Split the cases to reseal the case halves and found a chipped tooth on the second gear pinion,
Looks to be a press fit to the main axle. Anyone replaced the gear on the shaft? I am wondering
how much pressure it took to remove and install it. Since it drives the sprocket shaft, I wouldn't
want to have it loose. Maybe better off replacing the shaft and the gear. Looks like the rest of them
just slide off. Thoughts?

I've pressed them off & on but many years ago at work. Takes a good press to move it. I don't recal anything behind it so can support under the big gear to press. Fitting the new one you have to be careful or one bang too many will lock it up. My method be it right or wrong was to put a feeler blade in there, not sure how much but say 5 thou max & pump the press to a point where 'i used to' smack the press base or somewhere that'll shock it like the plate under the gear to creep it up incrementaly until it's touching but not locking the feeler blade. Sounds easy doesn't it--Not. They used that system a lot on other small models but also i was involved in one on a YFM250 i think it was. belonged to a mate & was a bugger of a total dismantle & the gear had moved sideways causing the next gear to move so probably wasn't tight in the first place & then it had no gear engagement or only enough for it to ramp out under power. Stupid idea 'that'!!. Had a V90 scooters do that too, just let go & spun so no--um-- second gear of three i think on those.

I have replaced several such gears on different models over the years.  It is not huge pressure required but a press is recommended.  I have done it using a hydraulic gear puller and a large vice before, but absolutely not recommended if you are unsure of how to proceed.  It is very tricky to do in a vice and hard work.

If you do not have the specs required, ensure you measure its position very carefuly before attempting to remove it and get the same position with the new.

Also, when changing the pinion, the corresponding gearwheel must also be changed.

I've got a shaft somewhere out of something Yamaha so i'll find it tomorrow/ Think the gear is off so out of sheer curiosity i'll smack it on to see if its possible then take it off again. Might be getting ahead of myself as will be driving from the threaded end to make it go on so maybe not a good plan.

Many years ago on our RD200DX, we stuck the shaft in the freezer overnight. Heating the cog would have the same effect.

that's too easy haha

I'm a big fan of easy.

I'm not a fan of acronyms, but I do like KISS - Keep It Simple, Stupid! 

Is it not an option to simply run it with a chipped tooth? Just make sure you find the loose chip. Don't ask....

Here's a video i dragged on to about 5.22 then dragged until he was pressing on.
Man he does great vids & i'd love a press like his.
But i'm not sure if he was just lucky or if it came up to a step so no fiddly stuff needede, but ended up with revolving clearance without having to creep it up the last bit.

Personaly i think i would have inverted the bearing remover tool & gone flat side under the big gear to push up to the little one seeing no circlip in there but no matter, he's great.