Boxerworks.com

Is the gearbox chapter in your favourite workshop manual embarrassingly free of oily fingerprints? Mine was until recently. I have ‘had a go’ at everything else my dear old R90 has thrown at me, as the tattered pages and fingerprints in the various manuals will attest.

This is a story about rebuilding a gearbox for my 1974 R90/6, or rather, putting together a usable box out of two non-usable boxes. I’m not a mechanic and do not have a mechanic’s workshop. I am also no expert on gearboxes – I’ve only played with a couple and only very recently. You need a bunch of tools to successfully undertake this job, and a bit of courage if it’s your first time. It is indicated as having a “four wrench” degree of difficulty in the manuals but if you can gather the essential tools together and devise some workarounds it is really quite straightforward, provided you are prepared to be particular about getting things right.

What I was faced with was a broken 1975 gearbox and another working but tired 1974 gearbox. The original 1974 box did not feel good – it felt sloppy, the input shaft was very obviously toast, there was the occasional false neutral, a tendency to slip out of gear and a significant whine in fourth gear. Also, the buggered input shaft splines made the clutch disagreeably ‘grabby’. I bought the ’75 box as a ‘reconditioned’ replacement in 2010 for AU$900. It rattled at idle, a noise that morphed into a sort of gentle grind at higher revs, and I worried that it was going to seize and buck me off one day.

However, it worked and went ok for 40 thousand kilometres but eventually developed a nasty clacking sound. I popped the cover off and found that one of the sliding gears on the layshaft had two dogs broken off but there was only one still in the case. That box had been put back together with a missing dog and sold as a reconditioned box. So it goes. I had a lot of fun riding that 40k kms (that is the circumference of the planet after all) so no problem from my side of the equation. But I needed a box that worked. A kind friend – Mal of Dubbo (Mal S7 here) – delivered a spare box so I could have a good autumn riding season while I pursued options. Thanks Mal, it’s been great, I’ve tested your box for nearly 5000kms and found it good.

Getting down to practicalities – to get the cover off you need a means to undo the big nut on the back of the output shaft that has been torqued down to a brutal 220Nm and a special puller to remove the output flange that is likely to be well stuck on the tapered output shaft under that nut. These tools you can buy, borrow, or if you have the skills, make. Fortunately for me, Jeff of Albury (ME109 here) lives locally, is a skilled welder and had already gone to the trouble of making the required tools to do this first part of the job. He also had a depth micrometer and infra-red thermometer that I was able to borrow. Jeff also had experience, having recently refurbished his 1981 box, and his help was invaluable. Thanks Jeff.

The holding bar and flange puller …



… and the underside of the puller …



If you visit Jörg Hau’s gearbox page http://jhau.maliwi.de/mot/gearbox.html you’ll note that he suggests using a length of angle section mild steel to make a simple holding jig. Since I don’t have much metal at hand I made one out of a length of hardwood and it worked just fine. With this bolted on, the case can be clamped vertically or horizontally depending on which task you are doing.





Ok, first thing is to get that big nut off the output shaft. Since I was working alone I found it useful to clamp a solid length of wood to the side of my bench as a stop for the holding tool while I used a breaker bar to crack that nut. There are small diameter but thick spacing washers under the holding bar to provide clearance. Using a 24mm socket (actually a 15/16” – don’t tell anyone) and a breaker bar with a length of water pipe the nut cracks easily enough.



The flange puller. Cranking that big bolt with a ‘shifter’ (adjustable wrench), the output flanges came off surprisingly easily from both boxes.



To be continued ...

Next task is to get the cover off. Remove the screws and start heating. I used a 2000W heat gun ($40) and found that it took an hour to get anywhere near the required 100C temperature. Pretty simple really – that case is a very efficient heat sink that will happily dissipate whatever heat you are trying to apply. I assume that this is why a lot of mechanics use a gas torch to apply heat. In the right hands a torch might be fine but I really didn’t want to risk warping the cover, so I tied some fibreglass insulation batt material around the box and managed to heat it up in 10 to 15 minutes. I’d urge you to get hold of an infrared thermometer gun ($50) so you can see where the heat is going.





Once the cover is hot enough give it a gentle tap or two and lift it off with a gloved hand. There are a couple of places where you can pry with a suitable tool if it is wanting to stick. Lift that lid carefully so the shims remain in their positions, either on top of the bearings or stuck in the pockets in the cover. It is good practice to note the location and thicknesses of these shims so you have something to compare with when re-shimming.

You should find that the input shaft will come out with a bit of wiggling.



Next thing is to remove the shifting mechanism. There are two dowelled bolts that have an interference fit in the case. Take care undoing them – they may want to turn in the case but not withdraw. This can crank one end of the shift mechanism away, possibly causing damage. With one of the boxes I had to clamp the shift mechanism to the case with a block of wood to get the bolts to withdraw.



And this is what you have once the shifting mechanism, the fork shaft and its two forks removed …



Removing the output and lay shafts is next. With the box mounted horizontally and once again wrapped in insulation, heat the front end until you get to that 100C again. You will find that some areas will take a lot of work to bring up to temp. Just keep at it, chasing the cold spots away with the heat gun. Once at or a bit over 100C, wiggle the shafts and pull them out with gloved hands.

To be continued ...

edited

SteveD wrote:

Am I the only one that cannot see Steve's links from motorcyclistcafe.com?

(I don't think they want you borrowing their smilies, Steve.)

Oh, looks great, Charlie.
If I cared, you guys would give me such an inferiority complex!

I hope I never have to open mine.

Good stuff Charlie! Keep it coming.

Ok, I'm keen to see whatever SteveD thought he was posting ...
Ah, I think I've just done what Rob must have done - right-click, inspect element, and I see that Steve has stolen images for 'thumbs up' and 'lurker'. Goodo.

Thanks for comments, you lot. I'd better push on before my memory thingy fails me ...

An aside: Went for lovely ride yesterday on a perfect sunny spring afternoon. I’m really enjoying the newly refurbished gearbox. Having been in there gives me a new understanding of its qualities and limitations. I have fondled each part and can visualise what is happening each time I move that gear lever. I feel greater kindness towards every part in there and my riding style is a little different. I’ll probably write more about this later …

Next episode:
Time to assess what you’ve got. This is where experience really helps. I had two boxes dismantled and all the parts together in their separate marked containers. With good light, a magnifying glass and Jeff’s help it was clear that the gear sets from the original 74 box were in better condition overall than their sisters in the 75 box. The good bits in the 75 box were the input shaft (by a mile!), the shift mechanism and the rear cover (the 74 box had a kickstarter that I did not wish to re-install – bloody stupid thing – what were they thinking? etc).

So here are the input and lay shafts from the two boxes. Note the missing dogs on the ’75 layshaft and the crappy splines on the ’74 input shaft on the right …



Also note the different widths of the channel for the shifting forks on the gear with the dogs. This was a change that occurred between the ’74 and ’75 boxes and was supposed to provide a stronger base for the dogs. Interestingly, a friend who is an experienced fitter/machinist looked at these and noted that the earlier dogs had a bigger radius fillet between the dog and the flange. The dogs on the ’75 box have virtually no filleting here. He was confident that this lack of fillet was the reason that the dogs broke so easily. I don’t have close-up photos of the difference, unfortunately. Might sort that out later for discussion purposes.

The dogs are not in very good condition on any of the sliding gears. That is, the leading edges are not square and crisp. There is rounding and some stepping common with well used dogs apparently. Replacing these gears is not a simple matter. BMW only sold complete lay shafts with all gears; they were stupidly expensive and are now no longer available. One could scrounge up good used gears and with a proper press, put together decent layshaft sets. In my situation it was more a matter of giving old parts the best chance of living a full life pushing a nice old bike around rather than creating an as-new gearbox.

It was time to order what I needed from Motobins. A complete bearing set (6 bearings), all seals, two cover gaskets (the extra just in case), and two each of the four different thickness shims they offer – all came to a total of nearly AU$400, delivered. I also bought a ‘bearing separator and puller kit’ from ebay ($50, delivered).



This cheap chinese kit is a bit rough but was totally adequate for the job. A quick wire-brushing and some anti-seize on the threads made it better. First task was to disassemble the two input shafts since I was replacing the roller bearing and swapping the helical gear and ‘cush’ spring and cam parts from the 74 shaft to the 75 shaft. This involves compressing that very strong spring enough to enable removal of a little spring clip at the rear end of the shaft. I set up the bearing separator like this …



… and found that it wasn’t easy to tighten the cranking bolt without everything twisting. The simple solution was clamping the black fork thingy in the vice (vise) like this …





… and it was pretty easy to remove the clip with a little screwdriver and a sharp pointy thing I found. This technique is a lot easier than what I have seen in the BMW and Clymer workshop manuals, where a length of flared pipe with a cutaway for access is shown.

I don’t have a pic of how I removed the roller bearing ‘inner’ (using the same puller), but here is how I pressed the new one on with my woodworker’s ‘press’. If I didn’t have this nice big vice on my bench I probably would have bolted up a timber frame and used a car scissor jack, or something like that. Grin. The metal tube was previously the stem of a dead pedestal fan, chopped with my angle grinder and cleaned up with a file:



The rest of the shaft was reassembled using the better of the two clips to hold it together. Probably should have used a new one I know, but I’m confident enough that it will last.

To be continued ...

G'day Charlie; Great thread mate I've bookmarked it just incase I need to use this
info in the future, looking forward to watching through to the finished box.

Regards Bob.

Bloody good stuff, Charlie.
It's fun discussing the art of pulling these boxes apart and working out how to fix them properly.
They're not quite the bogey man as historically portrayed.