that is a very manly holding tool RT!

MSAVITT, all my tappers on my bikes are pre-stuffed lol so gotta do em up tight.
A good taper holds well though.
I had a TT250, [quite a heavy flywheel] & ran it minus the crank key experimenting with more advance. Never looked like moving.
Back when we sold Husky mx bikes in the "60 & "70's the final drive sprocket was on a taper & no key.
Never slipped & was hard to break free from the taper when you wanted too.

Thanks for the advice.  I haven't pulled the flywheel magneto yet because my home-made 'holding tool' that I had used on others bent on me due to the force on this particular 250.  I have a new one coming off Ebay and in the meantime, more Blaster has been applied to the nut/shaft.

I just want to be sure when saying holding tool you mean to stop it turning while undoing the nut anticlock ie left turn.
If you mean puller to get it off the taper then please buy the right puller.
Sorry if i'm sounding stupid & not reading [i'm good at stupid--years of practice].
Also whatever 'Holding' tool you buy, be sure the tits don't go in far enough to rip the coils.

Yeah, I'm talking about the flywheel magneto holding tool. 
It's served me well on a Honda 50 and 350 but met its match on this 2-stroke bugger.