How to tell a bent shift fork


12 replies to this topic
  • fritzelman

Posted May 26, 2008 - 08:20 PM

#1

5th gear has been popping in and out on my 02 YZ 426 so I decided to tear it down and look at the tranny gears. The only thing I found wrong was some wear in a shift fork. Is there any way to tell how bent it is? Should it be noticeable or just by a few degrees? My theory was that the dogs on the 3rd and 5th pinion gears were wore from a bent shift fork. Any ideas what it might be? All the gear teeth look find, almost no wear.

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  • grayracer513

Posted May 26, 2008 - 08:45 PM

#2

The fork in the foreground of the first picture is bent. You can tell by the wear mark in the center of it. The fork was bent by the gears popping in and out under a load.

If you are looking for wear on the gear teeth, you might find some or you might not, but the teeth are not where your problem is. Look at the locking lugs on 3rd gear where it slides over against 5th to engage it. These dogs start out with a slight undercut so they hold together when driving force is applied. As they wear, the undercut becomes slightly reversed, and they force each other apart instead, flexing and bending the fork, because it cannot move aside to allow this. Replace those two gears, and the fork.

  • rufusz

Posted May 26, 2008 - 09:27 PM

#3

I had the same problem, but didn't replace anything :thumbsup: If you want to save money, you need to repair those locking lugs by grinding it with a hand-grinder (dremel-multitool or something like that). But you have to be very precise : all the lugs have to have the same length and that undercut grayracer was talking about. You have to look on the other gear too, where these locking lugs engage, and grind there too, doing the same undercut. Just be sure that the grindstones you're using are always flat, ie after grinding for a while straighten them with a special stone that is used for that (sorry don't know how it's called in english). The marks on the shifting fork come from the force of the gears wanting to be thrown out. It's not necessarily bent. I've repaired mine and now it runs just great.
OK, new gears and new shift fork would been a better way, but it costs more and my father likes challenges like these...he said if it won't work then he will buy the new gears...they work. damn it. :ride:

  • kheiney

Posted May 27, 2008 - 01:16 PM

#4

The 01 426 I just got had the tranny re-built because of this same issue. The guy that sold it to me was a mechanic. He said this is an issue with the 426 gear box and you will notice that when you get new parts a lot of them have updated parts numbers. Replace any gears with warn dogs and any worn forks

  • Yamaha4lyfe

Posted May 27, 2008 - 09:51 PM

#5

is this from shifting without a clutch? because i dont use my clutch usually for shifting but when i shift up or down i try my best to take the load off (let off the gas)

Thanks,

Ryan

  • grayracer513

Posted May 27, 2008 - 10:18 PM

#6

Not from clutchless shifting necessarily. Anything that causes the lugs to bash each other more often, especially when they are only partially engaged, will contribute to wear. To that end, a quick, full. positive shift is less likely to cause damage than a lazy, half-hearted one that causes the gears to "grind" with each other. Forcing the gears out of mesh under power can do more to damage the dogs than engaging under power if the engagement is quick and complete.

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  • fritzelman

Posted May 28, 2008 - 09:17 PM

#7

The fork in the foreground of the first picture is bent. You can tell by the wear mark in the center of it. The fork was bent by the gears popping in and out under a load.

If you are looking for wear on the gear teeth, you might find some or you might not, but the teeth are not where your problem is. Look at the locking lugs on 3rd gear where it slides over against 5th to engage it. These dogs start out with a slight undercut so they hold together when driving force is applied. As they wear, the undercut becomes slightly reversed, and they force each other apart instead, flexing and bending the fork, because it cannot move aside to allow this. Replace those two gears, and the fork.


Exactly what I was thinking. I ordered those exact parts last week and will be showing up thursday/friday. Is there anything else in there that I should look for thats common with these motors for breaking?

Thanks for your insight, its much appreciated :thumbsup:

  • grayracer513

Posted May 28, 2008 - 10:26 PM

#8

It's just prudent to replace the 8 major bearings (crank, main shaft, output shaft, and balancer) while your in there. If any one could be cited as more prone to fail than the rest, it would be the left side main shaft bearing, which only makes sense, since it's the hardest one to remove.

  • matt4x4

Posted May 29, 2008 - 04:57 AM

#9

Left side main shaft (forward one) was easy to get out, use a slide hammer style bearing puller to pull it - try removing just the outer race (all that was left of mine) on the output end - lots of dremel action to get a good groove to bite into so you can hammer it out from the back...as always - heat is your friend.

Also - you will want to replace ALL lockwashers, just as a safety precaution, they're cheap. Most have a second or third tab you can bend up, but when it's all said and done, it's cheap insurance.

  • grayracer513

Posted May 29, 2008 - 06:29 AM

#10

Left side main shaft (forward one) was easy to get out, use a slide hammer style bearing puller to pull it - try removing just the outer race (all that was left of mine) on the output end -


Having to heat and use a slide hammer is what makes the left MS bearing harder then the others. Not because that's really that difficult, just that that one is the only blind bearing pocket out of the 8. And, of course, most home tool boxes don't include a blind bearing puller, but many auto parts stores will rent you one. Heating cases is best done in an oven, so the temp can be controlled. It's important not to exceed 275 F to avoid weakening the aluminum. Go with 225-250. A propane torch will work, but it's trickier. Stop heating when spit bounces off the part.

Matt, if you ever run into another broken bearing that has left an inaccessible outer race behind like that, here's a tip:

Take an oxy/acetylene torch with a fair sized welding tip on it (I'd use a 2 or a 3 for the bearing you did), and get a normal flame going on it. Move the flame directly onto the steel race itself and heat one spot as wide as the race to a bright red as quickly as you can, then get the torch out. As soon as the red glow disappears quench the race with a wet rag or even an ice cube. The race may fall out right there, but in any case, this "heat treatment" will have permanently shrunk it to the point where it will be very loose in its pocket, and much easier to remove.

What makes this work is that the mass and the aluminum material of the case make it much harder to heat than the steel race, even in a spot like that, because of its size, and aluminum's ability to conduct and disperse heat, and that there is a thermal barrier between the race and the case simply because they are two separate parts. The steel heats to the point of being softened within the confines of the bore, crushing the race very slightly, then shrinks further still because of the metallurgical damage done to it. I've used this trick dozens of times in all kinds of places, and it works great.

  • matt4x4

Posted May 30, 2008 - 04:33 AM

#11

yeah - torches are great - if you have one!! But then, looking at your point on weakening the case, this leaves me wondering about the red hot heat though, it would heat the case in the spot behind the race well over 300F.....

I use the BBQ to heat my cases (less flack from the wife that way and it's got a thermometer on it - I usually keep it around 200-250 - LOW), throw a couple of hamburgers/hotdogs on at the same time and you're good to work through dinner.

Only problem is that for several weeks after the rebuild, you get these cravings for BBQ food every time you ride the bike - wonder what that could be from?!?!

  • grayracer513

Posted May 30, 2008 - 06:07 AM

#12

...But then, looking at your point on weakening the case, this leaves me wondering about the red hot heat though, it would heat the case in the spot behind the race well over 300F.....

Actually nowhere near it, if done as I described. The steel race can't transfer the heat effectively to the case, and a piece of aluminum the size of that is really hard to spot heat, even with an acetylene torch. Spot heating aluminum is like trying to make a pile of water in the middle of a bath tub. Aluminum conducts and radiates heat extraordinarily well, and in order to heat one spot on the case, almost the entire part has to be heated. If done right, you can pick the case up bare handed quite easily while the race is hot.

Like I say, I've done this more times that I can count and never had a problem with it.

  • matt4x4

Posted May 30, 2008 - 07:03 AM

#13

Thanks - good to know!





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