Restoring a 1973 BMW R75/5 Motorcycle

June 24, 2007

Let’s go see the wise man

Filed under: 1973, airhead, BMW motorcycle, motorcycle restoration, R75, R75/5 — Penforhire @ 8:44 pm

I did some sanding work inside my front wheel hub to smooth out the damage that prevented me from loading the guts last time. Measurements indicate something like an 8 mil interference fit at room temperature. Not a lot. So if you find yourself in my shoes be careful not to remove too much material. I reloaded everything while hot.

2077-front-wheel-hub.jpg

This time around I remembered a tool I had squirreled away.

2078-ir-thermometer.jpg

I originally bought this to help me tune tiny two-stroke motors on small scale remote-control cars. Those run on a mixture of alcohol and nitromethane, similar to top fuel dragsters. The engine output is amazing for their size but they can be very fussy to tune for the right fuel-air mixture. Measuring temperature is one indicator of right or wrong. This is not one of the most accurate infrared thermometers around but it should be close enough (looking for 220-ish degrees F) and much better than just watching water sizzle.

Anyway, I get everything in the front hub loaded but the back side still seems wrong. I added some arrows to show you a gap between the bearing and end of the hub that I think should not exist. I didnt pay enough attention when I disassembled it. Looking at the photo, the roller bearing can fall to the right, away from its race, maybe 1/8th inch until it hits the reduced opening for the grease seal. The repair manuals just talk about forcing the races all the way in. I’m missing a couple of parts, due next week from Hammersley, to finish assembly. It is possible the outer “top hat” spacer, held by the grease seal, presses this bearing in place properly. But I’m not sure.

2081-hub-backside-gap.jpg

My opinion is something is just not right. So I go up the hill to see the wise man, BMW Joe in my case. He doesn’t recall if this looks right or not. Same question for the other side , which is slightly proud of the hub surface. So Joe pulls a spare front wheel out of his parts. He has about a complete bike’s worth of parts in the rafters above his garage. Alas, it is a later spoked wheel, with disc brake, and I can tell the hub stack is different at a glance. We also can’t find his repair manual but that would not have mattered. The exploded view detail on mine are both insufficient to even tell which way the races should be loaded. I made a reasonable effort to seat everything.

I’ll wait to see how it looks later with the complete assembly. But I have the distinct feeling I’m going to be unloading the hub and reloading it bit-by-bit, which is really annoying when the whole hub is hot enough to boil water. Just tapping the bearing with a socket-and-hammer results in a hot-potato socket!

Hey it was Father’s Day a week ago and you know what that means, right? Yep, a good tool sale at Sears. I blame Alex for this one. He mentioned some ratcheting wrenches are handy to have.

2080-gearwrench.jpg

A starter set of metric Gearwrenches seemed like a good idea. They come is a wide variety but all have a 5 degree ratchet angle so they work in tight spaces. You can see the one in my photo has a pivot at the head so the arm can angle (but no reverse lever, you just flip the tool to reverse). The other end of this set is a plain open-end. They also make ’em in non-pivot, locking pivot, reverse lever, double-box-end, S-curve, half moon, … Seeming endless combinations. Well, after wrenching a bit with mine I can confidently say I’m hooked. Gosh these are nice to have. Now I’m spoiled. I need more. Curse you Alex! If you play with some of these, don’t say I didn’t warn you.

You may recall quite a while back I had to put a plug in my FJR’s back tire. Well its been several thousand miles and it still loses maybe 4 PSI overnight. I ordered a replacement Metzeler Z6 from my favorite tire supplier, SW Moto (swmototires.com). Normally I just take my FJR to the dealer to get tires changed and balanced but all this BMW wrenching makes me feel silly for not pulling my own rear off. I’ve also got the universal cones for Marc Parnes’ balancer. I’ve even got a how-to article from one of the FJR owners’ websites. So I go to remove my FJR’s rear wheel and I discover that I’m not the only monkey in town.

The wheel nut is on with at least 50% higher torque than the book value. Seems a professional ape at my local Yamaha dealer must have used an impact wrench to tighten it. I can get it to twist a little with my body weight on a breaker bar but the axle twists the same amount in the pinch clamp on the other side. Grrr. Time for my own impact wrench. I tried the one that Brad gave me (thanks Brad!) but it was too wimpy. I took this as an excuse to buy a serious impact wrench because, well, they’re intensely cool tools. Brad’s wrench is a 3/8″ drive with about 50 Ft-Lb. It’s a Black & Decker that looks like a private-label Chicago Pneumatic (design is too exactly the same and B&D doesn’t make air-drive wrenches any more). So I got a 1/2″ drive Ingersoll-Rand 231 Classic, pretty much the standard workhorse model of the last several decades. Nothing so fancy or expensive as their titanium-body models but it has an indestructable reputation. It claims about 400 Ft-Lb of peak torque and, sure enough, it made short work of loosening this axle nut.

2083-fjr-wheel.jpg

The FJR wheel is a bit more complicated than the R75’s. The disc brake and the ABS sensor wheel make for more ‘stuff’ hanging off the wheel. As near as I can tell the bearings are quite sealed. But the Yamaha shop manual does say to lube them. I’m just not sure how.

I got a chuckle out of the comparison to the R75’s rear tire.

2084-tire-comparison.jpg

Is that a monster tire or what?! Well, that machine has about triple the horsepower and a bit more weight so I guess it deserves a fat tire. Sure makes the R75 tire look like it belongs on a bicycle. I’ll get the tire changed during the week. I know, some of you are yelling at me to whip out the tire irons and do it myself. But not only am I not up to changing tubeless tires (breaking and seating the bead) but Metzeler changed this particular tire construction from “B” to “C” Spec just because it was so stiff too many mechanics were damaging rims! So it is still awfully stiff. I’m paying someone with a machine and some no-mar levers.

Back to the R75. I got around to painting the headlight bucket and tail light housing. I scuffed the entire surface with 600 grit sandpaper, masked a few things on the bucket with tape, and wiped it down with MEK solvent (methyl ethyl ketone). Krylon spray primer for rusty surfaces went on just fine.

2085-priming-the-bucket.jpg

Here’s the tail light housing, whose black paint was falling off the plastic badly.

2087-priming-tail-light-housing.jpg

The next stage, glossy Krylon epoxy spray, was a bit trickier. Following a tip from the 5 United board, I painted the inside of the bucket white.

2090-inside-bucket.jpg

That is supposed to make electrical wiring easier to see later. The outsides were all gloss black. Gary mentioned his experience with Krylon glossy spray bottles was difficulty between laying down a thick enough coating to get a gloss finish versus too much paint causing runs. Sure enough he was right. I got a decent result but there is at least one visible run. It is not perfect but is okay.

2091-bucket-outer.jpg

The surface is not as liquid-smooth as I would like. While I masked the “ears” I did not mask the various rivet heads. Too difficult to get a clean mask at their bases. So this bike loses concours originality points for that as well.

I decided I am going to buy a new seat. I think repro seats are about $300. You could say I’m just giving up because I can’t get the hinge screws out but that’s only part of the motivation. The more I worked on it the more I noticed issues with the seat covering and underlying foam. It would have needed more work. That’s what I’m telling myself anyway. I did recover the other hinge by grinding down and chiseling the bolt heads.

2086-recovering-the-hinge.jpg

None of this work is especially easy!

It has been long enough since painting so I got to waxing the tank and fenders with a carnuba wax. None of the carnuba waxes I found seemed as hard to work with as the old waxes my dad used to have so I went with Mother’s. I guess everyone mixes it with more dispersants and ‘vehicle’ these days, probably less actual carnuba. As long as I was waxing the tank I might as well assemble its bits.

The emblems just screw on (gently) with the original rubber backing pieces. The gas cap has a single pin holding it in place. You just have to careful which side of the cap the splines jam into (the larger opening). I also had to file away some paint at the hinge in the tank to insert the pin freely.

The rubber knee pads were a bit more of a hassle. I had soaked the outer faces with a surface restoring fluid similar to Armor All, “Formula 2000” if I recall correctly. They were not worn but were a bit faded and hard. I left ’em soaking-wet for months now, just the front sides, and that seems to have restored them nicely. These panels are glued to the tank. If you put oily stuff like this on the back sides it will not adhere well to the tank.

Most of the adhesive recommendations I found suggested automotive weatherstrip adhesive. This stuff seems like industrial strength rubber cement. I applied it liberally to the panels. You have to be careful near the edges. Too little and the edges will flop around. Too much and it will ooze out and be a pain to clean up. The difficulty is the tank is curved, the panels are stiff, and the glue takes many minutes to set.

I started out clamping the edges like this —

2088-clamping-tank-panel.jpg

But that doesn’t work too well. The tank is sloped in every direction and the clamps tend to slide off. I got better clamping from some bungee cords.

2089-bungee-tank-panel.jpg

I had to get more inventive to apply force to different edges and I had to add more adhesive to several flopping edges.

I decided on a shop for my engine work and sent them the cylinders, pistons, and heads. Motor Works in National City CA just south of San Diego (motorworkspromach.com) gets a thumbs up from several 5 United members. They also advertise flat rates for various work in the Airhead Club newsletter. They are not a BMW shop or even a motorcycle shop! They are a machine shop that specializes in European engines. Somewhere along the way they must have figured out how to do airheads right.

You may recall, Bob’s BMW recommended Bench Mark Works. And Bench Mark Works does not so their own machining. They recommended Bore Tech in Batavia OH (bore-tech.com) for cylinder work and Memphis Motor Werks in Cordova TN (memphismotorwerks.com) for head work. I went with Motor Works because in addition to /5 owner recommendations and reasonable pricing, I can drive down there if I have to.

So here’s the current state of the rear end —

2092-state-of-the-rear-end.jpg

Although I wanted to keep the painted fender out of harms way for as long as possible, too many things bolt up to it like the battery holder, tail light, and license plate. I did not have to remove the rear wheel to mount the fender but it would have been easier if I did. You can see I’ve also got the saddle bag mounts attached.

I’m at the point where stuff takes way to long to figure out because I didn’t take exhaustive photos or video when I tore everything down. Plus the hardware is mixed up. For instance I thought I had enough views of the headlight bucket guts (more than I showed you) but I’m wrong. The various lamp holders that fit into the speedometer are not identical. Three or four use a metal-sleeve body to make ground connection, with a single lug for power. Three other lamp holders are plastic-bodied with two lugs (power and ground). Two of the three plastic bodies are same-size as the metal bodied sockets. And all of those have grounded metal sleeves. None of my photos give a clear indication of which goes where. The electrical diagrams and exploded views in the manuals are no help. I’m thinking all the bulb holders might have originally been metal-bodied single-lugs because of all the matching metal sockets. You might recall the odd splicing I saw in the bucket before. Heck, I don’t even know which bulb color means what (though I’m sure I can find that out). Geez the wiring is going to be awful to do…

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2 Comments »

  1. I googled, “r75 wheel hub” and arrived at your blog. I have a 1973 R75/5 also. I like your bike. I would love to do the same to mine someday. I in the midst of discovering my front bearings are shot. I discovered it when re-installing the front tire after taking it to get a new tire put on. I did not realize how difficult reinstalling it was. I drove it around for about 200 miles then had a hunch I should kickstand it upp on a wood block to see if the tire rotated easily. KKKKscrapemetalgrind! Took of wheel and realized the cast alum drum plate was somehow sucked to tight to the hub and was grinding alumimum to aluminum. The BMW shop guy has never seen anything like it. He said to use a spacer (AKA washer to create more seperation. Then I got curious and removed the chrome hub cap and discovered the outer (leftside) bearing race was loose inside the hub. Crap.

    thanks for reading this far if you have. Any idea on where to find a good wheel/hub/bearing assembly to just slap a tire on and install it.

    Any comments would be great. I will check in on your project. I signed up to your blog.

    Kind Regards,

    Patrick K. Seattle Washington

    Comment by Patrick K — June 27, 2008 @ 4:41 pm

  2. Hello Patrick! I don’t have a cheap source for, well, just about anything! You can tell from my expense list. Those wheel parts sure are expensive. Maybe try the BMW owner’s boards, like 5 United?

    My project is pretty much done. I posted up my first >300 mile day on the R75/5 in my Jag resto blog along with other bike bits. I’ll probably keep up tidbits in that blog.

    But feel free to visit and mine this blog for all it’s worth!

    Comment by penforhire — June 27, 2008 @ 6:45 pm


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