I bought a RMZ 450 at the end of last year that had gearbox issues, see the original thread here http://www.biker.ie/forum/showthread.php?t=235369 long story short – 3rd and 5th driven gears had been replaced by the original owner, he had ran it on a few practice days before it locked solid on him, he traced this back to a couple of screws loose on a retaining plate on the selector mechanism, tightened them up and worked away again fine for a while until it locked solid again. This time the offending screws hadn’t worked loose so he got pissed off and sold it – to me.

When I collected the bike the gearbox had somehow freed itself, so after taking it for a gentle spin it was jumping out of second gear and making a grinding noise under any load.

I eventually got around to opening the crankcases last weekend expecting to find all sorts of horrors inside, but there is nothing very obviously wrong. My theory is that when the shift mech retaining plate came loose it caused the shift drum to overshift, maybe selecting two gears at once and locking the box, and this happening a couple of times has bent a couple of selector forks, which in turn has fucked the dogs on the driven second gear, along with the neighboring gear.

The only evidence of any damage inside the gearbox is a slight (and I mean slight, no more than 1mm deep) rounding on the corner of the dogs on nearly all gears, but I am assuming this is just normal wear and tear. Also two of the three selector forks have rub marks on the side of them, but the fork part, along with the groove in the gears, are all within spec width wise, they are not obviously bent either.

My plan is to change the two selector forks, but I’m in two minds about the gears, none seem bad, and at €50 a gear, I would rather not replace all 8 if it isn’t necessary, so my question is to all those that have experience rebuilding gearboxes, how much wear is acceptable on gear dogs?