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  02.02.07 BMW R100 GS PD Transmission Problems & Solutions Feb 2007

Transmission First Look

The transmission the first time I took it apart.


After a harrowing ride I made it to Hampi. Cecilia had called two days earlier to tell me that she was having technical problems with the bike. Well, after trying to diagnose the problem per SMS and telephone, it was decided that, in either case, it made sense to head over to where she was staying to have a look.

First a few words about the ride. The best way to get anywhere in India is by getting an early start, and that is exactly what I did. The ride was really great at least up to the border to Karnataka, where the road took a major turn for the worse. Anyone who has been on this stretch of road will never forget it. Ankle deep red powder, broken up by huge rocks, and boulders. Of course for a change of pace, there are plenty of very deep potholes. All of this is actually quite tolerable if it wasn't for the constant heavy truck traffic, with heavy dump trucks, three abreast going in one direction and one coming in the other. Your visibility is down to 5 feet. The rest of the ride was the usual Indian experience. Crowded and confusing towns, suicidal drivers, and plenty of interesting vistas.

Hampi is a fascinating place, not only the temples and ruins, but also the fact that it is a major tourist and pilgrim center. Check out the pics in the main picture area.

Ok, back to the transmission story. In Hampi I test rode the bike to see if I could definitively figure out what the problem was. The vibration and noise pointed to the drive shaft. The vibration was erratic, and a clanking noise was coming from the general vicinity of the upper u-joint of the drive shaft. Ok, no problem, I have a spare drive shaft with me so this should be simple. I went and got myself a paper, while Cecilia dismantled the swingarm and took out the drive shaft. Well, it turned out that the drive shaft was fine. The final drive was also fine, something we ascertained by turning it manually without anything connected to it. Since both the u-joints on the drive shaft where fine and the final drive was fine, that left the transmission as the only possible culprit.

So now we had to decide whether to do the work here or ship the bike back to Goa, or try and ride it back. After much debate and contemplating the various options, we decided to head back to Goa and do the work there. There were a number of reasons for this. Mainly, I already had all the resources I would need for something like this, checked out. Including a very good machine shop. Additionally, Hampi is a ways from the nearest town (Hospet) with any mechanics or infrastructure. So we had to put the bike back together anyway, and this being a BMW, the decision to ride the 350+ km wasn't all that hard.

Another day was spent just sightseeing before heading off very early in the morning. This allowed us to get through most of the towns without major problems or traffic and a little over 6 1/2hours later we were back in Benaulim, Goa.

The following day, Cecilia took the transmission out of the bike. The first problem with disassembling the transmission on a BMW is removing the output flange. That is the part that connects to the drive shaft and transfers the energy to the rear wheel. This part is held on by a large nut, which is very tight and requires a special tool to hold the drive shaft connector, while turning the nut. After that, the flange itself needs a puller to remove it from the shaft to which it is connected. So I took the whole transmission to the mechanic where I got the oil changed. He had a look at it, and pulled out a tool which is used to hold the clutch assembly on smaller bikes, and together with his assistant got the main nut off.

He then took a straight puller he had, and welded a cross section on to it so that he could pull the flange off the output shaft. This took a while since the first two attempts didn't hold. The flange was too tight on the shaft, eventually he managed to weld it good enough that it held, and off came the flange.

Now it was up to me. The first thing I had to do is heat the transmission casing to expand the metal so that the bearings at the end of the various shafts would come loose and I could open the transmission. This is normally done with a heat gun, but as you can see by the pictures a normal cooking stove also works fine.

Once I got the cover off the rest is pretty simple. Unfortunately, after looking at all the bearings and shafts, I was not able to determine what the cause was. None of the bearings had any problems, the shafts and gears all looked ok. The only thing was the small output bearing which for some reason had come off in with the casing when I open the transmission (normally the bearing will stay on the shaft). I re-heated the casing and got the bearing out, and sure enough, it was bad. It seemed incredible that the vibrations and noise came from the fact that this bearing was going bad. (It now had at least 500km since the vibrations/noise had started, and still felt serviceable for the most part. I assume that when it finally does go, it would do so quickly). Although we made it this far, I would definitely not recommend riding any further, in fact, if I hadn't been too lazy to find a suitable place near Hampi, and there weren't two of us, I would not have undertaken to ride back to Goa before fixing the problem locally.

With the help of our hosts, who telephoned around, we found a place in Margao who had the required bearing. So after a short trip into town we were back with a standard SKF 6304 bearing. This was too easy! In short order, I had the transmission back together again. I put it back together the same way I took it apart, and without checking the bearing clearances. (There are shims between the bearings and the case). So in a short space of time, I managed to make three major mistakes.

Nevertheless, Cecilia put the transmission back in the bike, and although it was very, very tight (difficult to turn), assured her that it would be fine. It wasn't! A short test ride went well; for the most part. No, noise, no vibration, but as soon as she tried climbing a hill the clutch slipped. Now this is something that never should happen, specially since we had these ultra cool (and expensive), ceramic clutches, which were still (nearly) brand new. Remembering how tight the transmission was, it was clearly the transmission and not the clutch which was the problem. So back to the drawing board.

In the meanwhile I had also gotten confirmation from a friend in Germany that the bearing had to be a "C3" bearings. So the standard SKF 6304 bearings were not enough, they had to be 6304 C3 bearings. The C3 denotes additional internal clearance that the bearings have, allowing them to expand in extreme temperature situations (i.e. transmissions). Our previous sources weren't able to provide any, so I turned to my mechanic, who said no problem. And sure enough the following day came up with the correct bearings, albeit an Indian version of a japanese bearing.

So now after Cecilia had again, disassembled the bike and removed the transmission, I took it apart with the help of the mechanic. Who by now had started to assemble a formidable collection of BMW tools and knowledge. This time he also had to fabricate a bearing puller to get the new (wrong) bearing off the shaft. This time it had come off on the shaft, as opposed to last time when it came of in the casing.

Additionally, after some consultation with the high priest of BMW transmissions, and a good friend of mine, Eric Demant, in Germany. It was decided to shave off a millimeter from the flange in order to compensate for "stretching" of the shaft. Something which took a whole 5 minutes at the local machine shop. While I was at it, they fabricated a number of shims in 0.03, and 0.05mm sizes so that I could re-shim the casing when I put the transmission back together.

So now, with the correct bearing, assembled correctly (with the bearing on the shaft), re-shimming the casing, and the minor modification to the flange, I put the transmission back together. Lo and behold, it turned nicely, made no noise and generally felt good. As far as transmissions can "feel" good.

Cecilia put the whole thing back in the bike and reassembled everything and we went for another ride. This time everything was fine. No noise, no vibration, no slippage, it felt "tight" (as in good). So we were done.

And what where the three errors you may be asking?

1) Re-assembling the transmission the way I got it apart. Since the bearing had come off in the case, I re-assembled it the same way. Wrong. The fact that this required some forcing of the casing onto the the transmission and bearing onto the shaft. With the casing taking most of the strain, I got real lucky that I didn't really screw something up.

2) Use the right bearings. The C3 mark was right there the whole time, I just ignored it. The consensus is that the non C3 bearings will work, for a while, but will go bad much, much sooner, and are basically not worth the effort of putting in even for a short. time.

3) Check and adjust the shims whenever taking the transmission apart. Specially if you have changed any bearings, or gasket. This should be a no-brainer. Well, for other people maybe. The whole process of re-shimming is documented in the BMW shop manual (you do have one don't you!!), and is pretty much straight forward, if a bit time consuming. Not to mention that you will need shims of the 0.03 and 0.05 sizes in order to make it work, and a good set of calipers.






Pictures / References
This article covers an episode which occurred in South India to Cecilia's motorcycle. Which in the end required "just" the replacement of a small bearing. As always, just click on any of the pictures for a larger version.

Here Cecilia is taking the bike apart in Hampi, Karnataka.

Removing a bearing from the transmission casing. Note the cooking stove!

My favorite mechanic in Goa in his shop.

The mechanic fabricating a puller to use for removing the bearing from the shaft.

The tool in action. (It worked)

The casing, note the green "old" style output seal. Also note that the breather hole has been closed. This is to avoid leaking gear oil into the swingarm.

The drive shaft flange before modification.

And after modification.

The transmission just before the final re-assembly. Note the bearing on the shaft, and it's the correct one this time. 6304 C3!!

Heating the transmission before the final assembly. The cooker works great, you can turn it on its side for maximum effect, or hold it in your hand while heating the object in question.

Cecilia, putting the whole thing back together again. For the last time she hopes!!




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