K-Nut Used for Exhaust manifold/header fixing.
The beauty of this bolt is in the design. It is manufactured so once tightened the threads slightly distort around the thread it is located over. This distortion of the thread makes a mechenical interference and stops the K-nut unscrewing due to vibration or heat cycling.
When you need to undo the K-nut just unscrew and the distortion changes shape to allow safe removal without damaging the thread on the mating part.
These are a one use component and should be changed once used.