It isnt a left hand thread by any chance?
get a longer lever (the Suzuki PE400 crank nut is l/h thread, btw)
If you read his initial post ? you will see it is the Nut on the clutch boss (main transmission shaft)(RH thread)That Oldbiker is troubled with.
If it is that tight or frozen(RH thread). Hey, Old biker? ? I would choose not to needlessly load that much duress through the transmission with a rattle gun. “Leaverage” is the correct practice. Apply “a lock” to the opposite force at the exact point of resistance (IE; to the clutch boss, as Yamaha call it). Yamaha did make a special tool, YAMAHA PART#90890-01022.
I have manufactured a stronger tool. I have two similar engines to get out very soon. I could post the tool to you? But you would have to post it “straight back”. Where are you? ? PM me with your details.
Trust me you will be pleased at how easy that nut loosens, when you lock the clutch boss with this tool.
PS; The nut that holds the clutch boss to the transmission mainshaftt is a RH thread, so apply a LH undoooing force.