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Rebuilding a /2 Boxer Transmission

Kees van der Heiden –
(original publish date unknown)


/2 Transmission Rebuild

When I bought my R69, the latest thing I thought about was the transmission. BMW transmissions are sturdy devices, ain’t they? Well, that’s true, but my bike is 37 years old, and what I concluded from various other parts, not very well treated. So I decided to rebuild it. Some of these hints are also useful for the post ’69 four speed gearboxes, because they are almost the same. Don’t be afraid, all this work isn’t anymore difficult than engine work for instance. The BMW workshop manual has a good chapter about how the box is assembled/disassembled. This is meant as an extension to the manual.

Before you start, clean out your workshop.

Opening the transmission is very well described in the BMW workshop manual. The only culprit is, they assume the use of some special tools. Because I haven’t those tools, I had to improvise.

First the output flange nut. It is a castilated nut. Only way is to make a castilated socket. You can use a piece of round steel, drill a hole in the middle, and with a hacksaw and a selection of files, or a grinder, cut out the four edges. Or you can use an old socket of the right size and cut out the four edges. Anyway, keep the remaining four teeth a tight fit in the nut. It is better to make the teeth slightly too big and hammer the socket into the nut.

Now you need something to keep the output flange from turning while you undo the nut. I had a steel plate of 10 mm thick, with a hole in the middle that is slightly bigger than your castilated socket, and two 10 mm holes on both sides to screw it onto the flange. A piece of heavy angle iron is also perfect.

The flange is pressed onto the output shaft and has a tapered fit. It is very tight. I made a nice tool, just like the BMW special tool on a lath, but it can be made simpler. Get the previous tool, weld an M20 nut over the big hole. Maybe it is better to use all four M10x1 bolts to attach your tool to the output flange, so drill two extra holes. You can’t use the original M10x1 bolts, because they are too short. The tool can’t go flat on the flange, so you must put some washers in between. Now protect the output shaft with a slice of bronze or aluminum, and insert a bold in the M20 nut. Tighten it, hit the bolt with a large hammer, tighten, hit, and off it comes.

That was part one. Now remove all the nuts of the transmission cover, and the two bolts under the airfilter housing. And remove the clutch actuating mechanism inside the input shaft. If you hadn’t already drained the transmission oil, DO it NOW. Heat the transmission in your oven, on your electrical cooking plate, under the infrared lamps, whatever you like, but don’t use a torch. The whole transmission should reach a temperature of 100 degrees C. Now you can remove the cover with a wooden drift on the protuberances of the cover. Take it easy, keep the cover parallel to the main casing and if the output shaft is binding in the cover, tap it back with a drift against the outer bearing races. This prevents damage to the selector forks. Tap the input shaft back on the shaft itself, because it’s bearing will remain in the cover.

You can look inside now, beautifull isn’t it? Mark the selector forks, the bushes inside them and the locking plate. Not only which one belongs where, but also how they are orientated. Pull the selector forks out of the selector plate, if they hadn’t fallen out yet, and remove the three shafts as a set. Watch out that the forks don’t hook somewhere. If the casing is warm enough you can wriggle the shafts out, otherwise jam the output shaft in a vice with padded jaws, and hit the casing with a rubber mallet. Watch out, don’t drop anything on the floor.

Now it’s time to study the amount of wear. Worst are usually the selector forks and their adjacent gear locking plates. The prongs of the selector forks have droppled shaped projections. Usually their sides are worn flat. If this isn’t too much, and there isn’t too much difference between the two prongs, you could use them again. Otherwise you can have them welded up and reground to shape by some specialists. For example, Uli’s Motorradladen, Frankfurt, Germany has them as an exchange part in stock. Check out the specialists. The gear locking plates have holes in their sides that fit into the projections of the gearwheels. These holes can be pretty much worn out, especially on the plate between firth and fourth gear. Again, you should decide on your own what is still acceptable. New plates are not so expensive. If the gearwheels themselves are heavily pitted, this will cause a lot of whining noises, and I advice you to look around for another gearbox, because new gearwheels are hard to find and very expensive.

Another thing to look at are the oil sealing faces on the in and output shaft. Of course you use new simmer rings, but the place where they run on the shaft should be spotless smooth. You can restore a pitted, scratched or rusted output shaft with a Speedi-sleeve from your bearing supplier. But the input shaft has a diameter of 29 mm. There is no such Speedi-sleeve. So I turned the shaft back to 28 mm, polished it and installed an undersize oil seal. You can buy bearings and oil seals from a bearing supplier, or from one of the vintage BMW specialists. The latter can also supply you a complete gasket and oil seal kit. This one also contains the special clutch actuating rod seals.

In my BMW parts book of 1963, special C3 bearings are listed. (C3 bearings have a little bit more play). But the specialists only supply standard bearings. I don’t know what is wise. Does anybody have practical experience?

The output flange is a tapered fit on the output shaft. If the flange has come loose some time in the past, it will be chewed up quite a bit. If it isn’t too bad, you can grind the flange and the shaft together with some grinding paste, until everything is nice and smooth again.

Disassembly of the output shaft is very well described in the BMW manual. When you disassemble everything, make careful notes about what belongs where, how was it orientated, which size of washers belong where, and on which side are they chamfered.

For the input shaft you need again a special tool. The shock absorber is secured with a wire clip. You must compress the spring in order to remove the clip. You could compress it in a large vice, but that isn’t particularly safe. Better is to get a length of M8 stud, a tube with an inside diameter of 26 mm, and two “inspection” gates through which you can release the wire clip with a small screwdriver. Put the stud inside the shaft, the tube over it and with some nuts and washers you can tighten the spring and release the clip. Now you can replace the bearing and maybe restore the oil seal flange.

Replacing the bearing near first gear on the intermediate shaft is also difficult, because it is a tight fit, and is close to the gear wheel, with which you should be very carefull not to damage it. I had some steel wedges in the workshop and a large vice, wich worked like a charm. To improvise, chissels, large screwdrivers, bank hammers, etc spring to mind. As long as it is wedge shaped. Avoid any pressure on the gearwheel tooth.

When you have everything up to spec, now it is assembly time. Again the assembly of the gear selection mechanism is very well described in the workshop manual, with nice descriptive pictures. Also install the neutral switch with a bit of silicone gasket putty. You can check the working of the switch with the transmission cover back in place and a dentists mirror through one of the holes in the cover.

When you replaced or revised your selector forks, selector plate or gear locking plates, you should adjust the selector forks. For this you must install the completely assembled outputshaft in the main casing. You must secure the other end of the shaft with BMW tool nr 540. Again this I improvised from 4 mm aluminium sheet, with a 47 mm hole cut out with a hole saw. Then I put the old bearing in the transmission cover, and used this as a template to get the position of the two 6mm holes that secures it.

Put the gear selection mechanism in neutral, and install the third-fourth gear selector fork. Check if the gear locking plate is “exactely” in the middle with the dentists mirror. If not remove the fork and bent the prongs a bit and reinstall it. Check again and repeat until it is in the middle. Now you should check if both prongs are parallel. Switch between 3rd and 4th gear up and down. Both should go equally easy. If one goes harsher it is because the locking plate is tilted and binds on the shaft. Because my “BMW tool 540” is a bit sloppy, I could see very well which prong had to be bend a bit. Do the same procedure with the other fork. To bend the forks, I clamped them in the padded vice, and used a large 24 mm ring spanner as a lever to bend them. But with this you scratch the forks. It is better to make a tool that spreads the load over as much surface as possible. Don’t use a hammer!

Now remove the output shaft. Install the input shaft. Then install the intermediate shaft, the output shaft and the selector forks as a set. For installing and removing shafts, you must heat up the casting. Watch out, the empty casing heats up pretty quick.

What now remains to be done is checking the endfloat of the shafts. This endfloat is necessary to prevent binding up of the shafts when the trans- mission gets hot. In the old days an endfloat of 0.2 mm for the outputshaft and 0.2 – 0.4 mm for the other two shafts was specified. But in the late sixties this was decreased to 0.1 mm for all shafts, in an attempt to get a quieter gearchange. When you get it between 0.1 and 0.2 mm you won’t go far off. The most important is the output shaft, so get it here as close to 0.1 mm as possible.

How do you messure the endfloat? If you have a depth gauge, it’s simple. First put the new gasket on the main casting. Then messure the difference in height between the outer bearing race and the face of the main casting. Messure in the transmission cover how deep the recess is, and subtract these two numbers. With the result you can determine which size of shim you need. The input shaft is a bit different. Here you put the bearing in the cover and you have a small ridge on the shaft. Now the documentation becomes a little bit diffuse, because there are two rings in this place. One ring holds the starter-gearwheel-spring in place, the other is a dish shaped ring that comes in different flavors of thickness. In my case I have a gap of 1.5 mm, a dished ring of 0.5 mm, so with a “spring-ring” of 0.8 – 0.9 mm it should be OK. At the other hand, another book let me believe that the “spring-ring” should be beyond the ridge, so the dished ring should be 1.4 mm. I guess on the first option.

If you don’t have a depth gauge (like me), but only a vernier caliper, you must find a steel bar with two really straigth and parallel sides. I could borrow something, but you could try the block end of an engineers square. Check it if is straight and parallel. Now lay it on the face of the casting (with the new gasket in between) next to the bearing and meassure with the depth gauge of your caliper. Subtract the thickness of the steel bar and your messurement, and the result is how much the bearing sticks out of the casting. Doublecheck all your meassurements and calculations, because it is important to get it right.

When you are sure that all components are in place, heat the cover, stick the shims with some grease in the cover and put the cover back. Install the output shaft oil seal and have it protruding about 1 mm. Put the flange back, and the nut and tighten it with 120 kgm if it is the old M14 nut, or 220 kgm if it is the newer M16 nut. If you made your tool from an old socket, you can put it on a big torque wrench. But I made my tool from scratch, so I had to weld a big nut on the back end.

Install the box in the bike and go for a ride (but I must wait until I get a license number). If it ain’t good, start all over again 🙁

/2 Transmission rebuild-Appendix

When I finished the gearbox, I was very proud of course, and planned to install both engine and box in the bike this weekend. I had of course send a detailed “trip report” to the list, full of useful advice for others.

Then I touched the input shaft a bit forcefully and felt it moving inward. After a bit of wiggling, it moved easy in and out for about 1.5 mm. Baffled as I was, I stared at it, and my own words hammered in my head: Double check all your measurements and calculations.

There were two things wrong. First it shouldn’t move in, because there is a bearing that has a tight fit in the case. Second, it shouldn’t be anything more than 0.2 mm.

The first was obvious, because the bearing WASN’T a tight fit. Obviously the box is so far worn out that the bearing is a loose fit. Just for this purpose Loctite’s stud and bearing compound is available. I used this because I didn’t want to buy another case. The other two shafts are happily not so bad. I am only a bit worried about the future, because I suppose the bearing is now attached quite permanent.

But the second point was puzzling. It took a day before I found out what had gone wrong. When I studied my calculations I saw that, for instance, the intermediate shaft bearing was sticking out 12.6 mm, while the recess in the transmission cover was 11.5 mm. I had decided that there was a gap of 12.6 – 11.5 = 1.1 mm, but only now I saw that there was a negative gap.

So bear in mind when you meassure the recess in the transmission cover: The edge of the recess is DEEPER than the edge of the transmission cover.

The funny thing is, the previous person who has rebuild this box, has made exactely the same (among other) mistakes. Only, he didn’t correct them.


/2 Transmission rebuild-Appendix II

In both my postings about the /2 gearbox, I haven’t mentioned anything about the “over play” of the gear selection mechanism. This is the play between the selector plate, and the notch that keeps the gear selected (this is all in the foot-pedal mechanism, and has nothing to do with the shafts), when you press the pedal further than necessary to select the gear.

This play should be something like 2 mm in both directions. When you look at it, while playing with the pedal, it should be obvious where this play should occur. This play is necessary to be sure that the spring loaded notch is always in it’s groove, when you release the pedal. Not somewhere before or after it.

The play is adjusted with washers under two screws. One of these screws is clearly visible when you look inside the tranny, and stops the pedal travel on it’s way upwards, the other is screwed in from the underside and sits next to the drain plug. Of course, in my box, one of the washers was not there, so I had to make it myself. It was the washer on the screw for the upward motion. This screw is secured with a locking tab, that was rather butchered. I meassured that I needed 1 mm, so I made a locking washer that was 1mm thick. But that didn’t work. When you remove the screw, you see a round indent under it, that takes the shim washer. When you put a larger washer here (and the locking washer is larger, other- wise it won’t be functional) and screw the screw tight, that washer is pressed inside the indent and you loose your setup.

So I made a new locking tab of .5 mm and a shim washer that fitted inside the indent (an M6 washer, bored out to 8mm and grinded to the right thickness). This turned out to be correct.

While assembling the gearbox, I found it difficult to direct the 3rd-4th gear selector fork in the right place. If you have the gearbox on your bench with the open end facing upwards, the locking plate on the output shaft falls down into 4th gear. So if you have neutral selected, you must wiggle the plate up on the shaft, while you can’t realy reach it, hold it there, align the tabs on the bush, in the selector fork, with the indents of the locking steel strip, hold the steel strip up, push the other end of the selector fork in the selector plate, and insert the screw. You need at least five hands for this, while there isn’t even room for two very slim hands.

Maybe it’s best to make a rig so you can put the box upside down with the open end facing forward to you, AND you can still insert the screw. Now you can put everything in position, and it won’t fall down immediately. Because I have slim long fingers, I opted for another aproach. With the open end facing upwards, I selected 4th gear. Then I bolted the steel strip loosely to the house with the 1th-2nd selector fork bolt. Then I could align the bush in the 3rd-4th selector fork with the indents in the steel strip and insert the screw, while holding the bush in place with a long screwdriver. Screw in for about one turn. Now I had to reach to both sides of the locking plate on the output shaft and wiggle it up a little bit, and screw the bolt fully home.

The other selector fork is easy, because you have full access. All this work assumes that you haven’t yet installed the kickstarter ratched mechanism on the input shaft, because it gets in the way. If the bolts and nuts are clean, it should be possible to turn them fully home with your fingers. If it doesn’t go, the locking plate is binding on the outputshaft in the wrong place. Unscrew the bolt, and wiggle the plate until correct and try again.

I hope my meanderings are understandable. It will be very vague when you just read it, but with the subject opened and in front of you, everything should be obvious. If not, ask more!

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