426 piston and 5th gear help

First thing I seen a similar post about a 4th gear issue and I'm having the same problem but in 5th. When I'm riding and get to 5th it's like it's jumping in and out of neutral. I've replaced all three shift forks and shift drum last summer and it has helped because it was completely dropping out of 5th. I'm guessing I need to replace the 5th gears ? any other gears to replace that could cause this ?

Second issue I'm concerned about. As I said I just rebuilt the motor last summer and have about 3 hrs on the new piston (complete head rebuild too) and it has some small pitting started on the rim above the exhaust ports. What could cause that?


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Your fifth gear problem is due to the locking lugs on the sides of the third and fifth speed gears on the main shaft being worn. Your new shift fork is likely already damaged as a result.

The piston looks like it "chewed on" something that didn't belong in the engine.

ok, so I need to replace 3rd and 5th on the main. I keep reading good reviews on wr 5th gears. Do you have anything bad to say about replacing yz 5th gear with the wr 5th?

And about the piston, that could of been any kind of debris that caused the chewing correct? Do I need to replace it again or will the piston be ok?

No, gearbox debris cannot normally find it's way into the combustion chamber.  Dirt or carbon, more likely.


Using the WR 5th by itself creates a massive gap between 4th and 5th, RPM wise.  Changing both 4th and 5th works pretty well.


Remember that if you want the WR gears, both gears in the pair have to be replaced as a set.  And you still need the main shaft 3rd gear.

Ok well if I start replacing a bunch of gears its going to get expensive quick so I'll probably just stick with replacing 5th and 3rd on main and looking at the manual shift fork 1 ®. ?

Do you recommend replacing the piston or not really sure?

"Shift fork 2" in the parts book, 5BE-18502-10-00, the one that moves 3rd gear into either 4th or 5th to engage them.


The damage to the piston is insignificant as long as the top ring is still free in its groove, and there are no small burrs of metal sticking up around any of the little dings.  Knock them down with a small file if present.

Awesome, thanks for the help!! Time for me to get to work.

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