Howdy BMWMOA Forum,
I was hoping to get some recommendations, warnings, advice, etc. on a (hopefully) simple transmission job that I am planning on doing in the coming weeks/months. I got a great priced ($200) 4-speed gearbox off of Ebay; the owner alleges it shifts great and was taken out because a non-kickstart transmission was installed to accommodate a side car. Only (aesthetic) flaw is a welding job done to one of the ears that holds the clutch control arm. See pics to get an idea of the weld job. I can take some more if that is needed.
Got the transmission, and attempted to drop in the stock clutch pushrod assembly into the Ebay transmission, my piston (part #23 21 1 230 107) does not fit into the mouth of the channel on the transmission cover, at least not smoothly and with no signs that the piston will actually be able to slide along the channel with grease and with the rest of the assembly.
I checked to make sure that my piston was the right diameter following a great article from largiader.com. My piston is the appropriate 23mm diameter for 4-speed /5 gearboxes vs. the 23.5mm diameter (part # 23 13 1 232 087) for five-speed gearboxes going up to '80.
So my thought process is this:
1) Replace the gearbox cover with one that has not been welded, with the hope that an unaltered cover will accomodate the piston,
2) (Only after some positive reinforcement from the forum) Attempt to widen the first few lengths of the channel where there is interference with the piston.
I feel more comfortable with the first option, but only because I think changing the cover on the transmission should be pretty straight forward. I'm not planning on gettin' up in the guts of the transmission or anything thing like that. I think I can pop the top, marvel at the innards of the gearbox, and then install the new cover with new seals and a new gasket and bolt it back up. Is it naive to assume that a cover change is as straightforward as that?
I'm aware that the cover houses some bearings and shims, can those be removed and installed into the new cover without too much trouble? If I keep the orientation of the gearbox in such a way, can I avoid having to realign the transmission gear shafts and internal bits? What would that look like, would I need to work on the gearbox on its end (input shaft pointing towards the ground) or on its side (how it sits when installed on the bike)?
Any suggestions of what to be aware/mindful of when doing this would be very much appreciated or any votes for option 2.
I'm a bit bummed to have to do this work as the gearbox looks great, save for the welding. It even has a bolt installed at the rear of the gearbox cover to secure the kickstarter idler gear shaft, which Snowbum recommends for 4-speed gearboxes on his website.
So as always thanks for your help, I really do appreciate the awesome braintrust that is the BMWMOA community. I tend to really get into the weeds and read all I can on these projects, but when that fails it's nice to know that I can bug a few people here who know this stuff inside and out, aspirational goals of a 24-year old first time BMW owner.