Crank pinion nut

[Tigger52]

Hi all,

Twice now I've had the crank pinion nut come loose and lock up the motor as it alllows the crank to move sideways - towards the flywheel!

It's been torqued to the recommended 40lbs but it's just come loose again and ruined another days riding.

Am I missing something, should there be a lock washer of some sort behind the nut to stop this happening?

Suggestions very welcome as this is really p***ing me off now.

Many thanks