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2000 YZ125 popping out of 2nd gear??

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Quick Background: When I bought the bike it had a bad left countershaft bearing (bearing behind the front sprocket). I split the cases and replaced all of the bearings and replaced the transmission with a cleaner unit I got on ebay. I replaced shift fork 3 with a new one and resued shift forks 1, 2 and the shift drum as they looked to be in good condition.

The bike ran great for about 2.5 hours. By hour 3.5 it was slipping/popping out of 2nd pretty bad.

I found that 2nd gear was pushing up against the left countershaft bearing pretty hard as it wore in the bearing keepers pretty bad. Basically it wasn't fully engaging with the dogs on 6th gear. I looked at a diagram and I think I left out a washer that sits in between second gear and the countershaft bearing. I'm guessing this is fairly thin and couldn't be the total cause of my problem. Shift fork 2 also shows wear and looks bent, although I'm pretty sure it wasn't bent when I installed it.

Any ideas on what might of went wrong, besides the missing washer?

Worn countershaft bearing keepers

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Worn face of 2nd gear

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Worn/bent shift fork 2

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Worn dogs on gear 6 and inside of gear 2

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That washer will throw everything out of wack. Keep in mind the tranny is very precisely designed. A mm off and it will throw everything else off. Id be willing to bet that if you install that washer and reassemble it doesnt rub anymore. There was a thread on here about a month ago with the exact same damage, guy replaced the washer and had no further issues.

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I was that guy. Also with a 2000 yz125. When i split my cases the shift fork and gears showed the EXACT same wear. I'm not even exaggerating. And I too was missing that washer.

For your reference, note its the same washer that sits behind your clutch basket. Its a good 0.5mm thick, maybe a little more from memory.

Note that without that washer, you also get wear on the shift fork which (I theorize) is also allowing the shift forks to bend. No washer + bent shift fork = 2nd gear problems.

New washer + shift fork + 6th gear = around $100. May need to replace second too.

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Lol thanks for chiming in, I couldnt remember who it was but knew it was the exact same problem. The reason this happens is because without this washer it allows the gears to float, causing uneven pressure which pushes everything out of true. Not allowing gears to mesh as they should.

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Thanks a bunch guys. Ugh, that was my first time splitting the cases and going through everything. I thought I was being so careful and meticulous. At least it's going a lot faster now. Yeah, $100 and I'll be on my way.

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Your lucky man, could have been alot worse. Ive seen teeth shear off gears from that exact same mistake. Cost alot more than $100 to replace the whole tranny, bearings, and crank.

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No worries buddy. Hope it all runs well for you. The second time around it goes so much quicker, you've already done it before and got all the little things you need, plus you've had some practice at applying the liquid gasket. Good luck :bonk:

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Ha, I opened up the rag that held the transmission that I pulled out previously, there was the washer, staring me in the face. It's 2.0mm thick, and yes, identical to the one behind the clutch basket.

I cheaped out and bought a used countershaft with all the gears for $15 on ebay and a used set of shift forks for $10...hoping they'll be in good enough shape.

Yeah, sealing the cases. I used Threebond 1211 and it seamed like when I split them they came apart fairly easily although there were no leaks at all. Should I use something else?

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Ha, I opened up the rag that held the transmission that I pulled out previously, there was the washer, staring me in the face. It's 2.0mm thick, and yes, identical to the one behind the clutch basket.

fair enough, I don't remember it being that thick lol

I cheaped out and bought a used countershaft with all the gears for $15 on ebay and a used set of shift forks for $10...hoping they'll be in good enough shape.

you may find that you'll get gear problems sooner rather than later then. From what I understand it only takes the slightest of wear on shift forks to start getting issues, especiallly considering how tight the mx transmission is. But if you don't mind splitting the cases then in the long run a few $15 shift forks but well work out cheaper

Yeah, sealing the cases. I used Threebond 1211 and it seamed like when I split them they came apart fairly easily although there were no leaks at all. Should I use something else?

Again up to you, I cheaped out a bit last time and now I am a bit suspicisious that I may have an air leak somewhere due to some weirdly intermittent 'jetting' problems, and possible air leak problems can be so hard to effectively diagnose that next time I will got the proper stuff. When I get some time I plan to make a pressure tester to confirm if I do or don't have an air leak

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Ha ha, so the countershaft and gears I bought were from a '96. This shaft has some different shaped splines so I can't use gears 2 and 6 on my '00 countershaft. When comparing both shafts the gears all sit in the exact same spots but there are some slight differences in the tapers and widths of some of the gears...Anyone had any success using the older style countershaft and gears? I'm sure the answer is no, just thought I'd check.

1211 is the expensive stuff, about $25 a tube. I've got some 1194 also, it seems to set up a little more firm, not as flexible.

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