PRM gearbox

I have an 150 and it's 18mm but be careful, all the 18mm spanners and sockets that I use want to round the nut off, almost as if it's not really 18mm.

Your not wrong.......I can't move it even with a new, long snap-on spanner. Now what? Not keen on using a blowtorch down that hole.
It is 18mm but almost like its tapered!
 
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