Restoring a 1973 BMW R75/5 Motorcycle

March 4, 2007

Call me abrasive

Filed under: airhead, BMW motorcycle, motorcycle restoration, R75/5 — Penforhire @ 6:18 pm

Now that the tank is squared away it is time to deal with the headlight bucket. I equivocated about tearing it down completely or not. In the end analysis it came down to “why do it at all if you don’t do it right.” For some reason that doesn’t always stop me from screwing it up.

Here’s the two threaded tall nuts holding the speedometer inside the bucket (see arrows). Sorry for the mis-focus.

1874-speedometer-retainers.jpg

I found it extremely odd that these were 5/16″ instead of a metric size. Back of the speedo says made in Germany so WTF?

Here’s the speedo on my bench.

1875-speedo-on-bench.jpg

You can see the busted tach needle. The glass also needs cleaning on the inside. Unfortunately the chromed ring is crimped onto the face! Apparently if I am a master mechanic I can gently pry the ring off without splitting it. Doesn’t sound like me, does it? I’m going to ask Palo Alto Speedometer about it when I send it out for service. The ring itself cleaned up better than expected with a little wad of Nevr Dull. The rubber seal on the backside of the ring needs replacement. It feels more like baked clay than rubber.

Here’s a view of the ignition switch assembly inside the bucket. The red arrow points to one of four tabs that have to be bent straight to release it.

1876-ignition-switch-in-bucket.jpg

These tabs are integral to the bucket so welding work or a new bucket will be required if I break any. This type of assembly seems atypical for BMW, too maintenance-unfriendly. I can sort of see the design review now, “Ach! Fritz, what kacke did you make for our ignition? You affenschwantz!”

Here’s the hidden side of the switch and the stack of parts above it. No tabs were broken in the making of this blog.

1877-hidden-side-of-ignition-switch.jpg

Here’s a picture of the actual contacts of the switch (see red arrow).

1880-ignition-switch-contacts.jpg

There is something Rube Goldberg-ish about the whole arrangement. There is an oddly-shaped cross-bar and the ignition key presses the two contacts together. The rest of the “rotation” switching is just for lights (you can start the engine with no lights on or vice versa). I do not understand the intrinsic value of this design. Must be Fritz’s work too?

Here’s a picture showing the cross-bar (left-right here) and the looping spring that is riveted at the right side.

1881-ignition-switch-close-up.jpg

Here’s a photo of the switch retaining tabs in the bucket. The red arrow points to one of the similar tabs holding the sliding cover in place on the outside of the bucket.

1878-ignition-retainers-in-bucket.jpg

Here’s the underside of said sliding cover. This also shows the spring mechanism for the slider. It is a coil that is trapped behind a riveted stud on the bucket. Seems like another odd design, instead of a straight spring, but it works.

1883-ignition-switch-cover.jpg

And the top side.

1884-ignition-switch-cover-top.jpg

I was worried about the icky brown plastic slider but the whole thing cleaned up nicely with Nevr Dull. The plastic part returned to a glossy black. I have no idea what it was coated with but I thought it was just faded by the sun.

Here’s the outside of the bucket showing where the slider was mounted and the riveted stud at the right.

1885-bucket-where-switch-was.jpg

Okay, this was all hugely successful at emptying out the headlight bucket. There are still a bunch of riveted pieces inside but I’m thinking I can powdercoat over the rivets rather than try to extract them all. There is also a rubber part epoxied in place to hold two small terminal strips but we’ll see if this can take some heat. I got to toss the bucket on the pile of to-be-powdercoated parts. Woo-hoo!

Aside from cracking open the tranny most of the work I have left to do until the frame is coated and the wheels are laced (all TBD) is clean-up. You all know how successful I’ve been at the cast parts (not). I figured I’d give a go at some other aluminum parts. Well, it took a lot longer than I expected.

First I figured out the final metal polish is not abrasive enough to remove any pitting. It just polishes flat surfaces. So I switched to some white polishing compound and a cloth wheel on my drill. Meh. That doesn’t cut much metal either. So I switched to red rubbing compound. Geez, that still doesn’t move much metal! I tried some 220 grit sandpaper. It says extra-fine but don’t believe it. Big-time gouges. I tried 600 grit sandpaper. It still seemed too abrasive. It did a good job of reducing the scratches I made with 220 grit but rubbing compound never seems to get all the 600 grit marks out. “This is the polishing police, put down the paper and back away slowly!”

So I’m working on the same two-piece handlebar clamp for hours. I think I went through two talk-shows on my AM radio as I’m polishing. When I pull up on this bike and you’re standing across the parking lot you’re going to think, “man, do those bar clamps shine!” But when you look closely you’ll be thinking, “old, eh?” I got to thinking I could carve new clamps sooner than polish the old ones. But that’s just a slight exaggeration. Here are a couple of photos because shiny surfaces are hard to capture well.

1887-bar-clamps.jpg

1888-bar-clamps-another-angle.jpg

I am SO not looking forward to polishing the other bar clamp (and a million other aluminum pieces)! So… anyone have suggestions on how a lazy polisher can get the job done? Is there some common grit between red rubbing compound and 600 sandpaper? I know of sandpaper down to 1200 grit but it sure is expensive and loads up fast. Should I just be more aggressive with 600 grit?

Now I have a shiny side to keep up.

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6 Comments »

  1. Eric,
    I think if you look in one of the boxes I brought you, you will find two sets of clamps that are better then yours. One tall set and one stock.
    You know you could also put them in the powercoat pile. They look good in black.

    I started some of my polishing with scotchbrite and steelwool and a little rubbing compound, then moved to, Mothers, white compound, never dull, or whatever seemed to work the best.
    As for the switch plate, it looks new, you should have seen mine
    on my first one.

    Joe

    Comment by BMW Joe — March 5, 2007 @ 11:54 am

  2. Great blog. Thx for sharing this with the public. Its really motivating me to restore my dad’s r75/5 as well. Quick note, I once polished many aluminum parts on the /5 before such as the handlebar clamps, wheels and handlebar with a high quality german polish from the bmw dealer. After a month or so they all went back to looking dull again. Maybe a professional polishing might be in order to get them to stay shiny????. The last thing you want to happen is all your hard work getting dull on you. Its great exercise though….

    Comment by Ozzie — March 6, 2007 @ 1:41 pm

  3. yeah I was wondering about that also. to my mind, aluminum might take a nice polish, but it immediately wants to turn back int Al2O or AlO or whatever oxide it is. Does one have to put a coat of polyurethane or somesuch to keep it shiny?

    Comment by Mark — March 30, 2012 @ 10:21 pm

  4. I had good luck waxing those surfaces but I did rewax regularly. I got a preference for harder carnuba-based waxes from my dad. I know of clear urethane coatings but my history with those is they eventually yellow. Some metal polishing pastes claim to leave a protective layer but I don’t trust those either.

    Comment by Penforhire — April 2, 2012 @ 1:37 pm

  5. I think 0000 steel wool is the best bet for aluminum, the trick is to keep it oily with some wd-40 and work at it for a while, then hit it with mothers.

    Comment by dan. — May 21, 2012 @ 5:42 pm

  6. I’m hoping you might have an answer to a mystery I’m working on solving. I’m trying to re-wire a 1971 R75/5 that most of the wires in the headlight melted… probably from a short. I’ve been sorting most of them out while replacing them. However, there’s one wire I can’t find on any wiring diagram including the factory one that is still with the bike. You show it in your picture on this page of the ignition switch assembly. The white wire, just above the red arrow you drew… it’s soldered to the board; however, I can’t find where it goes to, and I’m losing my mind. Please, if you might know where that connects, I’d appreciate it. It is powered when the key is turned… but, goes nowhere on my bike. Any help is appreciated!!

    Comment by Bill Karitis — January 14, 2017 @ 10:29 am


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