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Does anyone know the thread size for the oil pressure sender that is on late 70's gm engines? Is it npt or is it bspt? (P.s. I got a 1978 pontiac sunbird with the 2.5 iron duke... a low budget turbo project.)

1/8" NPT

Welcome aboard Boosted! Sorry I some how missed this thread. I don't know for sure, but as I recall it should be easily visible [if yours isn't the cross flow head] on the passenger side of the engine near the lifter cover. But all of these at that time would have been NPT on that engine [based off of the "Chevy II" which is a short Inline 6] and I remember those as 1/8NPT. 1/8NPT tap drill is "Q" about 21/64 so the outside of the switch will measure about 3/8" dia. or slightly larger. 1/4NPT tap drill is 7/16" so its' outside dia. will be just over 1/2" Sorry I cant give you an exact as the switch isn't usually made from pipe and will have a rolled thread so it could be made to a number of sizes. Harry

Harry Bitched wrote: Damn it Spencer you Treed me again!

I'm having problems sealing the fitting going into the block. How should I seal it properly?

How to seal the sender has an "it depends" answer. If it's the standard h-body 3 wire fuel pump control one, go ahead and use the sealant of your choice; liquid pipe sealant, teflon tape, or even rtv. Ditto for a mechanical gauge line adapter. But if it's a single wire "idiot light" sender, they are designed to be installed dry so there is a good ground connection to the block.
Whatever it is, it has to be TIGHT.