Horror Picture - YZ450F gearbox bearing.

Look away now if you are mechanically squeamish.


My lad was going very well at Loton Park Speed Hill Climb yesterday on the tuned 04 YZ450F. Then on his first timed run after practice, changing up after giving it the beans out of the first corner and laying it in to the next,  the back end locked up and it spat him off. Fortunately he wasn't going that quickly.


It was impossible to get the bike in to neutral so he had to be recovered back to the paddock. The kickstart was solid with the clutch in or out so that tended to point to a gearbox failure rather than a top or bottom end seizure.


So this morning I had the clutch cover off and scratched my head a bit faced with the STM Evoluzione slipper clutch. Figured out I had to pop the circlip out to get the outer plate off and remembered that the bike came with a box of bits including the special tool to undo the clutch holding nut. I assume the clutch should then come off as a unit but it wouldn't budge. So I had to pull the various components out (catching the six balls) until I was left with just the basket. That still wouldn't come off but already I could tell there was huge play in the gearbox main shaft. After a jiggle of the back wheel something dropped with a clink in to the bottom of the side cover and the basket then pulled off to reveal the horror below. OK , to be fair, not all the balls you can see had dropped out at this stage, but more did as I foolishly (though what more harm could be done by now) twiddled the mainshaft a bit. You can also see the pieces of inner bearing race. One of those was presumably jammed in the gear teeth on the basket and preventing it pulling off.


I was intending having an engine rebuild at the end of this season but obviously left it too late. So I fear this is the end of the road for this ex Dani Mueller motor. A rebuild is probably beyond my capabilities, and looking at the basket it's going to need a new one as well as all the additional horrors that may be revealed when opened up. Time to start looking round for something much more recent I think.



Edited by 72degrees

Messy, but you might be surprised at the condition of things inside the transmission as you open it up.  May not be the disaster you see it as right now.


Way to break stuff, though. :thumbsup:

Thanks for the encouragement. I don't trust myself to do a bottom end rebuild so I'll drop it out and take it to my local engine builder for an estimate but I suspect it may be too high to make it worth having done.



Not as bad as it could have been. Nick at Revs says that apart from the failed bearing the rest of the internals do look as if they have had not that many hours use with frequent oil changes. The main transmission axle needs replacing (along with the primary gear/basket) as the broken bearing inner race bits have damaged the teeth. Otherwise all the gears, selectors and drum are OK. No obvious clue as to what caused the bearing to fail.


The total parts bill including precautionary bearing replacements is still pretty eye watering at UK OEM parts prices.

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