Topic: BMW R100/7 Transmission Question

[millla03]

Recently rebuilt my 1978 BMWs transmission. Sat with water in the oil before I got it, resulting in some rusted parts and bearings that were starting to go.

All new bearings and typical replacement parts (oil seals, circlips, etc), and also an OEM 4th gear and reproduction 15 degree helical gear on input shaft (I think made in Switzerland?).

Shimmed using shim plate and depth mic. Input shaft could be turned by fingers with some resistance and output shaft turned freely after assembly.

Just back on the road, about 5 miles on it, and there is an audible whine while under load, goes away during coasting or deceleration, and when clutch is pulled in. Shifts and goes down the road fine. If anyone is familiar with these, is this noise normal for new gears wearing in or did I mess something up? I'd think a bearing noise would persist even while coasting in gear, so it might be gear noise, but this is just a guess. Im fine pulling it back apart, but looking for opinions before doing so. And I know, I should just ride the 750 and be done with this German money pit : )

Thanks in advance.

[Terry in Australia]

Ha ha, the worst part about an old 1970's (or 80's, or 90's) BMW gearbox is that you can rebuild it with the best parts, and it's still an agricultural piece of crap compared to a jap bike of the same era. Even the gearboxes in my K100RS or my R1100GS are like a tractors box when compared to a CB750. I can understand it whining under load and not when you back off and coast along, but maybe heavier oil might be enough to quieten it before you pull it apart again? Cheers, Terry.

[millla03]

Thanks for the reply Terry. I agree the old BMWs are like a tractor compared to the Japanese stuff, but they are cool for what they are. The amount of speciality tools you have to make or buy just to work on them is stupid. Makes you appreciate the Hondas design.

I asked one of the suppliers I bought parts from and they didnt seem too concerned. Rode it to work today and it shifts better than it used to. Hasnt grenaded on me yet. Just gonna ride it and keep an eye on things for now.

[simon#42]

i think you may have shimmed it too tight , or you changed the bearings and didnt re shim . you may well get away with it but it will cause the bearings to wear prematurely  .
use a hypoid gear oil they dont realy like light modern oil .

[millla03]

Simon,

Bearings were shimmed. Used a shim plate from Cycleworks and a good quality depth mic. Measured everything several times to verify too. Its possible I made a mistake, but I did everything within my means to avoid it. Any other methods of checking endplay besides the plate method? The Clymer suggests measuring the outer race against the rear housing surface, which seems wildly inaccurate.

Still not planning on plucking the transmission out right away at this point, but would like to be prepared when i do.

Using 80w/90 gear oil, same as before the rebuild.

[Terry in Australia]

Thanks for the reply Terry. I agree the old BMWs are like a tractor compared to the Japanese stuff, but they are cool for what they are. The amount of speciality tools you have to make or buy just to work on them is stupid. Makes you appreciate the Hondas design.

I asked one of the suppliers I bought parts from and they didnt seem too concerned. Rode it to work today and it shifts better than it used to. Hasnt grenaded on me yet. Just gonna ride it and keep an eye on things for now.

No worries mate, don't get me wrong, I love my R1100GS and K100RS, but I jump off them and hop on to a Jap bike and it's like chalk and cheese. Cheers, Terry.

[millla03]

The saga continues....Simon i think you may be right about improper shimming. Asked a couple more shops, and while they couldnt diagnose it over the phone, they did say that the noise is not expected after a rebuild and it could be bearing noise from improper shimming. I dont trust it now and am going to pull the trans to be safe before something goes. Luckily I have another new set of bearings on hand. Will update when i find out what i did wrong.

Appreciate the help.

[simon#42]

luke , the way you have done it is correct but i used to do loads of these for a local dealer . in the end i found it much faster and more accurate to use plastigauge .
the down side is you have to take it apart to get your measurement the plus side you get it spot on
did you check the size of the plate from cycleworks , i have two and they are different !

[millla03]

Yep I measured the plate with a mic to use in my calculations. 0.750" right on the money. Thanks for the tip on the plastigauge. I've seen guys use solder wire, but hadn't considered that. It'll be good to have another way to check the end float. Any trouble with it melting from the heated cover when it is installed?

It's also possible the cover is warped or something, and the measurements I'm taking from the bearing area are different than the actual gasket surface. That's one thing I neglected to check. Plastigauge should account for that. Another thought I had is that I didn't torque the bolts holding the plate (with a new gasket under it) down to spec, just snug. Maybe the gasket compressed a bit more at normal torque and that tightened things up. I think it's possible given the tiny 0.1 mm clearance you're shooting for. Careless mistakes, in hindsight.

Rode it some more, about 100 miles total, and the noise is still present. Getting harder to blame new gears wearing in. Been a learning experience, for sure. I now see why a lot of people send them off to be rebuilt, but where's the fun in that? I'll get it figured out in the end.

[simon#42]

heat should be ok , i dont use much i just stick it in front of a space heater for a few minutes . i have never come across a warped endplate but am sure such a thing does exist , you would have to be unlucky
to have one and im sure it would leak bad if it was . the easiest shaft to check is the input shaft , the front bearing is a roller and you should be able to feel that 0.1 mm with your hand when you move it in and out .
look on the bright side , when you have worked out a method to do them properly you can do them for all your friends !

[millla03]

Just putting some closure on this topic. Installed new bearings and redid the shimming. Ended up with about .080" difference in countershaft bearing height, other two shimmed out about the same. Suspect one of the bearings on countershaft wasnt fully seated on the shaft, causing preload on the bearings. Triple checked everything this time, and input shaft also turned easily by hand after assembly. Thanks again for the help!

[Goofaroo]

Just putting some closure on this topic. Installed new bearings and redid the shimming. Ended up with about .080" difference in countershaft bearing height, other two shimmed out about the same. Suspect one of the bearings on countershaft wasnt fully seated on the shaft, causing preload on the bearings. Triple checked everything this time, and input shaft also turned easily by hand after assembly. Thanks again for the help!

I rebuilt the tranny on my 78 R100S several years ago and it was a real pain doing all the measuring, math, and shimming. In the end, it was worth the effort.  I hesitate to say that it shifts just like a jap bike but it’s pretty close. I’ve had a few airhead guys ride it and one commented that it shifted as easily as his CR125.  I’m not sure about that but it’s the best shifting airhead I’ve ever ridden.

Kudos on tackling the rebuild.  I know how much effort goes into it.