Magneto - Ruston PB

I've spent days trying to remove the magneto sprocket from my RH-PB 1 1/2 hp engine.
The manual says
"Give it two turns ACWs until it goes tight again, then give it a tap with a spanner"
Well I've tried that, I've cleaned everything, spayed it with copious quantities of WD40, tried the gear pullers, tried just keeping turning the nut.

Absolutely nothing.
The sprocket pullers got so tight they slipped and rounded the aluminium where they were placed.
I have a block stopping the engine turning, placed a ratchet on the nut and thwacked it enough timed to wake Mr Hornsby himself, but it just will not shift.
I'm getting concerned that if I use any more desperate measures I will do so serious damage.

Am I missing something?
Help

is it keyed or taper fit as there not usually threaded on
is there a grubb key hiding away from you
try heating the sprocket with a small heat tourch and see if the oil gets in better
other than that with out a pic  were in the dark other than the men with said engine with  i.e wico a mag or????
us one of the message options in my box to the right side of this message and i can post it on the post for you --------------------------------->
kev

It just needed a bigger sprocket puller in the end.
Mine was too small and slipping. 

Da_Bunn wrote:It just needed a bigger sprocket puller in the end.
Mine was too small and slipping. 

well atleast its off now though mate
  
kev