im installing a pyro probe size i 1/4 npt what size tap do i need

 

The answer to your question is in your question

 

NPT is "National Pipe Thread". That's different than NC (National Coarse) or NF (National Fine). Anyway...the threads are tapered so the further you thread it together to tighter it gets...in a hurry. You can get a tap at most hardware stores.

 

thanks guys thats wat i thought but some guys told me different so i had to ask just in case

 

i got mixed up i asked for tap size but what i ment was drill bit size anyway i got the answer with the purchase of tap its 7/16 thanks anyways guys

 

Just don't go too far with the tap or you'll be welding the thermocouple to the manifold like I had to do OOPS!!

 

What

 

NPT rule of thumb...
run tap in till 5 threads left ABOVE surface (5 threads on tap remaining)

There are NPT reamers that add the taper to drilled hole prior to tapping, Not required on smaller sizes but they do ease the effort and improve the threads.

another trick to reduce chips falling into manifold, is to coat tap well with grease, and back it up frequently to get a better thead (especially important with cast iron, as chips are crunchy and can jam up the works (potentially breaking tap on removal))