A couple of weeks ago I was on a trackday at Cadwell Park. I had a massive problem with 4th gear.
Every time I went for 4th I just got nothing. Then struggled to get 3rd or 5th.

So I have decided to strip the gearbox to see what the problems is. I have found that 4th gear is badly worn where the teeth go into the gear.

My question is that the other gear have some wear. The edges have rounded off. I didn't have any problems with the other gears.
So is some wear acceptable?

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Could be a problem with that shift fork not allowing the gear dogs to enter that pocket enough, and as a result it just pops out of gear and rounds over that edge.

Ideally they should be nice and crisp sharp edge but will usually show some wear, post up a picture of the teeth that enter that pocket and the wear on that.

Yamahas can suffer a similar problem on 2nd gear.
There is some useful info on gear undercutting HERE

HTH

Neil

nine times out of 10 if one gear has gone the undercut on the matching one will have worn also check the forks havent picked up pop a photo on hear if in doubt also look at the drum to see if its is slightly worn it doesnt take much i get my parts from powersport pluss in the usa a lot cheaper than the uk

From the photos you've shown, that would not stop gear engagement. I think you need to look elsewhere, as damage in that area of the gears would cause them to jump out. Not stop engagement.

I had a circlip failure in my box, see this thread: http://www.locostbuilders.co.uk/viewthread.php?tid=191667

I would check the shift drum, forks, and the shift rods. If it is a Gen1 engine these were known to bend over time especially in a car application. Also check the pins that engage in to the shift drum as they can wear causing poor engagement, the shift fork tang thickness is also critical.

To me it sounds like you might have missed a gear, and bent a shift fork/rod which would make it very difficult to engage certain gears.

John

The pictures above are of 2nd gear. Which did seem to be ok. So I guess that would mean the gear isn't to badly damaged.

This is 4th gear, the one that was a problem.

4th

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looking at the bottom one it looks like the undercut might have been recut hard to see by photo but i would replace both only bit that im a bit confused is the gear on the drive (output shaft) is a double gear for 3rd and 4th

The 4th gear selector doesn't look bent but it has definatly made contact with something.

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After some more investigation I have found this.

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I think I might need a new gearbox.
Anyone got a 2nd hand one for sale?

John was correct with his diagnosis.
The selector fork is actually burnt due to the massive sideways pressure created when a gear tries to jump over the dogs. It becomes like a hammer drill and the percussion causes burning. The real damage is on the selector drum and the fork. The dog damage isn't that bad. Depth of engagement is vital to prevent gears jumping out. You also need to check the position of the selector drum when it is installed. Lay all the gears assembled into the bottom half of the cases and engage each gear. Check that the position of the cam is central to the gears. Sometimes you will see that all the gears engaged to the right side are well engaged and all the gears to the left are not so deeply engaged. If this is the case, you can shim the selector drum over to correct the alignment. Shims are NOT available you will need to make one.
Finally, take a look at the modified shift star and spring. These really do work and are worth installing. The spring is much stronger than the standard item and will ensure good engagement. http://www.factorypro.com/products/shift_kit,Factory_EVO.html