Another special feature are the different core diameters of the threads. The side that is screwed into the engine housing has a slightly larger core diameter than the cylinder head side. Due to this oversize, the bolts can be screwed into the housing relatively tightly, but they always remain in the housing and do not rotate even when the cylinder head nuts are loosened.
The side that should face the cylinder head is marked with a narrow turned in circular groove. Older bolts have a grain knock on the upper side of the bolt.
Some Piaggio bolts already have a factory-applied threadlocker (usually blue). This marks the side to be screwed into the motor housing.