What is this thread?

[iMacThere4iAm]

Good evening folks, another identification question for you...
I have this oil pressure gauge but I'm not sure what thread it uses on the connector. I guess it's either 1/8" NPT or 1/8" BSP but I'm not sure how to tell since I don't have a thread gauge. The two standards are too close to reliably tell apart with a vernier gauge.

 

[Azazello]

Find a couple of nuts of the possible thread and look for the right one?

 

[DavidN]

I think it is no NPT thread as it looks perfectly straight to me.

The NPT is a conical, self sealing thread used for drain plugs etc.

Here is one for example: