could believe. Joe thinks if splices were necessary it would have been a fancy BMW connector block like you can see at the upper left. Yes, I took photos of the wire layout on that block and elsewhere. Otherwise my odds of proper re-harnessing would have dropped quite a bit.

I’ll be tearing down this bucket a little further. The tach needle is broken down to a stub. I think that was common but I don’t know why. I am also thinking I might throw an ignition key kit into it. Otherwise any mook with a 10-penny nail can start the bike (ignition uses a universal key, not really “keyed”). Someone developed a slick lock retrofit that hides under the original sliding latch. Looks stock.

Here is my rear axle.


Note the ubiquitous rust on the left side. The repair manual was just wrong about removing this piece. It talked about drifting it out from the left side. A few taps later, when nothing moved, I thought again about why there would be a nut on the right side and only a clamp on the left if that were true. If I was a better mechanic I would have thought of that sooner. Anyway, no harm done (I think).

Here is the rear wheel.


Funny thing about the rear wheel. The manual says after getting the axle out just pop the wheel off the rear drive unit and roll it out. Well the wheel popped off the drive unit just fine but it was NOT going to just roll out of the swingarm. The brake assembly leaves a clearance maybe an inch less than the inflated tire diameter. I called Joe just to make sure I wasn’t missing anything. Nope. You have to deflate the tire and squeeze it past the brakes. The whole fit back there is so tight that it limits the choice of replacement tires. A larger more rounded tire than the typical “flat-top” Continental is said to improve handling. This rubber is old enough that I was planning to replace both tires (and inner tubes) anyway, even before I spotted a couple of nail heads in the rear tire.

The manual says the brake drum starts as 200 mm in diameter and should be replaced at 201.5 mm. Unfortunately the longest caliper I own stops at 150 mm. Eyeballing a 1.5 mm diameter difference with a ruler is not doable. So until I find a bigger caliper I can’t be sure about the drum, though Joe says they almost never wear out.

Here are the rear brakes, still assembled on the final drive unit.


The manual said to pull on the brake shoe centers to get them off. They didn’t have any proper image of what direction to pull though. So I prodded them for a while before I yanked straight back along the axle’s direction. Then they popped off like a bear trap (both shoes levering toward the center)! Can’t work on these things if you have a bad heart. Or value your fingers.

Here’s one of the rear brake shoes.


The manual says to replace these when the lining gets to 1.5 mm thick. I measure these as 2.4 to 2.5 mm so I figure to re-use these shoes. If you think I’m wrong please throw a comment up here to correct me. I notice the manual also speaks of a rubber block added around the springs and a different shoe (different spring mount location) in later years to reduce brake squealing. I can’t tell from the manual if these benefit a stock /5 or not. Anyone know?

Here is the complete rear drive unit. I need to bust this pumpkin open eventually and see if the gears are still happy.


Here’s a view from behind the bike at the end of today.


You might also notice the control cables, electricals, and lights are stripped off. They weren’t too exciting so I didn’t take pictures to post. I can add specific part images if you insist. I haven’t figured out how to remove the neutral gear indicator signal from the tranny. That’s my last electrical connection still attached. And the front turn signal harness is still threaded somehow through the forks. I had to cut the rear harness off because it was threaded through the frame and a kink just refused to pull through the frame.

Today was a productive day, at least for me. I washed up three times. My wife spotted some grease on a carpet I must have tracked in. That’s when the sky shook for a little while. She is obsessive about a clean house. Well, I’m glad someone is. Do you guys work in a moon-suit and decontaminate when you go in the house or what? I don’t have a sink out in the garage (or heat or enough electricity for that matter) but I can see how useful that’d be. That Liquid Glove coating almost works. It seems okay on non-wear locations (say the wrist) but it must wipe off right away on the fingers.

I’m thinking my next bike-work-day I’ll strip off the shaft drive and the odd joint behind it. Then the tranny is ready to fall .



  1. Eric,
    About measuring the ID of the brake drum. You have the brake shoes off, soooo, can’t you simply measure the arc of the brake shoe, calculate the radius and with a few more calculations, figure the ID of the drum???????
    About wearing moon suits.
    I say, HA!
    Be a man!
    Stand up to her!
    Simply let her know whose Boss!!!
    PS….DUCK! and RUN!
    PSS.. You will now be able to stay in the garage as long as you want.

    BMW Joe

    Comment by Joe — January 22, 2007 @ 1:34 pm

  2. Eric,

    If you don’t alraedy have, get some orange waterless hand cleaner, lots of blue paper towels. Rub on, wipe off, and 90% are gone before you actually go into the house. Eaiser to clean you hands once over the sink, and keeps most of the black stuff off the sink hidding the evidence as well.


    Comment by Bryan — January 22, 2007 @ 2:24 pm

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