This is an oil pump nut which can be secured to the oil pump sprocket with safety wire (provided).  This will fit the E36 M3, 325, and 328 cars.  The engine model application is M50, M52, S50, and S52.

The nut holding the drive sprocket on your oil pump can become loose over time. We have personally seen this on more than a couple M3 motors, but caught the problem before the nut became too loose.  Others have not had such luck.  There are countless stories of oil pump nuts coming loose and causing a no oil pressure situation costing thousands in repairs or replacement.  We place this on every turbo car we build and every car we pull the oil pan from.  It's cheap insurance compared to an engine rebuild and eliminates the need for tack welding the oil pump nut which is another securing method used.

This oil pump nut has a hole EDM'd (Electrical Discharge Machined) on the flat to allow it to be safety wired to the oil pump sprocket.  The kit includes one oil pump nut and a longer than needed length of safety wire to secure it.  It's a small price to pay for peace of mind.