Mystery fastener du jour.
 

Looks like it goes on a stud. Do not remember removing it, found under driver's side front. Recently replaced power steering pressure hose and had to remove alternator and PS pump. Also still putting things back together after pulling intake, cleaning valley, replacing throttle body shaft o-rings etc.

Car is an 86.

Thanks

Otto: I would check your suspension, and tie rods.

Keeps the muffler bearing tight.

Should have included something in the pictures for scale. The nut portion is 10 mm, awfully small for steering and suspension.

Not the muffler bearings, I lubed them last week with the grease Greg used to sell for $160 for a tube.

I know I have seen one of those ,
maybe from the back side of the power steering pump mount,
or the arm where its fastened to the block.