Restoring a 1973 BMW R75/5 Motorcycle

February 18, 2007

Start me up

Filed under: 1973, airhead, BMW motorcycle, motorcycle restoration, R75/5 — Penforhire @ 9:37 pm

I promised some wrenching and today I deliver.

I began by taking apart the starter. Now, I have no idea if the starter was okay as-is but I figured it could use some TLC. I bought a refresher parts kit, from Eurotech Motorsports I believe, to see if I could at all avoid a $350 replacement so I was determined to dive in.

One side note about Eurotech — this week I received a rather large parts shipment I ordered from them. So far my only real complaint with them as a parts vendor is they do not label every part with its part number. So, not being an expert in what seal goes where and such, I did have to use deductive reasoning to inventory the order versus the shipment and re-bag the parts with some ID’s. As it turns out, the throttle cable is not the P/N I ordered so we’ll see how their return system works, again. You may recall I had to return some carb diaphragms to them before but it was pretty painless. And one incorrect part in thirty or so isn’t too bad. Is it?

Here is the “top” section of the starter on my bench.


So far the only tricky thing was that brush holder at the lower right.

Here’s a close-up of the commutator.


I see some lightly gouged circular wear tracks so the brushes need attention. The Haynes repair manual does a funny sort of verbal dance about not knowing what the correct brush length is… but they should be replaced if worn over 50%. How lame is that? The replacement kit had brushes and springs so I could tell the original brushes were no more than 25% worn. I sanded them smoother and did not replace them. I also had fresh brush springs so I did replace those.

Here are the windings and field coils.


Even leaving the solenoid alone, I was starting to see what makes these things worth such a high price. The manual says if the field coils are charred that professional help is needed. Good thing that red coating looks fine, just grimy. Just for the heck of it I tried to loosen one of them and that big screw was not going anywhere. Okay, okay, back together you go.

Here’s one of the two replacement bushings. The manual and renewal kit both said to soak these in motor oil.


One source said overnight, the other said 30 minutes. They must be a sintered bronze or similar bushing. I used a just-right-sized socket to tap the old ones out and a soft mallet to tap the new ones in. There were all sorts of warnings about how fragile these are. The old bushings didn’t look trashed but everything was gummy inside the starter so a clean-up can’t hurt. Well, unless I screw something up.

Here’s the whole beast reassembled and ready for action. I hope. It’ll be a while before this thing turns in anger. I decided not to repaint the outside. There is some surface rust but nothing deep. And this whole part is hidden under a motor cover. It looks like someone already repainted it once because some of the black paint is flaking off badly.


Remember that 36 mm fork spring nut that resisted my attempts to charm it off the bike? Well I took a tip from Bob who mentioned using his floor jack handle as a breaker-bar extension. I think my handle is aluminum so I worried about bending it. I got Julie to hold the socket on the nut and help prevent the frame from moving then gave it the old heave-ho. The nut broke free so suddenly Julie thought the socket slipped off. Nope, it just did the trick. Woo-hoo, no nut can beat me! A few more nuts to loosen and voila —


Forks ala mode! Did I mention I forget to drain the fork oil first? Oh yes, there was a mess. But I was so happy to get this far I didn’t mind. Here’s the bottom steering yoke, with one of two bearings shown at the bottom.


Joe happened to call when I had just set these forks aside and we chatted a little bit about the bearings. I’ll have to double-check but I think the same exact bearings are used at the top & bottom of the steering head and at the swing arm. As long as I am tearing the whole bike down I feel a duty to throw new bearings in there. Here’s the lower race of one steering bearing, still stuck in the frame.


If you look closely you can see the individual roller bearing marks. Steering has relatively little motion but a lot of straight ahead vibration. So the failure mode tends to be local damage. I believe this is called brinelling which is odd because I know of the Brinell hardness test, named after some fellow named, well, Brinell. Anyway, it looks like I’ve got some. Okay, so here we’ve come to needing another weird tool. Those races are not about to just pop out of there. I tried. Not as if I’m reusing them so what the heck, give it a go, right? I’m not sure how they’re fitted at such a shallow angle but I did get a clue when the manual talked about freezing races overnight to help fit them back in. So now I need an expanding bearing puller or some such utter horse crap. Sigh.

Onward and upward. I drilled out the two rivets holding the bike’s ID plate to the front of the frame (between the steering yokes). The frame has to be completely stripped since I intend on powder coating it.


I could be mistaken but this plate looks like that black paint was etched away to expose the base metal to make lettering and images. Does anyone know a source for replacement plates? This looks like a serious pain to otherwise restore and I can see it making a significant visual difference. Maybe I can use a photographic process to mask on a new black layer? I dunno. Right now this is one of the Mysteries. I’ll try searching the 5 United board and post a question if nothing pops up.

Here’s another sticky point.


This is the column lock. I think I remember some comments about pulling this lock. I just need to find those. Because I seem to be lacking a key I bought a replacement set of both column and seat locks, keyed alike. So I think I’m drilling this lock out. We’ll see. And that pivoting cover looks like a rivet too. Rivets. Rats, that’s another tool I don’t own yet. But just this lock and the two steering bearing races stand between me and a frame ready to process (blast & powder coat). Did I mention how heavy this frame is? I’m down to the bare frame but it must still weigh fourty pounds. That’s the result of pre-finite element analysis and low-porosity casting technology. Probably means it’ll last longer but it can still break a toe if I drop it.

While I was staring at this lock it reminded me to take a look at how it engaged the steering column.


You can see the slot in the column shows some wear. I think that is inevitable, given how this lock system functions. Doesn’t look too bad.

That’s it for this weekend. All in all I feel good about today. Feels like I’m winning the war.


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