Restoring a 1973 BMW R75/5 Motorcycle

June 17, 2007

T’aint easy being a monkey

Filed under: /5, 1973, airhead, BMW motorcycle, motorcycle, motorcycle restoration, R75, R75/5 — Penforhire @ 12:52 pm

Did you see the loose wheel guts stack in my last post? BMW Joe calls me up, “Eh, you know, that left side bearing is backwards.” The two conical roller cages are supposed to face each other. Now somehow I muffed that up for the photo but got it correct for the next photo, showing the preload stack. Otherwise the left race would have had zero pressure on it and would have fallen off to roll away and hide on the floor. So I sort of know I got the final assembly right. I think. Joe knows someone who installed one backward and, while it worked for a while, his wheel fell apart on the road.

Hey, where were the rest of you eagle-eyes? You’re supposed to post a snide comment when you see me drop the ball like that! I can always delete it if I’m THAT embarassed.

I got my speedometer back from Palo Alto Speedometer.

2070-restored-speedometer.jpg

They fixed the broken tach mechanism and needle, replaced the faded dial and colored lights, either cleaned or replaced the glass (looks like new), calibrated, and put on a new crimp-bezel and gasket. Turn around was very fast, maybe four days. They are known as a good but expensive shop and this was a $350 bill with shipping (ouch). I think you can save some money with other shops (search on the 5 United board) but PA Speedo goes way back in my memory and Palo Alto is one of my favorite California cities (I went to High School next door in Redwood City).

I got to working on my front end some more. There is an article on alignment by Duane and he points to a more comprehensive article by Randy Glass (Google for it). The BMW specification for fork parallelism is 4 mils. But it seems many instances of funky handling have been cured by adjusting to near zero tolerance, removing slight binding. I read the articles and was a bit intimidated by a process that could take a week (!) and requires that sort of precision. Oh well. I stripped off the fork lower legs (and gaitors) I had assembled before I read about the process. Hey, at least the forks weren’t already mounted in the tree.

First thing I noticed after pulling one lower leg off was —

2071-where-is-my-crush-washer.jpg

Where’s my crush washer? I remember assembling with new aluminum crush washers at the fork bottom seals. This one maybe fell off when I took it apart but I couldn’t find it. The alternative is too stupid to contemplate. Off to NAPA Auto Parts, who have expensive copper washers but not in that precise size. Close enough. Put crush washer assortment on the list of things to look for (like o-rings, snap-rings, washers, lock washers, e-clips, cotter pins…).

Apparently fork alignment is affected by almost all the various tightenings at the top plate of the triple tree. That is a bit of a surprise to me. I figured it didn’t matter how you twist and pull on top of that flat plate but it seemes the top plate is a bit thin (about 0.2″) and soft by design. Most of the proposed adjustments involve cold flow of that plate, twisting the legs whichever way and keeping the pressure on overnight by tying some 2×4’s in place. They say alignment is suspect any time you release the forks completely from just about any part of the triple tree. I’ve also seen a really beefy-looking machined-billet replacement plate somewhere. It does not look at all original but if you want improvement you may want one of those. Someone else suggested stacking two OEM plates but there was no consensus on whether that would be good or bad (and your handle bar clamps would need longer studs).

Now BMW Joe says just feeling for non-binding motion of the fully assembled forks with the springs removed is good enough and the lower fork brace can be used to make small adjustments. One repair manual mentions another technique with the springs in (completely assembled). If you have a partner to help then you can sit on the bike, bounce the front end down and see where it comes to rest, then compare to lifting the front and seeing where it rests. The two positions should be no more than 1/2 to 3/4 inch from each other. The difference is stiction and alignment is the assumed culprit. I decided to give the most complex tests a go since I could always just give up and monkey my way through.

The first axis test uses a glass plate.

2072-glass-plate-test.jpg

Sorry for the crappy focus. If the legs are not parallel front-to-back then this glass plate will rest on only three corners and “tap” at the fourth corner, depending on how you press on it. I was very surprised to find virtually perfect alignment in this axis. At both the top and bottom of the legs I could not get any tapping feel at any corner. I ran this same sort of test on the lower fork brace’s mounting surfaces. Could not detect any warp there. So far so good.

My next test got a bit Rube Goldberg. You are supposed to slide a dial indicator between the legs to check the gap at top, middle, and bottom of the forks. I even have such a dial indicator and a magnetic stand (thanks to Harbor Freight!). But my stand’s post was too long to fit between the legs. I tried a bunch of pretzel positions with a second arm but nothing got me a consistent reading. There is, of course, a proper BMW tool and a few aftermarket fixtures (e.g. Ed Korn) but not yet in my toolbox. So —

2073-gap-check.jpg

I was able to get consistent readings with a micrometer by looking for the minimum value while holding one end in place and swinging the other up-and-down (something harder to do with a digital indicator). The challenge is not to use too much side pressure. They say even the pressure of a typical dial indicator is enough to splay the legs wider by a mil or more when measuring near the unsupported bottom. So this caliper technique is not recommended. Anyway, I figure my forks were out of parallel in this axis by about 3 mils.

I decided to assemble the rest of it to try the no-springs sliding-feel test. On go the lower legs and gaitors. This is something more of a hassle doing it on the bike than it was on the bench. You have to put some goop on the crush washers to hold them in place while putting the lower legs on. And oddly enough, the bottom seal crush washer nuts don’t really want to torque up properly. Thinking about the assembly, there isn’t anything to keep the damper rods from spinning, and the bottom nuts turn on those rods. I believe friction between the crush washer, where it seats on the rod, and the fork bottom cap is supposed to be the locking force. Probably a bad idea to use a little oil or grease to hold the washers in place then, eh? But that is exactly what the manuals said to do. In a horizonal bench assembly no goop was required. Grrr.

That’s not such a big deal. I figure I’ll fix it if it leaks. But it is at this point that the monkey strikes again. See, I’m putting the front axle in position. In a perfect world it would just slide through the holes in both legs with almost no resistance. In my world the first thing I notice is, I think, the axle is sliding through from the wrong side.

2074-wrong-side-fork-leg.jpg

The larger clamping hole is on the right leg. I double check the manuals. Darn! That leg is supposed to be on the other side. But, as you can see in my picture I even marked the lower fork leg with an R for Right Leg. Hmm, somehow I mis-marked the legs as I took them off. T’aint easy being a monkey. Off come the lower legs, re-goop the crush washers, back on in the correct positions, re-seal the lower crush washers… This whole thing just passed the “unhappy” mark on my personal frustration meter. I didn’t want to take the upper legs off and bench-assemble because I measured such nice alignment. By the way, not having the engine in place yet, the whole frame is rickety on the center stand and constantly wants to tip over or dive forward. Grrr. Sitting here now, I’m thinking I could have laid the whole frame on its side and gotten a horizontal assembly.

Okay, at this point I feel the action of the complete no-springs front end. As near as I can tell I don’t feel any more resistance than the fresh oiled fork seals, which do have significant sliding friction but I felt them individually before assembly. As far as I’m concerned this is mission accomplished regarding alignment. If I’m not happy with the front end later I’ll take it to someone less simian than me.

Now I might as well clean up a prior monkey-mess so I take off the rear wheel and flip the brake shoes upside-down. Well what-do-you-know, the technique of snapping the shoes in place with springs already loaded worked okay, much easier than stretching the springs after positioning the shoes. And now the rear brake seems to work in the right direction. Yee-haw (oo-oo, gimme a banana?). I torqued the rear axle nut to the book spec and I’m sort of hoping that’s the last time I remove the rear axle for a while. It was only the fourth or fifth time.

Back to the front forks. Finally time to put oil in.

2075-fork-oil.jpg

Some fine tuning of the suspension can be accomplished using different oils or weights. According to the members at 5 United not everyone’s 5 weight is really the same viscosity! This Bel-Ray oil is middle-of-the-road and I have to start somewhere. I’ve got a cheesy plastic 100 ml graduated cylinder so I’m thinking I’m ready to measure out 280 ml of oil into each leg. So as I go to dump the first 100 ml into the leg I discover my first words of good advice on this topic. Pour slower than I did.

I got an overflow and spilled an unspecified amount of oil on the floor. I guestimated the amount and finished the job. It would not have been more accurate to drain the leg and start over since 280 ml is only correct for a dry assembly, not a drain-and-fill with wetted components. I read somewhere about a dipstick technique to fine tune damping by adjusting the air pocket based strictly on fluid height in the leg but I just go ahead and slap the top caps on with new top crush seals.

A tip I read somewhere else came in handy. The pin-drive top cap was easy to torque using a needle-nose pliers in the pin holes, turned by a channel-lock wrench holding the pliers near the tips. At least it was easier than using the pin wrench in the BMW tool kit.

By the way, I am using new Progressive front springs and they require a different spacer under the top cap than the metal BMW part. Progressive provides the correct length of PVC and washers to fit between the PVC and the spring. I read some people don’t like the lifespan of PVC spacers but I think they’re comparing number of decades of hard use. We’ll see. Not everyone likes Progressive’s springs. Some prefer BMW’s Heavy Duty optional springs. All I have to compare the ride with is my fuzzy memories of how much of a death-trap this machine always felt like. So it can’t come out worse than my memories. Unless I crash.

I got to mounting the rear sub-frame and shocks. I went with Progressive shocks and springs (pre-assembled for me by Bob’s BMW). I did not buy the optional chrome vanity caps. That was just one step too much form-over-function for my wallet. I was hoping to locate some of the correct length Koni shocks, the hot-ticket upgrade back when (and rebuildable), but I kept losing e-bay bids. A modern alternative, Ikon, supposedly built to similar specs, was in scarce supply where I shopped so I went with Progressive. They are a relatively inexpensive alternative with only spring preload adjustment. The box comes with four different sizes of metal bushings (and two sizes of large washers). The original BMW shock bushings are metal-in-rubber. I’m assuming this means shock motion will be more exact but perhaps more vibration will also be communicated? Anyway, same as for original bushings, add a little moly grease when you assemble. I’m a fan of Honda Moly 60 grease but it sure gets everywhere. Be careful what you touch!

Here’s the state of the union —

2076-assembly-so-far.jpg

I’m thinking I’m not going to send anything out for chroming now. Several of the important parts cleaned up better than expected with Nev-R-Dull polishing cloth. The rust on the chrome sort of bloomed over the surface from relatively small pits. I can still see the small pits and some parts may rust again but I’m down to needing just a few improvements. For instance I have a new front hubcap on order since the old one was banged up in addition to rusted. We’ll see about the mirrors.

I still haven’t sent out my heads, cylinders, and pistons yet. Bench Mark Works does not perform head or cylinder machine shop work themselves. They recommended two different shops for those jobs and I’m still checking out my options. I’ll post up more info on machine shops after I decide on mine.

Hey, I also just joined the Airhead Owner’s Club. They’ve got a monthly newsletter, Airmail, that might be useful to me, with tech tips and airhead advertisers. I’m not sure my pile of parts is an airhead yet. Well, my valid license plate should be enough even though I can’t really play in their gatherings yet. I can pretend and make vroom-vroom sounds, right?

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2 Comments »

  1. I found it easier to put the crush washers in the fork legs prior to putting the legs back on and centering them with a small screwdriver, viewed from below, to ensure that the end of the damper passes through them. The end of the damper rod should be held in place with an allen key when the retaining nut is fitted, to prevent the damper from turning.

    Comment by Richard — June 20, 2007 @ 1:55 pm

  2. Ah! Thank you for mentioning the allen key. I should have known that but one side tightened so well without it.

    Comment by penforhire — June 20, 2007 @ 2:05 pm


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