1/4" BSP HSS Die Nut
Parallel (straight) thread, British Standard Pipe Parallel thread BSP; originally also known as British Standard Pipe Fitting thread/BSPF which have a constant diameter. Die nuts are suitable for repairing damaged or crossed threads or clearing swarf from threads.
When using a die nut it is not necessary to apply the same level of precise initial alignment required to form a new thread (i.e. with a die) as the nut will follow the existing threads.
They are supplied as hexagon allowing them to be turned by a standard spanner or adjustable wrench without the need for a die stock.