Restoring a 1973 BMW R75/5 Motorcycle

May 28, 2007

Turning point

Filed under: airhead, BMW motorcycle, motorcycle restoration, R75/5 — Penforhire @ 8:04 pm

I saw BMW Joe this weekend. I dropped off his prizes from the last ride. It was on my way to drop the wheels off at Buchanan’s. Can you believe I forgot to take my bolts-stuck-in-holes seat with me? I think it was subconscious since I want to take another shot at it with a proper tap wrench to twist the screw extractor (I think I can, I think I can).

Joe had a little surprise for me. A five years back I owned a BMW R1200C and belonged to the BMW Owner’s Club. They publish a monthly magazine with all sorts of interesting stuff inside. I tossed mine after I sold that BMW (not one of their more reliable machines plus I didn’t like the cruiser riding position). Joe did not toss his. He loaned me his pile going back to about 2002. Here’s one example of an article I copied this weekend —

2014-bmw-owners-news.jpg

Pretty directly applicable eh? There are other articles about valve adjustments, clutch behavior, and such. There’s even ANOTHER article that talked about not forcing those exhaust nuts if they get frozen. So this is a fine addition to the “pile of knowledge.”

Funny thing about Buchanan’s. They are normally open on Saturday but it appears they closed for the holiday. But I really did intend to go there. See inside my car’s trunk?

2015-wheels-in-trunk.jpg

Joe convinced me to consider polished stainless spokes and nipples. I was thinking to go natural steel but the wheels will be extra shiny if they’re polished. And someone else will do the work this time.

Here’s some of the excess powder coating in the steering tube. Ignore any white powder you may see. That’s just beadblast material gone walkabout all over my garage.

2016-powder-coat-excess.jpg

A press fit bearing is NOT going to like that extra thick paint. So grab some sandpaper and do what it does —

2017-powder-coat-removed.jpg

I was careful to avoid damaging the finish elsewhere but it was gratifyingly difficult to remove the powder coat layer here and in a few other necessary spots. They masked every screw thread but not any unthreaded areas.

It feels like a I hit the turning point on this project, where I stop taking things apart and start putting them together. It is a bit daunting. It sure was easier to take things apart. There is also less of an obvious path than during disassembly. So I celebrated by adding a replacement VIN plate to the frame.

2018-new-vin-plate.jpg

The rivets that hold it are, guess what, not rivets at all. They are press pins. Hans at Hucky’s had them with the plate. I need to find some number punches to add the frame number to the plate. Thought I had a set of punches at the perfect size but it turned out to be the alphabet, no numbers. You may say, “Hey, why didn’t you stamp it before putting on the frame?” Well, the plate comes pre-curved so it needs to stamped against a frame-size backing piece anyway.

I’m still not done bead blasting but I got several more parts completed. Here is the front brake cast cover in mid-blast.

2020-bead-blast-line.jpg

You can clearly see the line between dark corrosion and lighter blasted alloy. I used tape to protect the axle sleeve but those tiny glass beads get everywhere. You must blow and wash thoroughly because it won’t take much abrasive media to ruin something. I underestimated how much grit and how widely the grit travels in the garage and I’ll need to flush the engine’s bottom end pretty thoroughly before I assemble it (and likely do multiple oil changes during break-in just to be sure).

Some of the hazards of buying parts in anticipation of needs are not having exactly what you need (I think I’m short a few bearings) or, sometimes, buying two of something.

2021-fork-rebuild-kit-plus-bush.jpg

On the left is a complete front end rebuild kit. On the right is a single replacement bushing I just knew I needed, suspiciously similar to the bushings you can see in the kit. Oh well, it is only a $0.40 part. Curiously many parts do not look exactly like what they replace.

2024-bushes.jpg

The replacement bush is on the left and the chewed up original is on the right. I’m hoping the material is an improvement, not just a lack of original molded rubber.

Here is a replacement crush washer, with the original copper disk still on the damper rod.

2023-crush-washers.jpg

Copper is an excellent material for this (we still use them in super-fancy semiconductor manufacturing equipment) so I am suspicious that the replacement is a downgrade. That original crushed copper washer was a bitch to remove.

And here’s a case of “something’s just not right.”

2022-orifices.jpg

The part on the right is the original orifice, that sandwiches between two other disks at the bottom of the fork. The part on the left appears to be a replacement with a completely different ID and thickness. In this case the original part was in okay condition so I just reused it. I suspect that other disk belongs to a later-year fork. The original fork oil is unusually thing, something like 3 weight according to the repair book. That should be, ah, interesting to locate outside of BMW parts vendors.

I finished rebuilding one fork and forgot to take some more photos. The triple sealing rings of the damper inside the fork are, um, interesting to reassemble given that they are a bit fragile and a spring-fit against the walls (like a piston’s rings). Haynes manual suggested sliding it in using a couple of feeler gauges as ramps (the proper BMW tool is some sort of ramped widget) and that is what I did. We’ll see if I remember to take more pics when I assemble the other side.

The new 13 rib (replica) fork gaitor or boot looks great. But, uh, it is making an airtight seal so it bunches up strangely on compression and sucks in on extension. I get the feeling I need to punch a small air hole near the bottom but holes are entry points for road sludge. What should I do?

Dunno about the bike but now is the time to honor our fallen military men and women. I intend to hoist a few tonight and enjoy our hard-earned liberty.

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6 Comments »

  1. Great blog, keep us posted with your hard work.

    It looks like a great example!

    Comment by Lee — May 29, 2007 @ 5:24 am

  2. 1. IMHO, I think those intake ports should be polished ! Geez, they look awful. Every ring dinger I ever worked on had polished intakes from the factory. I guess some might make a case for better turbulance with a a rough surface, but I am used to having polished intakes.

    2. Your inability to turn the engine over with the kick starter smells like trouble to me ! Since you were able to rotate the flywheel, things are not bound up. Perhaps the kickstart engaging mechanism or gear is bad ?

    3. Now for your amazement: The stylized “SK” on the top of the piston represents “Kolben Schmidt”, a manufacturer of BMW pistons starting at /2 over and going to 5 over. SK does not appear to be used as BMW OEM pistons, that contract is with a company called “Nural”, which also make oversizes. On a blog I used to get this information, (searched SK pistons, BMW pistons, etc) the author describes on earlier BMW’s, R50, R60, the land area above the rings was often nearly as large a diameter as the skirt. This would often lead to siezing and other bad things. Another maker was “Kolben”, but the author did not know much about those. You may already have this fellow bookmarked-he seems to be of the same vein as Kevin Cameron from Cycle World; http://www.w6rec.com/duane/bmw/piston/index.htm.

    4. I am much impressed (IAMI ?) with your little tribute to Stanley Kubrick. That is another one of my very favorite movies in my collection. On the other hand, methinks you might have spent WAY too much time in the garage with your open can of carb cleaner !

    Comment by Brad — July 30, 2007 @ 12:43 pm

  3. I’m working on my ’71 /5 front forks and bought the same rebuild kit (from Eurotech?). I’m trying to figure out where the white bushing (shown above) goes…
    Chris at Eurotech thought it might go under the spring. However, I found no evidence of the original bushing.

    Could it have disintegrated? It would seem to be a rough life under the spring (with no washer?).

    Can anybody help?

    Comment by Keith — July 23, 2008 @ 4:10 pm

  4. If you go to Page 5 of the blog ( https://penforhire.wordpress.com/page/5 ) and look at my “Back in the Saddle” post you can see the stack of parts ( “fork guts” ) laid out just as they came off.

    Maybe yours got left out in prior service?

    Comment by penforhire — July 24, 2008 @ 9:55 am

  5. Found your link while searching the web for an answer to a question: Is it OK to powder coat an R75 series engine? Wanted to powder coat silver instead of polishing. Thanks

    Comment by Kevin Quinn — September 28, 2008 @ 5:42 pm

  6. Hello Kevin,

    Good question. I don’t know the proper answer but I imagine powder coating would work fine. You would need to be careful not to get the coating into threads of in between mating surfaces.

    I have seen examples that were polished to a bright finish and I seem to recall a painted example. very non-original of course. My airhead guru tried some clear anodizing to preserve the original surface but the finish is slightly tinted and the tint varies.

    My point of view is the natural finish lasts long enough if you bead blast it to start with a good-looking surface and keep it clean.

    Comment by Penforhire — September 29, 2008 @ 9:54 am


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