The Thin Locking, or “Jam”, Nut is used to lock the Fuel Tap into place once the correct position is established on the Petrol Tank.
The Jam Nut is a Thin Nut so that it doesn’t occupy too many threads on the Fuel Tap, allowing enough threads to retain the Petcock in the Petrol Tank. 1/4 BSP threads.
The Jam Nut is installed onto the threads of the Petcock and is used to lock the Petcock into position on the Petrol Tank.