Vespa / Piaggio Self-Locking Nut - M10 - Sprint / Primavera - 1C005302

You will find this nut in the subframe, also called a swing arm or engine mounting.

This means that great forces will be exerted on it, and of course, any loosening of the material is out of the question.

So even though this nut is on the expensive side:

Always use this original lock nut to ensure it is properly secured!