1987 K75S Gearbox Rebuild Due to Inability to Downshift

[gone_ape]

Motobins.co.uk sells full sets of gearbox seals as well as individuals.... They ship to the US and are fast, reasonable and freight isn't too bad.
They don't have 'every' part BMW, but peruse their site and you may want to add something else to your order....
https://www.motobins.co.uk/bmw-parts.php?model=K%20Series&bikeref=K100

[jay1622]

Motobins.co.uk sells full sets of gearbox seals as well as individuals....

Good call, and much less expensive. I spoke with them this morning, and it appears that they're using new-old-stock, i.e. the part that was superseded by the current run. While I couldn't find any of the current runs, there's two old ones on eBay. Same dimensions withstanding, any thoughts of using a replaced run?

[gone_ape]

Good question, that.  I installed a MotoBins gearbox seal set a little over 2 years ago and it has been perfect at every seal. 


There's the school of thought that "a seal is a seal is a seal" and therefore one can find them at a bearing supply house etc.  Then there's the school of "you must use this type seal because of the type of fluid it's holding back cuz it needs this kind of rubber or that kind of polymer." The obvious question being: Which numbers are stamped on the original seal to cross reference to an aftermarket.  Fooey.  If this bike were a race machine or some high end unobtainable exotic, then I would worry.  But it's not, so I don't.  I feel Motobins has done the legwork to provide seals that work....good enough for me.


That being said, I guess it really comes down to the individual....can you get defective new parts?  Just have a look at the "Monkey nutz orientation" post in this section....only, in this case, it's a pita to replace a "new-failed" trans seal !!!

[jay1622]

Successfully rebuilt and tested...

That's what it was! It was that little (1/4 size of a tic-tac) steel pin that sets the shifting plate to the shifting roll. My only symptom was my inability to downshift. Smooth as butter now, and better than ever (fresh Staburags on all splines/SAF-XO in the box). To reiterate... The shifting plate is steel. The setting pin is steel. The shifting roll, which is aluminum is much softer (little research quickly reveals too soft in the early years), and the force of the pin eventually created an oval shape in it's (originally circular) recess within the roll. In an up-shifting condition, the shifting roll would properly align and the gearbox would shift like normal. In a down-shifting condition, the shifting roll wouldn't properly align; thus, not setting the shifting forks where they needed to be to properly set the individual gears to switch over.

[Martin]

It's always good to see a positive resolution. :2thumbup: :clap:
Regards Martin.

[jay1622]

You guys, and this forum, have helped me quite a bit with this Brick... A lot of my posts are just situation reports, but I hope someone with similar issues (this one for instance) stumbles upon these threads and benefits from them.

[jay1622]

First test-run of the gearbox under power...  And yes, that's without any part of the exhaust.

[Keven1100]

Hi guys that the only forurm that i found that...i would like to know where exactly goes those 3metal shim in the picture.

Im asking because i open my k100 transmission and i dont remmenber where those goes ..

Thanks you

[jay1622]

Hello. I hope my reply helps you, but you’re not going to like what I say...

Those shims are for one, the other, or both of the bearing pockets circled in the picture below on the case cover of the gearbox.

Based on your post, I am going to assume you do not recall which shim(s) went where. Unfortunately, the exact placement of those shims is very important since each may have different thicknesses. You need to do your best to compare (think ballistic forensic reconstruction) what metal surfaces matched up with what part/shim. Look at the bearing, shims and pockets, and do your best to see what was originally where. You want the tolerances to be the same as they were before.

Anybody else have any recommendations short of using the BMW clearance tool?