The saga continues with a different box:
As part of tidying up after the above exercise I decided to clean the borrowed gearbox, ready to pack and return it to its owner. I set to with solvent, brush and rags and all was going well until I turned it over and heard a little rattle. I picked it up and shook it and “rattle rattle” it went. I knew what the sound meant.
Short story: I pulled the cover off and there it was, a broken dog …
Not only that – there were three dogs missing from the same sliding gear on the intermediate shaft (as featured in the above story) and only one in the case. This gearbox has unknown provenance. Like the refurbished box that is now back in my bike it is stamped with a “73” which means it is one of the first 5 speed boxes, built for the 1974 model. This one has no kickstarter but the input shaft has the teeth for one and they are in used condition, so it had clearly been opened up previously.
Other issues – the front input shaft roller bearing is quite sloppy, more so than others I’ve felt. Also the big bearing on the front end of the output shaft is badly worn (outer shell can twist quite a lot in relation to the inner shell) and the inner shell slides back and forth on the shaft (about 1mm). The circlip is also broken. There is no snap ring under the circlip but there appears to be room for one.
There are some good things. The input shaft splines and ‘cush’ cams are in excellent condition. The gear teeth on all shafts look good enough. And this is the really good thing:
The spare sliding gear in that pic is the one that Disston (of ADV) kindly donated to the cause. It is the right one for this box but there is no hope of fitting it with the equipment I have. That’s ok, I’ll take it to the local motorcycle-friendly engineer – he has a proper heavy duty press.
Here’s a question: Is there special information I should pass on about disassembling and pressing the gears back on again? Or is it just a matter of applying brute force?
I’ll have to dismantle the output shaft to see what damage might have been done to the shaft by that sliding bearing. I do have a spare shaft from my dead 1975 box that will hopefully do the job if it is damaged.
I’ll be ordering another bearing kit and seals from Motobins and another gearbox will get a new lease of life. This box felt pretty good, by the way. No crazy clunking, no false neutrals, no slipping out of gear and neutral was easy to find. It is likely that the last dog broke while in my care – I did notice a funny rattle that lasted for a few seconds while in neutral a few hundred kms ago. I had ridden that box for nearly 5000kms, most of it keeping up with Jeff (ME109) on some great rides through the mountains.
So how do you tell how good a gearbox is going to be? If it is out of the bike I’ll be picking it up and shaking it like a cocktail shaker, listening for telltale rattles as a first check. Running it through the bench testing would be next. I really don’t know what one should do with an unknown box apart from opening it up to see the condition of gears and to feel the bearings. If all is good, re-shimming and assembly would take half a day if one has the necessary tools.
Good bearings will obviously help every other part to work for longer and early detection of problems is important. I’m now wondering how long a gearbox typically lasts before bearings should be replaced as a matter of course.
