5/16-24 six point steel nylock jam nut. These can be used virtually anywhere on your race car that uses a 5/16-24 thread and are looking to save weight as the jam nut does not carry the weight of a full size nut.
Nylon at the top of the thread on the nut provides a safe way to fasten parts on the race car that could come loose to due to tire shake or are in an area that doesn't get regular between rounds maintenance.