OSL - It's Nuts (or rather ... Bolts)
It's a very simple shader, it doesn't really follow the proper physics of bolt design (using a sine curve for the shape of the thread itself) but it's fun to use.
NOTE: My vRay settings were COMPLETELY WRONG FOR INTERACTIVE RENDERING. PLEASE DON'T TAKE T HIS AS A "PERFORMANCE COMPARISION" IN ANY WAY ! !
The shader itself? Of course it's on GitHub!
Just click here :)