Restoring a 1973 BMW R75/5 Motorcycle

January 20, 2007

Lucky number 13

Filed under: 1973, airhead, BMW, motorcycle, R75/5, restoration — Penforhire @ 4:20 pm

By the way, the repair manual call the rings holding wrist pins on the pistons “gudgeon” clips. What is some stubborn French part doing on my German bike?!

Here’s a picture to show that right side header pipe I whined about earlier. This is the part that refuses to come out of the head.


You can see how I “peeled back” the cross-pipe clamp from the other header pipe. Today I, um, borrowed my wife’s hair dryer and heated up the head. After fifteen minutes or so it got toasty. I then grabbed an icepack and chilled the header tube. Sad to say, it made no difference. That sucker is still stuck in there. Next time I plan to saw off most of the pipe and try peeling the remainder out of the head. I see Bob’s BMW charges $104 (ouch!) to restore the threads I smeared but I don’t see “unstick header pipe” on their menu of services.

Here’s a close up of the valves on this side.


Not quite the burnt casserole that I had on the left side but not so pristine either.

Hey look! Here’s the diode board. Glamorous, I know.


This is the sort of part that I’m debating whether to replace or not, even if it tests okay. Electrical gremlins are the bane of old vehicles so I have the notion to re-harness the entire bike, given that unstable connections are the very worst of those gremlins.

Here’s the starter motor mounted on the bike. I drew a little red arrow pointing to a dead spider, one of many that seem to be holding things together.


On the subject of wild animals, Here’s a shot of a wanna-be beehive started on the underside of the rear fender. Yeah, I got the fender off. I’m surprised too.


There must be something attractive to bees about motorcycle fenders. When I lived in upstate NY I rode a beat-up Yamaha XT600, a single-cylinder dual-sport bike that was perfect for the rugged terrain in the far suburbs of Syracuse. I vividly remember jumping on it, starting it up, and wondering why there was a cloud of insects buzzing around me. It took me a few seconds to realize that was an entire hive of bees and they weren’t happy! While they were not Africanized killer bees they also were not peace loving make-honey-not-war bees. I ran off in a screaming hurry, leaving the bike running, and only got three stings. Those bees managed to make a full-size hive under the rear fender in about a week!

Now here’s something you don’t find on 1973 bikes every day —


This is an SAE connector, probably added to connect a trickle charger to the battery. This or something like it will get re-installed.


Shocking! At this isn’t a Brit bike so I don’t have to call ’em dampers. The bodies are so far gone I can’t say for sure if these are original or what. I know the hot ticket back when was a pair of rebuildable Koni’s. There are Hagon’s available now but many prefer the optional Heavy Duty BMW shocks. I might pull these apart for giggles but I recall my dad never strictly maintained things like shocks or brakes. That “suicide sensation” I remember in corners is at least partly due to these crappy shocks.

Here’s where we get to the title of this post, lucky number 13. Millimeters that is. The starter is held in place with two 13 mm nuts. I just can’t seem to figure out how to remove them. Here’s a picture of my socket almost on the nut.


You’ll notice the webbing that holds the driver shaft at a bad angle. I tried a knuckle-joint, my new fancy impact sockets, and even a single u-joint driver I happen to have. Nothing doing. Not enough room.

Here’s a picture of my combination wrench on the nut perfectly.


What’s wrong with this picture? Hint — which way does the nut turn to unscrew? The wrench is right up against the engine casting with no room to turn. There is also not enough angle to move the wrench back one step. Don’t tell me, time for a 12 point wrench? I dunno. I asked Joe about it and he doesn’t remember any drama in removing the starter. So it must just be me. Chalk this one up with the other handful of things waiting for Joe to come take a look. Stick and move. Sting like a bee.

Here’s a pretty good image of the corrosion on all the cast aluminum surfaces (most of the engine/tranny and wheel hubs). Once again, I am thankful this is not a Brit bike or I’d have to say aluminium.


This is whitish powdery surface corrosion, probably aluminum oxide. The casting are not pure aluminum but rather a Si-Cu alloy of aluminum. That is the reason, according to some, that no wet chemical dip is known to properly restore this surface. The best option I know of is to glass bead blast this surface. Joe went one step further on his last rebuild and clear-anodized after blasting. He had some difficulty, getting slight variable hues from the anodizing, so I’m more likely to just blast ’em.

Some people complain blasting leaves the surface brighter-than-original. This darkens over time and I don’t care about matching factory specification that much. The best sequence suggested on the 5 United message board is to steam clean, blast with a fine glass and alumina mix (have to be careful not to abrade too much metal), and finish with a heavy glass at lower pressure to avoid a “powdery” look. At the end you have to rinse thoroughly to remove any traces of blasting media. You can imagine how harmful it would be to have some loose alumina or glass powder in the valves or cylinders!

My parting image shows how nicely stripped down the back end of the bike is looking right now.


And that darn starter, and the pistons, and the…



  1. Does anyone know if it is possible to clean these Aluminum parts with ultrasonic cleaning – if so, what does the finish look like?

    Comment by Chuck — July 21, 2007 @ 8:58 pm

  2. I did some exhaustive searching on the 5 United board. I even tried ultrasonics on a small part with Simple Green. No go.

    I brass-bristle scrubbed with Eagle One etching wheel cleaner (I think I covered this somewhere) and it just darkened the surface.

    Glass bead blasting is the only thing that did the trick properly for me.

    Comment by penforhire — July 22, 2007 @ 2:12 pm

  3. If you still have the original shocks do you want to sell or give them away? Thanks, Rob,

    Comment by Robert Haag — April 3, 2016 @ 8:41 am

  4. please let me know by email, Thanks, Rob,

    Comment by Robert Haag — April 3, 2016 @ 8:42 am

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