Vespa VBB LMLPX150

Hi all,

I have a Vespa VBB de 1968 with a 4 speed gearbox on a LMLPX150 engine.

The rear hub nut keeps coming loose, I've retightened it a couple of times.

Does anyone know the torque figure for this rear nut? I'm wondering if I'm not tightening it up enough.

Or is there a known problem and it needs a new rear hub etc. (hope not)