Lost third gear

1988 CR125R
I bought an abused bike in feb, and rode it once and noticed that third gear didn't engage, it was as if the bike were in neutral

I tore the motor apart and noticed that the shift fork had a grove worn into it. what was happening when third gear is selected, is the fork isn't pushing the dog teeth on the gear far enough into the other idle gear so the bike is essentially in neutral (no power being transmitted to the output/counter shaft).. I filed minimal material off of the shift fork, so that the gear sat flush again in the fork in hopes that it would engage itself again. While the cases were still split, i tested the shifting and activation of third gear and it worked flawlessly.. However the first time I rode the bike, I shifted into third and my "missing gear" problem was back. The wierd part is that the gear will take load while engine braking, however I can't apply power.. When i roll on the throttle I'm guessing there is enough lateral thrust to disengage the dog teeth from the face of the other gear.. I'm assuming this was an on-going problem with the bike before I bought it, because the dog teeth have been slightly rounded on the leading edge. The gear face that they insert to shows no wear.

I have been looking for a while to find used parts (since I first diagnosed the problem in feb), and finally have come across a guy parting out his bike..

My question is, do you think a new/used shift fork will fix my problem? The guy is willing to sell me all three shift forks, the shift fork rails and the gear select drum.

My secondary question is what would cause the gear to not sit flush on shift fork?

Sorry I have no pictures because I never thought to take any while I had the cases split, and the motor currently assembled in the bike. The motor, tranny and clutch all work MINT otherwise. I just wish I had third gear, the distance between second and fourth is pretty big while trail riding, and it's annoying having to shift 2-4 every time.

edit. I should have put this in the Mod/Performance/Repair section, can a Moderator please move this for me?

Thanks Vintage Bob.. For the record I haven't been engine braking, just stating that the gears do positively engage under reverse load, just can't take forward load.. WHen I kick it into third gear, using the clutch or not i can feel it positively engage, but they can't take thrust..

Thanks for all the info.. I assumed the forks held the gears in place, didn't realize the slight undercut.. Is there any kind of mexineering way to fix these gears? I'll ask buddy if he has a tranny available..

I'm assuming thats why the shift fork got messed up.. The thrust is pushing the gear outwards against the fork..

I'm gonna buy some gears, but not install them untill I need to tear the motor apart for some reason, most likely top end..

My buddy has a mill, I'll see if he can help me out..

I'm not too sure about the availability of new parts, I'm going to call and iquire with the dealership tomorrow.. But I will most likely buy the used set of gears, and just swap over the 27 and 29 tooth gears on to my counter shaft.. I've checked a few online sources and the individual gears cost about $85 a piece.. thats making my budget bike too expensive for my taste haha

I was thinking that using just the gears I need would be a better idea, since I know that mine (aside from the two fudged ones) are in good shape and hold.. I won't have to reassemble the motor, and run it again before I find out that maybe second gear doesn't work in the donor tranny.. another reason to get the entire tranny is spare parts.. Maybe something else will go all FUBAR on me..

if it was $45 new I'd buy it, save me 3 hours of driving.. Gotta love the "currency exchange" between Canada / USA..

I just sgot off the phone with the Honda Powerhouse Dealership near me, they have no problem at all ordering those parts for me.. to the tune of $94.62... EACH!

I have no buddies in the states..

I'm most likely gonna make the trip next week sometime to get that parts tranny.. I'm gonna bring a straight edge with me to check that there is undercut in the gears I require..