Dunning Kruger: How to almost ruin a gearbox…

Having successfully completed a full rebuild of another monolever BMW, I occasionally get slapped upside the head with what I think I know, vs what I know. I have watched videos, read articles, studied technical manuals and more for all the work I do on these bikes. In all, I’ve done well into the hundreds of hours of learning. All of that is for naught if you don’t actually do the work of each task. It’s one thing to know what needs to be done and another to be able to do it.

Blue is a low mileage bike and mechanically, I am really just doing a few upgrades and making sure it has fresh seals. Superficially, this is all straight forward, since I am not looking too deep into what parts need to be replaced due to wear or improper prior assembly. This, and a bad habit to do work well into the evening when I should be resting can lead to some complacency, or overconfidence.

For the gearbox, I wanted to add the taller fifth gear for some added highway top-end speed, adding the output shaft circlip that BMW began omitting in the mid-80s, and replace the roller in the shift mechanism from the stock plastic version to an actual bearing that will allow for smoother shifting and a longer life. Other than that, it’s just redoing the seals. All simple tasks that are easily done. I don’t have a mill, so the output shaft had to be sent out for the groove, but the rest I can do. No problem. If you pay attention.

The teardown was uneventful. I found exactly what I expected: a perfectly serviceable unit with no signs of wear or anything to worry about. All as expected for <15,000 miles. It was the reassembly that nearly led to disaster. This provided the opportunity for some lessons learned.

1. Double check the parts you receive. Not just the parts you order. In this case, I ordered my taller fifth gear from Siebenrock. I’ve ordered numerous parts from them and they are an excellent vendor. Still, everybody can goof. These gears come in two varieties with the main difference being the angle that the gears are cut at. Older models have a different angle. The difference is small, 15 degrees for older and 17.5 degrees for newer. Not something you would quickly notice. The newer version which I ordered and was listed on the packing slip are marked with an X to indicate the revision. After assembling the shafts and installing them, I discovered some binding when trying to work the gears. It would lock up after a short amount of rotation. This meant removing all the bits, tearing down that shaft again, ordering and installing a correct gear, and putting it all back together. Fun. No, practice. 🙂
2. Don’t reuse any circlip, snap-ring, or other spring steel retainers. When fitting the bearing on the shift mechanism, I reused the retainer. It still felt firm and I was sure it was properly seated. Still, after installing, I noticed the bearing drifting when I was testing the sift action. This let me know that the retainer had fallen off. This was another round of partially disassembling. I ended up replacing all the retainers and springs on the mechanism and reinstalling.
3. Pay attention to the order of assembly. In a rush, I installed the output and intermediate shaft before installing the input shaft bearing. That bearing should be the first thing you put into the case, since it will not install later. Again, tear it down and redo it.

All of these goofs meant additional cycles of heating the case, letting it cool, and other tedium. It meant I spent several hours on tasks that should be done in less than one. I did spend a lot of time measuring for the shims needed on the shafts that aren’t in that calculation.

Shimming is essential to provide the appropriate range of end play for each of the three shafts. It involves using a precision milled measuring plate to determine where the shafts have ended up once assembled and installed. Then, determining the available space for each of the shafts in the gearbox cover. From there, you can calculate how much space needs to be filled with shims to arrive at the correct specification. I did this multiple times.

Partly, I wanted to be sure of my work. Partly, I was finding that I needed different shims than what was in the gearbox from the factory. I expected this for the output shaft which had been fully reassembled, but the input and intermediate shafts were left intact and I expected I would be able to simply use the factory shims. I will assume that in even 15K miles, things can shift enough to change ideal shimming, or the factory uses a different measuring method. There are two main schools of thought on that.

Some measure the depth of each shaft pocket to the top of the pocket itself. This approach is fine if you know your cover is perfectly flat across all edges. If your cover is slightly bowed, then the outer edge of the cover will prevent the pocket from engaging the shafts by a small margin. I used the measuring plate on the cover as well to test the pocket depth based on the outside edge of the cover. This would provide a more accurate measurement if the cover has a slight bow. Mine clearly has this bow.

After triple-checking the measurements and assembling the right combination of shims, I got it all back together correctly. It shifts well and is now ready to reinstall.

My video of some of this work is here.