Brand: Racetech Gauges
This special adapter has 3/8 BSP (BSPP, British Standard Pipe Parallel) male threads on one end and 5/8-18 male threads on the other end. It is designed to replace the 3/8 BSP - 3/8 BSP adapter supplied with Racetech gauges. The 5/8-18 thread is now a more common size for mechanical temperature bulbs, particularly in the USA.
Note: 5/8-18 and 3/8 BSP threads are hard to tell apart by sight.
A 3/8 BSP fitting has a 0.66" OD and 19 threads per inch. It will not quite fit in a 5/8-18 hole.
A 5/8-18 fitting has a 0.625" OD and 18 threads per inch. It will be loose in a 3/8 BSP hole.
If you install this adapter backwards on a Racetech gauge, it will feel very loose.