Standard 52mm/2" diameter.
If you have a performance engine and really want to keep in touch with all aspects then a quality means to measure oil temperature is a must. While never stokc, it is almost always found in race applications. The capillary tube connection (Included with the gauge) serves as the temperature sender and requires a mechanical connection to the block or transmission.
You will require a HPS8 union for the sump plug hole if you are fitting this to the gearbox.