Oil pressure switch thread ?

[grewth]

Can someone please tell me what the thread size/form is for the series three type oil pressure switch ?
Yeah there's got to be something where I can look this up like a parts catalogue or whatever, but it's been a long day, and I'm too tired to rummage through a pile of paperwork.
I want to plumb in a Smiths oil pressure gauge, so I'll be needing a T piece etc etc

 

[kermit_rr]

I think it's 3/8bsf.
Power number for the switch is 90519864 and was used on all the 2.25 and 2.6 engines