This 3/8 BSP - 5/18-18 Temperature Bulb Adapter replaces the 3/8 BSP - 3/8 BSP adapter supplied with Racetech gauges. 5/8-18 thread is now a more common size for mechanical temperature bulbs.
5/8-18 and 3/8 BSP are hard to tell apart by sight.
A 3/8 BSP fitting has a 0.66" OD and will not quite fit in a 5/8-18 hole.
A 5/8-18 fitting has a 0.625" OD and will be loose in a 3/8 BSP hole.