Posts
55
Joined
3/9/2015
Location
Childress, TX
US
A buddy dropped off a 2005 YZ250 that lost second gear. Split the cases to find that it snapped the 2nd gear shift fork shaft right where its pinned. I also found somebody put a wide ratio transmission in it. They didn’t want to replace gears so I dropped in new shift forks and got everything back together. It was shifting fine on a test ride but ended up snapping the 2nd gear shift fork shaft again two engine hours later. Any ideas on what would be causing this? All forks were replaced, shaft bores are smooth with no catches, no visible damage to the drum. The gears are absolutely ate up at this point but I figured I’d check with the vitards before telling him every component needs replaced. 
I should also add that he just got the bike and the previous owner advertised it as freshly rebuilt. The cases were beat apart with a hammer so I don’t trust that anything is correct. Some of the gears had sharpie writing on them so I’d assume the wide ratio gears were added at this time also.
I'd be looking at these...
2nd and 4th are definitely ate to crap on the countershaft. I don’t have much experience with transmissions so I guess I’m just confused on why it’s causing it to snap the fork shaft. I assume it’s missing engagement and slamming in eventually transferring the force to the weakest point being the pin slot in the shift fork shaft?
That's likely to be the cause.
The 4th gear's dogs engage the 2nd gear, it's dogs don't look great, if the countershaft is also damaged, they may all need to be replaced.
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10-4, that’s the only conclusion I could come up with but wanting to bounce it off another mind since I haven’t done much with transmissions. I appreciate it!
I've typed up a thread on the YZ250 gearbox in this tech section (maybe a year ago) and mentioned some of the issues they have and how I fix them. The biggest issue is the main shaft 3rd gear and 4th gear dogs as well as the 3rd gear shift fork marked C (for center). I replace the entire gearbox with better parts when I build a bottom end.
Most of the time, if you don't replace gears and the associated forks together as a pair, you'll have repeated problems.
The rounded engagement edges on the dogs that @Last2Stroke pointed out can eventually result in a bent or damaged fork as they fail to bite on a partial shift or under heavy shock-loads.
If your fork isn't bent and you want to try to rescue it, PM me and plan to cover shipping on a free to you, good/used pin or pins.
(I don't use those stock pins, so I have a few in a box full of gears and parts up on the shelf).
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