New Transmission?

Discussion in 'Airheads' started by pthomas, Jul 5, 2012.

  1. pthomas

    pthomas Been here awhile

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    What to look for when buying a new to me transmission? What fits a 1974 R90/6? I'm aware that the 74 is a bad year, but is there anything to "fix" the problem should I open things up and fix it myself? I've never opened a transmission, so I'll be in unknown territory.

    I was ridding home last night (1974 R90/6) and broke down around 10:00 p.m. I was clearly ready for the trip with nothing more than the clothe on my back -a polo and shorts-, my wallet, and a cellphone running out of charge.

    I was on I-75 when, cruising along when I lost propulsion. RPM didn't change but I was clearly losing speed and more throttle didn't change anything. Shifting down didn't help but I could see that I putting some power to the ground; sadly that was short lived. Being the only tool in sight, I spent the night on top of the bike, and waited for U-Haul to open up and finished the "ride" home this morning.

    I have yet to open things up to diagnose the problem, but my belief is that it's a single failure point at the transmission or beyond (driveshaft). I heard no noise or rattling, so I don't have much to go on but I believe I'm going to be in for a new transmission.

    Thanks for the help.
    #1
  2. Solo Lobo

    Solo Lobo airhead or nothing

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    Sounds like a clutch to me...

    That said, if it is a trans I would always spend the extra $$ on having my trans rebuilt by on ok of the known experts (not just anyone who says they can do it) as opposed to buying a used and unknown one.

    You may pay $400 for a replacement or (if you get lucky) $600 for a rebuild of yours
    #2
  3. Wirespokes

    Wirespokes Beemerholics Anonymous

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    I agree - sounds like a clutch issue to me. Perhaps the clutch got oil soaked - they don't normally go south that quickly.

    When transmissions go they tend to make lots of bad noises.
    #3
  4. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Ran out of gas?

    Don't do that. It's dangerous.
    #4
  5. disston

    disston ShadeTreeExpert

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    Another vote for this is not a transmission problem. True there are problems with the 1974 transmission but this current problem does not seem to be one of them.

    Now that you are home can you start the bike? (good call Bill)

    But I also think clutch. So sitting on the bike with the engine running can you put it in gear and what happens when you let the clutch lever out?

    Look on the rear of the trans for the clutch throw out arm. The clutch cable goes to it. And when the cable pulls the arm pushes in on the throw out parts under the small rubber boot. Is the throw out arm working? Sometimes the pin holding it in place can fall out.

    I hope you wear a helmet. And no sandals, no slip on shoes. Boots preferred. And, and, and....Oh never mind.
    #5
  6. pthomas

    pthomas Been here awhile

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    I normally wear more gear... and have more in the saddlebags; the one time I'm not ready and I get burned.

    The engine still still has gas... I already know that feels like :deal

    The engine still run and I thought it might be a clutch issue but don't think it is. There is slippage in the drivetrain but pulling the clutch in removes it all. I would think that a clutch failure would mean that I can't disengage the driveline, not that it all of the sudden disengage. Does anyone know the failure mode on these?

    The more I've been thinking about it, I agree that it should not be a transmission issue but something else in the driveline... investigatory work will tell the story.
    #6
  7. Airhead Wrangler

    Airhead Wrangler Long timer

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    Sounds like maybe you popped the taper loose on your trans output flange. Still a little friction to give you a little go, but nowhere near a solid mechanical connection. If you spun that taper for any length of time you'll need to pull it apart and see if it's even salvageable.
    #7
  8. mark1305

    mark1305 Old Enough To Know Better

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    Sounds like you are confusing the two failure modes for the clutch.

    Your stated presumption that clutch failure would stop you from disengaging the driveline would in reality mean a failure in the clutch cable, lever, or the other lever at the transmission. That would keep the driveline from disengaging.

    The other failure mode would be as described by several folks above - the clutch has started slipping and no longer transmits power through the drivetrain to the rear wheel.

    You stated that you were able to downshift (without results) and that it seemed some power still made it to the ground. But you also stated that pulling in the clutch removes all the slippage????? Do you mean that pulling in the clutch removes all driving force (as it should) and thus the slippage goes away along with the driving force? Or that pulling in the clutch transmits full driving force to the rear wheel?
    #8
  9. DoktorT

    DoktorT BigBrowedNeandereer

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    I think you must pull the tranny to do an inspection of the several clutch parts that can have wear that explains your symptom. If the clutch parts meet specs, then look for the output flange problem. If that's the culprit, I would look for a 76 through 79 tranny for replacement. With a good clutch and updated tranny, you will be good to go for a loooong time. Bite the bullet, fix it right once and be done with it.
    #9
  10. wirewrkr

    wirewrkr the thread-killer

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    actually, 1970-through 1980 all interchange
    I prefer to find transmissions from R65 bikes. They don't seem to be pounded inside as hard as R100 Transmissions. They're all the same fitment wise in the proper year range.
    .
    I
    #10
  11. pthomas

    pthomas Been here awhile

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    Pulling the clutch removes all driving force.

    I'll read up on the clutch and takes things apart; I didn't have time to do it this weekend as my car also broke down this week and I need it for work... it was one of those weeks.

    The good news is that this will force me to take the bike apart and learn more about it, do a top on the engine, and see what else needs to be done. It will move me further along the restoration I want to do on this bike but don't have the funds for.
    #11
  12. disston

    disston ShadeTreeExpert

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    It is a good way to learn Airheads to learn how to repair it as you go along. Then when it's working you ride. Then when it's broken you repair. After a few years you will know more about what you are up against as far as this "restoration" goes. And you should have the bike working properly by then.

    So here's part of the deal. Right now we are going to repair what is broken right now. It seems there is a problem with the clutch or (have we eliminated this one?) the output taper. If you haven't done this yet I think you should first take off the swing arm and with the drive shaft out of the way see if the universal flange is firmly attached to the rear of the trans, to the output taper. If it is it is time to remove trans and look at clutch. Taking the swing arm off is part of removing the trans so you are proceeding in a logical manner.

    If top end repair is what you mean by, "do a top on the engine"? Then don't do this. It is another part of the engine, it is at the other end of the bike, it has nothing to do with the current problem. Fix the current problem. If you want, after this is done, and have any money left, then we'll go on to other repairs needed.

    Since I brought it up maybe I should mention that any of the systems on these bikes is not cheap to repair. Transmissions can go over a thousand dollars, a clutch can easily cost five hundred, or more. I think you should stick with one thing at a time.

    Charlie
    #12
  13. mark1305

    mark1305 Old Enough To Know Better

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    pthomas, peruse this thread I wrote a while back. It should give you some insight into the inner marvels of the BMW 5 speed boxes. Also check the links in which I credit some other folks with lots of valuable info that helped me.

    I picked up an Ebay box to replace my shot transmission and rebuilt it myself. Still works great.
    http://www.advrider.com/forums/showthread.php?t=435511
    #13
  14. pthomas

    pthomas Been here awhile

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    Reading your thread is making me think I need to farm out that work; I've feel okay doing an engine but the trans scares me for some reason. Worst case, I open it up and find out I need to send it away because I can't do it.

    I'll be working on the bike this weekend. I've been putting long days at the office all week but I should have a free weekend :clap
    #14
  15. pthomas

    pthomas Been here awhile

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    Given the amount of metal out of the transmission, it needs to be opened :cry
    #15
  16. JonnyCash

    JonnyCash turd polisher

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    I've got a good one, I listed it in the airhead flea market. Check it out!
    #16
  17. pthomas

    pthomas Been here awhile

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    I not yet ready to pull the trigger on a used transmission; mine only had 30k from the factory. I'd like to open it up and survey the damage, even if it's just to learn before I shop for one.

    The transmission is ready to come out, when the swing arm gets out of the way... I don't have a 27mm thin wall socket and didn't want to a good one to the grinder, if I had a lathe it would have been in a different story. Sears doesn't carry thin wall sockets so I'm not sure where to grab one that will fit locally. I might have to order the BMW Airhead Swingarm Tool from Northwood.
    #17
  18. disston

    disston ShadeTreeExpert

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    There is a wrench in the tool kit that will work on the swing arm lock nuts. It is the tool intended for this work. The BMW part # for this tool is;

    71 11 1 237 857

    The thin wall socket is a regular 12 point socket turned down to meet the size limit of this application. The tool kit tool will work if you have that tool in your tool kit. It should be in your tool kit but then individual tools are often missing. The problem with the tool kit tool and the reason why there has appeared the after market socket is that people over tighten the lock nut and the tool kit tool gets broken. It also has a short handle and people have a hard time using it.

    I have the socket but almost always do this job with the tool kit tool. I'm explain how I do this. I use a small hammer to hit the wrench so that I can make it work. I don't hit it hard but hard enough to loosen the nut after several whacks. This is the Red Neck impact wrench. I haven't broken a tool kit 27 mm wrench yet.

    Here is a page from Max BMW that shows the tool kit tool. It is #19 on this page. You may already have this tool.


    http://www.maxbmwmotorcycles.com/fiche/DiagramsMain.aspx?vid=51889&rnd=05012012
    #18
  19. pthomas

    pthomas Been here awhile

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    I don't have a BMW tool kit but I'm familiar with the Red Neck impact wrench; I didn't have much when I grew up and had to improvise regularly. Ordering a socket is cheaper than the official tool and will take less space when on the road so that's what I will do.

    The delay in getting the transmission out will give me a chance to read up on what broken or damage things I'm looking for on the clutch side of things.
    #19
  20. disston

    disston ShadeTreeExpert

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    That is an expensive tool, I agree. The after market socket is cheaper.

    To recap; You have a running BMW motorcycle that does not transmit power to the rear wheel? You are going to remove the transmission and check the clutch?

    Have you looked at the transmission output flange yet? This is under the boot at the junction of the trans to the drive shaft, at the swing arm.

    Part of removing the trans is to disconnect the drive shaft and remove the swing arm with the drive shaft. The flange that the drive shaft bolts to is a taper fit to the output of the trans. This taper is held onto the trans by a large nut. If this large nut is loose the taper has spun and is loose. It will be easier to see when the trans is out. It is still a possible source of your problem, if I have correctly summarized what is going on.
    #20