Hi Jack and thank you for your reply.

I should have specified that I have the 2 stroke version. I have attached a photo of someone elses mower but mine is the same one. I spoke to a person who has completed a restoration on one of these machines and he told me there would be a grub screw holding the two parts of the centrifugal clutch together. I have had a look but it looks like someone may have taken this apart once before and used a bolt that i could swear is welded in. I may be stuck with trying to remove the whole engine and clutch in one piece ( if i can remove the sprocket off the shaft)

[Linked Image]

Jed