I can kill 2 birds with one stone - go up one tooth on the gearbox sprocket and hopefully replace the oil seal.

Only problem is that the nut holding the sprocket will not budge, I assume it is a right hand thread.

It may be the OE nut that has been there since the bike was built so the thread may be corroded.

Any helpful tips for removing the nut - looking like I may have to use a nut splitter to remove it.

Oh and I assume I can replace the gearbox oil seal without splitting the cases.

Any / all responses appreciated.

I don't know of any seals on a Yamaha Enduro that require case splitting to replace.  All normal disassembly/maintenance items are RH thread as well.

Chain on and an electric or air impact should zip nut loose?

Thanks Mark - I am still learning about these engines.

I think we all are. I know I'm still learning about them.

Andy, probably obvious but make sure you have bent the tab washer back so it is fully flattened on to the drive sprocket flat face. Also soak some penetrating fluid on the thread/nut.  Original torque for that nut was about 68Nm so you've got to overcome that plus the resistance of years of crud..!! As Mark T said keep your drive chain on attached to the back wheel, to hold the front sprocket spindle from turning when you give the nut an anti-clockwise heave-ho. Good luck with it.

Got the nut off using the equivalent of an air gun - it was a battery powered version borrowed from a mechanis friend of mine.

New oil seal, spacer and 15t sprocket on order.

I did let it soak overnight in some penetrating fluid.