'72 BMW R75/5 4-speed Gearbox Cover Change - What to Expect?

[jrussakis]

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.

Cheers,
Josh

 

[20774]

Josh -

Personally, I wouldn't worry about the welded on ear. If it's done well and not cracked, and it aligns with the other ear, it's just going to get dirty and no one will really notice it. Maybe the rider will though! But really, I wouldn't worry.

As for getting the pushrod in, it should be put in from the front. Put the felt around the rod at the right spot, put some lube on it, and begin pushing in from the front. I think trying this from the back is not going to work.

And IMO, I don't think one can just pop the back cover and put a new gasket and put the cover back on. If you're going that far, you might as well check things out. And there's no guarantee that the box was properly shimmed initially.

My sig line has a link to some resources...there are several links to websites that deal with transmission overhauls and shimming.

 

[happy wanderer]

Wondering why a kickstart gearbox would be a problem for a sidecar rig.
- I've never seen a BMW with anything other than a LEFT side kickstarter.
- I've never seen a BMW with anything other than a RIGHT side sidecar.

 

[jrussakis]

Josh -

Personally, I wouldn't worry about the welded on ear. If it's done well and not cracked, and it aligns with the other ear, it's just going to get dirty and no one will really notice it. Maybe the rider will though! But really, I wouldn't worry.

As for getting the pushrod in, it should be put in from the front. Put the felt around the rod at the right spot, put some lube on it, and begin pushing in from the front. I think trying this from the back is not going to work.

And IMO, I don't think one can just pop the back cover and put a new gasket and put the cover back on. If you're going that far, you might as well check things out. And there's no guarantee that the box was properly shimmed initially.

My sig line has a link to some resources...there are several links to websites that deal with transmission overhauls and shimming.

Thanks for the advice, yeah the ear doesn't bother me and I totally agree, I'd like to keep the cover on if at all possible. What I am concerned about is that the piston does not move in that hole. In the picture above the piston is in there, but that took some tapping to get it in that far and the thing was stuck to the point that getting it out required some pliers and a bit of heat around the hole. I was under the impression that those things should slide pretty easily through that channel to activate and deactivate the pushrod.

But it seems like, and please correct me if I'm drawing too much from your previous post, you would recommend option 2 in that I try to get that channel wide enough where there is interference to facilitate the movement of the piston rather than replacing the cover.

Thanks!

P.S. I appreciate the link recommendations, I will definitely give those a look.

 

[jrussakis]

Wondering why a kickstart gearbox would be a problem for a sidecar rig.
- I've never seen a BMW with anything other than a LEFT side kickstarter.
- I've never seen a BMW with anything other than a RIGHT side sidecar.

hahaha a very good question when you put it that way. I'm glad to have those two observations in my memory; I know nothing about sidecar placement on BMWs.

The description of the transmission was this verbatim:

"This Transmission Came Out Of a 1964 R60 That A 1971 R 75/5 Engine Installed For More Power, An Electric Start Transmission Was Installed Because A Royal Side Car Was Added.The Original R 75/5 Trans Was Removed in Great Working Condition.Once Removed A small Hairline Crack Was Found And Repaired.The Crack Was At The Rear Of The Trans Where The Clutch Rod Mechanism Pivots,One Of The Two Ears."

It's definitely not the most intelligible string of sentences and clauses ever written, but I think I interpreted it correctly. Is a "Royal side car" a brand of sidecar? One that is installed on the left side of a motorcycle perhaps? All guesses and no answers on my end.

Best,
J

 

[20774]

I guess I wasn't thinking clearly...I thought you were trying to install the long pushrod, not just the items at the rear. The pushrod is to be installed from the front. The other items do get installed from the rear.

When installed, the larger OD goes in to be flush with the opening in the transmission. In operation, it doesn't move that far...maybe 1/4 to 1/2 an inch at most.

All those items should easily install from the rear. If they don't and you're using the right/good parts for the transmission, then something is wrong with the transmission. It's possible that due to the welding, the normally circular opening in the rear of the transmission is not oval due to the heat from the welding. I would look into that.

If you have to physically open up that hole, then the piston and other items will be too small for the opening. Not to mention if you do some grinding, you're likely to get swarf that may get into the transmission.

 

[jrussakis]

All those items should easily install from the rear. If they don't and you're using the right/good parts for the transmission, then something is wrong with the transmission. It's possible that due to the welding, the normally circular opening in the rear of the transmission is not oval due to the heat from the welding. I would look into that.

If you have to physically open up that hole, then the piston and other items will be too small for the opening. Not to mention if you do some grinding, you're likely to get swarf that may get into the transmission.

No worries at all Kurt,

Knowing me, I was probably so intent on clearly explaining my problem that I probably didn't explain it at all - typical .

That was my thought that during the weld job the opening was distorted. It would have been nice to know that before buying from the seller on (fl)E(a)bay, but I guess this keeps things interesting and ensures that I'll have had my hands in every part of this bike. So there's that.

Thanks for clearly putting all my initial hesitations for manually opening that hole into words. I would not be 100% sure that I could get that grinding material out of the opening.

So with all that said, it seems like the proper course of action is to get a new (relatively speaking) cover and swap it for the old deformed one.

I've already begun looking at the transmission articles in your profile, very helpful and they will guide me in that process. I figure while I'm at it I will add new seals to the cover and maybe, if I'm feeling really brave check for axial play in the shafts and determine correct shim thickness. But even writing that sentence gave me a sweat...

 

[20774]

But even writing that sentence gave me a sweat...

Nice! At least you're going in with eyes wide open!!

 

[lkchris]

So, if that transmission "came out of a 1964 R60" it's a /2 transmission designed to mate to a different engine than you have in your /5. If it's /5 parts you're trying to fit, this may explain why they don't.

 

[James.A]

The description of the transmission was this verbatim:

"This Transmission Came Out Of a 1964 R60 That A 1971 R 75/5 Engine Installed For More Power, An Electric Start Transmission Was Installed Because A Royal Side Car Was Added.The Original R 75/5 Trans Was Removed in Great Working Condition.Once Removed A small Hairline Crack Was Found And Repaired.The Crack Was At The Rear Of The Trans Where The Clutch Rod Mechanism Pivots,One Of The Two Ears."

That is definitely a 4speed transmission from a /5. A /2 transmission would have the base of a round air filter formed in the top of the casting.

The quoted description is all wrong. A /5 has electric start also. The transmission(s) have nothing to do with electric starters. The electric starters engage the flywheel. Your E-bay seller was either ignorant, or attempting to practice a deception with that description. I am not convinced that a sidecar set up has anything to do with any given transmission. Trans in great working condition,maybe? The heat applied to the cover by welding has distorted cavity where the piston does its work.

That being said, if it were my bike, I would try to lightly hone the cavity where the clutch throw out piston is installed in the back cover. You want the throw out piston to slide easily in the bore without a tendency to bind up.

 

[fxray]

. . . 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.

To answer your question, yes, it would be naive to assume that a cover change is as straightforward as that! The special tools you would need to do the job would quickly exceed what you paid for that ebay transmission (and probably exceed double what you paid). You cannot do the cover change without those tools. I would venture to say that a lot of people could not do it successfully even with the tools at hand. Even though you don't plan to dive into the guts of the gearbox, changing the cover would mean measuring and re-shimming the end play on the shafts. For some idea of the tooling involved, you could watch this video. It is the first of a six part series that is pretty good.

Ray

 

[jrussakis]

Hello and sorry all for the delayed appreciation of all your advice and recommendations. After some closer studying of the issue, I was able to determine that two burs left over from the weld job were to blame for the tight fit on the piston. I packed the opening with shop towels, used some small files to remove the burs and used a powerful shop vac to remove the leftover shavings. I'm happy to say that the piston moves in the chamber a whole hell of a lot better than before.

Thanks Fxray for the video link, I've bookmarked it and I think that procedure will be a great project worth taking on this winter. As it stands I'd actually like to ride my first airhead before the season is over . sigh, a lad can dream...

P.S. I realize this might be a PM matter/one for an entirely different thread on the forum, but I was wondering if anyone in the airhead/bmwmoa community had done business with a Ted W. at bmwboxermoto@gmail.com through Ibmwr.org? If so wondering what your experience was. Thanks in advance!

Best,
Josh