Restoring a 1973 BMW R75/5 Motorcycle

February 4, 2007

Gifts that go!

Filed under: 1973, airhead, BMW motorcycle, motorcycle restoration, R75/5 — Penforhire @ 12:30 pm

I called Joe today when I was ready to remove the engine. I was down to two cross bolts through engine and frame. I said, “hey, I don’t remember you calling me to help lift your engine out or in when you did your last rebuild. Did you use a cherry picker or what?” The center stand location make it a bit awkward to jack the engine up so I used two jacks, fore and aft.

1805-no-engine-frame.jpg

While he didn’t use these exact words, the message amounted to “be a man, just bust a nut and do the deadlift.” I eyeballed the engine for a while, lifting one end and then the other to get a feel for it. I wanted to avoid any excitement like having it half-in half-out of the frame, hands trapped somewhere. I was still suspicious but it turned out to weigh maybe 70 pounds (I’m guessing, don’t comment that it weighs 20 lbs because I’m trying to brag here), not too much for me to wrestle out of the frame by myself.

The title of this post is an advertising slogan I recall Honda used around the holiday season to promote their dirt bikes, especially for kids. “Give a gift that goes!” Well that’s the jingle that came to mind when I saw this on my bench.

1804-engine-on-bench.jpg

There is still a massive amount of work to do, stripping down the engine, since I want to refinish the outer case (even if the crank and cam are fine). I decided to start on the ignition points, just below the stuck rotor on the front of the engine. Here is what one of the points surfaces looks like. Check out the size of that arc-formed nugget!

1807-points-nugget.jpg

While I do plan to add an electronic ignition I still want to have a working set of points as a back-up system. The most common electronic retrofit, the Dyna III, still uses the orginal mechanical spark advance.

1808-mechanical-advance.jpg

Two precision-weighted arms fly-out against springs to deliver a spark advance dependent on RPM. The concept sounds reasonable though there are some ignition retrofits that respond to RPM electronically. I’m a fan of full-electronic but I’ll let you know which way I go. Any sticking of the mechanical action would be bad hysteresis of the advance curve. This particular advance seemed a bit sticky to me so I disassembled it.

1809-exploded-advance.jpg

Pretty easy to see how it operates. I don’t know if you notice in the photo but, someone used a permanent marker to cross off the last digit of the stamped P/N on the plate and marked it 10 instead of 7. Any clue what’s going on there? Did someone update or backdate this part?

I didn’t take a picture of it but there’s also a leather (leather?) lubrication pad to hold & distribute special grease on the points cam. Is that really the best material for the job? What is this, an old church pipe organ?

Now I’m staring at that darn stuck rotor, right at eye level on my bench! My local Kragen auto parts store has a tool loaner program so maybe next weekend I’ll borrow their slide hammer and give it a go, not really needing a $100 slide hammer for anything else in the garage. Or the front end (front wheel, forks, steering head) will get removed. Lots of choices left to bang on.

Today the Super Bowl awaits. Time to go clean up and see if Julie bought some chips.

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5 Comments »

  1. I really like your blog. I have a 60/7 that is not quite ready for restoration and you are helping me avoid it :). I pulled out an old magazine (in German) detailing a r60/7 restoration and they used ultrasonic cleaning for the engine case. It sounds more exotic than it is and you might be able to find someone local that can do it at a reasonable cost. The outcome are parts that look like they did the day they were made, it shakes the dirt and crud off. Bead blasting can change the characteristics and appearance of the metal not to mention the dimensions. A case of beer might get some sympathy from your local machine shop next time you need to turn down a socket wall, or get some expert welding. I don’t know how far you are from Detroit but we have every small tool and die shop on ten mile road to do anything ever needed. A lot of these guys are bikers and will take mercy on you. Good luck!

    Comment by chuck carman — February 6, 2007 @ 8:52 pm

  2. Hmm, I’ll try it. I have a small industrial ultrasonic cleaner on loan from Joe. I can probably fit one of the smallest cast pieces in there to try. I wonder what is the best solution? I can start with soapy water but I’ll bet there is something better…

    Thanks!

    Comment by penforhire — February 7, 2007 @ 8:27 am

  3. My points advance plate is identical to yours with the number crossed off in pen and everything.

    In 73 they were a number of changes, SWB vs LWB and I know BMW modified old parts to new design specs (my 73 LWB is just a SWB with a spacer welded in the swingarm.

    My guess is that a different spark advance curve was needed for either US market or maybe even SMOG requirements. So BMW changed the curve on a bunch of old advance units before the bikes were assembled. Another possible answer to the mystery is that it was a dealer retrofit kit in response to a service bulletin due to excess detonation in some climates \ fuel octane supply issues.

    As far as the engine case, personally when I pull my engine, I am going for the polished Alumnium look. Not stock, but its damn flashy in a good way. Maybe not the fins, but at least the engine case and tranny. Still haven’t found a good way to keep the shine for a long period of time, but there has got to be a clear coat, heat resistant paint \ coating out there. Or else maybe a anodized surface would look good.

    I am running a transistorized point system. It basically uses a micro current through the points to fire my bosch blue coils. It was a $15 build it yourself kit and so far, it’s working great. I did change plugs to compensate for the hotter spark.

    Good luck!

    Comment by Toby — February 8, 2007 @ 5:18 pm

  4. Oh and the slide hammer is not a great choice. The Haynes or the Workshop manual shows you how to remove the rotor with just the right length metric bolt and maybe a washer or 2.

    The bolt threads through the rotor and bottoms out, so all you have to do is go easy (dont bugger the threads). Tighten a quarter turn and wait a minute or two. And repeat. Liquid wrench also helps.

    Comment by Toby — February 8, 2007 @ 5:23 pm

  5. Dude,
    You’ll wanna remove that centerstand anyways to clean it up and repaint it along with the frame. Leave it off till very last when reassembling. Set up the chassis with the wheels on and the bike upright in a suitable secure wheelstand.
    Then take the swivel cup off of cone of your trolly jacks and affix a piece of plywood approx. 4″ X 10″ to the end of the jack using a carriage bolt (rounded dome shaped head). Set the jack under the frame and jack up the plywood platform till it is near level with the lower frame tuibes. Using the cylinder studs as grab handles (you are familiar with this by now :), set the engine, with oil pan freshly installed and tightened, onto the plywood. Now use the jack to raise/lower/move the engine block till the front through-bolt lines up. Insert the through-bolt and put the spacers in. Now raise/lower the jack until the rear through-bolt lines up. Easy as pie, nes pas?
    For cleaning the engine, if the crank and cam bearings are good, which they usually are, being way over engineered, just use chunks of foam (like from an old couch cushion) to block the holes and get some Simple Green Foaming BBQ and Grill Cleaner. This stuff is not only environmentally friendly, but it cleans away all the soaked in oil and road crud withoput etching the aluminum. It will still require lots of elbow grease and you must use BRASS wire brushes but the effect is a nice clean, uniformly colored aluminum surface. I do not care for it to look new. If I want new, I’ll buy new. besides, then you have to try to KEEP it looking new. I used an assortment of brass wire tools. Included in my collection were three sizes of standard wire brushes, a toothbrush sized one (went through three of those from the Dollar Store), a full size one, and a full size one that I cut the end off of. Also used two of the ones you put in an electric drill, one radial one and one cup type,. I also found some baby bottle brush types at an industrial supply store. I bought one that was about 3/4″ in diameter by 4″ long. the other one was about 2″ in diameter and 4″ long. They both had 36″ long twisted wire handles which I cut off so I could use them in my drill. I went back for another one, about 1/2″ in diameter and cut the handle off it as well. That one was superior for cleaning between the fins on the heads and cylinders. Used a cordless drill and lots of Simple Green. Cleaning between the fins was the second hardest part. Cleaning the carbon from the combustion chambers and the piston tops was the toughest. I used hot tap water to heat the parts, applied the foaming cleaner, waited for ten minutes, then rinsed and used the brass radial wire brush. Repeated many times to get them clean but there was no damage to the surfaces. For the really stubborn stuff around the exhaust valve seats, I used a small stainless steel radial brush in my Dremel, taking care to stay away from the aluminum and brush just the valves and seats.
    As for ignition, I went with the Boyer Microtronic (red control box). Set it to advance at 4K rpm and it has given no problems, runs sweet right through the power band and I have all my stock bits saved as on-the-road emergency spares. In fact, where I could, I left the stock components in place.

    Comment by Brent Schapansky — February 8, 2007 @ 7:15 pm


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