OK so a few of you know what happened. I was driving along when the gearbox locked up solid. This was at about 60mph and was in 4th (as you'll see!). I though it was the belt at first as I am experimenting with that a bit, but despite gearbox seizure the belt was ok and interestingly the engine spun the clutch up a bit rather than stall (which it did when I got off the throttle). There was a grinding noise from the gearbox and then this was followed by a sudden locking of the rear wheel, then brief unlocking and more locking up. Putting the clutch in had no effect so I was basically a passenger at that point. Thankfully it was a straight road and nobody was behind me...
So lets take a looksee what happened.
I opened the top of the box and found this -
- Stripped gear
Not looking good so far, the gear you see at the top left with no teeth is 4th. So the box definately needs to come apart. I dismantle the clutch and outboard support bearing to find this -
- Output shaft
This was strange. Also on the sprocket (not pictured) were a spacer and nut. The nut was loose on the output shaft and spacer could not fit anymore as the threads were flush to the outer face of the sprocket. I checked the sprocket was seated and it was. That could only mean the output shaft had moved somehow. I went on to pull the rest of the internals out of the box -
- Internals removed
Here we get a better look at the stripped gear. The rest of the gears however seemed fine, I am sure the box would be good if this gear was replaced and put back together with new gaskets, clips etc. Not sure if the parts are available separately. Here is what got scooped out of the bottom of the box -
- teeth
Teeth all seem to have come off pretty completely. This would indicate the box was completely in gear when the event happend and it happened suddenly as opposed to being over a longer period of time. Lets pull that output shaft to have a look -
- Output shaft
It becomes a little clearer now. The pic is the back side of the output shaft where the dogs engage for 2nd gear. So what is my conclusion?
The nut on the sprocket came loose. It was loctited on but obviously this was not enough. This nut is left hand thread. Eventually it loosened enough to rub against my outboard bearing support holder. The nut rubbed this and combined with the counter-clockwise rotation of the shaft it unscrewed itself with some force. This pressed the output shaft into the gearbox (it is removed this way for disassembley and there is no circlip etc to keep it in place). When this happend the dogs on 2nd gear engaged to the output shaft despite the fact that the gearbox was already in 4th. We can see the marks on the back of the output shaft gear where this happened (on the left of each tooth). At that point everything was doomed. The weakest part of the box let go first which happened to be the 4th gear.
Lesson - make sure the spocket nut is tightened and check it regularly. Perhaps on the original bike there is enough clearance between sprocket and outboard bearing that if the nut comes loose it just sits there rather than pressing the output shaft. I'll also redesign the setup so that there is a locking tab or bolt in there to stop the nut coming loose.
Live and learn (the expensive way).
Neil
Another hard to find bit. May try buffing out the cover to see if it will clear the new internals tho too.