This Ni Nut-Lock, or Nylock Nut, is a Silver 1/4 x 20 metal nut. Sold individually, it can be adjusted using a wrench or socket and torque wrench to the manufacturer specfications, and is used to secure the sheer pin in place, on the impeller assembly. These nuts are a one-time use part due to their locking mechanism. You'll know the nut has broken when it appears to have damaged threads or rounded off edges. Customers found this to be an easy repair.