The nut half of an AN3 bulkhead assembly, machined from SUS303 stainless. AN3 is the small line size used for brake and clutch circuits, gauge feeds and boost references, so this nut generally lives on a firewall or bulkhead panel where a hard line crosses from engine bay to cabin. Slide the bulkhead fitting through the drilled panel and draw this nut up from the far side to lock the body square and take strain off the line. Stainless suits under-bonnet and underbody positions where a plated steel nut eventually rusts into the panel. Also handy as a replacement when an existing nut is lost or damaged during a rebuild.