What is the thread that is in the block for the oil pressure switch.
1/8" BSP (28 TPI) or 1/8" NPT (27 TPI).

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Find a job you love and you will never have to go to work !

1/8 BSP.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks Kev
That's what I thought it would be being a British Motor car.
However, the T piece I got from Minispares has 27 TPI (NPT). Not only that but the adaptor for the oil pressure gauge is also NPT ?
While their web site doesn't state what thread they are my thread gauge does!
http://minispares.com/product/Classic/E ... 0to%20shop
When I removed the T piece I noticed that the thread looked distorted (27 TPI into a 28 TPI hole)

How can they get this wrong

_________________
Find a job you love and you will never have to go to work !

It has always been 1/8 BSP. NPT is slightly larger in diameter, yes it will (start to) screw in but gets distorted.
1/8 NPT though is the most common thread for oil pressure switches and senders in the world. Minispares only sells stuff they buy, they don't spec it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.