[Healeys] Thread size on pipe from oil pump by pass

[richard mayor]

Listmates,
There is a pipe that screws into the bottom of the block on 6 cylinder
engines. It is the oil discharge pipe from the oil pump pressure relief valve.
At first impression it appears to be an American  1/4 x 18 pipe thread.
Although close, it is not.  Does anyone know what the thread pitch is on that
little pipe?  Is it some kind of British pipe thread?

Richard Mayor
BN7L-466  Vintage Racer
Portland, Oregon