Wiper vacuum pickoff thread size

[1939mars]

I want to plug the vacuum pickoff from the intake manifold on my '31 sport coupe to see if I have a vacuum leak in the wiper system. I would really like to not mess up the threads on the manifold with the wrong type of plug. I have been told that it is 1/4 in pipe thread. Can anyone confirm or know the correct size?

Thanks,
Scott

 

[Bob C]

1/8" pipe thread.

 

[katy]

Aka: 1/8-27 npt