Question on oil sender on BBC

[ptc]

Ok Im perplexed...

On a BBC block (mines actually a Merlin BBC Clone) right above the oil filter are two threaded holes that look to be about 1/4" NPT. But they arent. In actuality a 1/2"-13 bolt threads into them both. But it seems to me that this is where the Oil Sender unit goes on these engine blocks yes?

So is 1/2"-13 thread the same as 1/4" Pipe Straight thread? or what am I missing?

Im looking to put a tee into this hole with a male thread into the block and then the two runs be 1/8" NPT - does anybody make this sort of tee?

Thank you gents!

sorry for the sideways pic