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 ...
