The standard threading size at the end of a temperature gauge capillary tube is 5/8-18 UNF (Union Nut).
The 1/2 A Bulb Adapter Nut is a replacement part for the standard adapter that ships with every new 20, 25, A20 or A25 Series temperature gage.
The inner threads of this 1/2 A Bulb Adapter Nut measure 5/8-18 UNF, to make the correct connection to the stationary end of the temperature gauge capillary tube. The outer threads measure 1/2-14 NPT allowing you to make a non-standard connection to match your application, which is generally a connection to the coolant bath of your engine block.
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