drill and tap size???

[nwcove]

im getting ready to drill and tap a bypass cover for a conversion to single line fuel pump, from what i can tell i need to use a 15/32 bit to drill a hole for a 1/2 npt thread? does that sound right?

 

[minuteman62-64]

Re: drill and tap size???

My chart shows 1/2 x 14 npt requires 23/32 drill with an alternate size of 3/4.

 

[minuteman62-64]

Re: drill and tap size???

Another thought. Not sure exactly what you are doing - but, 1/2 NPT (National Pipe Thread) is thread for 1/2 inch pipe - which is about 3/4 inch OD. Are you sure that's what you will be using?

1/2 x 13 NC (National Coarse) tap (for a 1/2 inch bolt) takes a 27/64 hole - which sounds more like what you are looking at. Are you installing a bolt or a piece of pipe?

 

[nwcove]

Re: drill and tap size???

My chart shows 1/2 x 14 npt requires 23/32 drill with an alternate size of 3/4.

thats why im leary of drilling the hole!! the fitting i have will fit in a 1/2 " hole on a gauge, it threads loosely into a 31/64 hole in the same gauge, and the 15/32 hole seems like it would be the one for the right drill size for threading to accept a 1/2" npt fitting. ??? i do see there are different sizes for different materials, and the charts show different things.

 

[minuteman62-64]

Re: drill and tap size???

See http://www.newmantools.com/tapdrill.htm for both tapered and straight pipe thread specs. If your fitting will fit into a 1/2" hole it is not a 1/2 inch pipe fitting. Pipe fittings are based in internal diameter. The threads are on the outside diameter - for example 1/2" pipe is about 3/4" in outside diameter.

A 1/4" x 18 npt tap takes a 7/16" hole (real close to 15/32"). 1/4" pipe would fit loosely in a 1/2" hole (and maybe a 31/64 hole).

How did you determine you needed 1/2" npt tap?

 

[nwcove]

Re: drill and tap size???

i had a couple of fittings laying around and just measured them, but i did pick up another one today at uap, and its the same as the others as far as size goes, but does say on the package, 1/4" MNPT.

 

[minuteman62-64]

Re: drill and tap size???

Ah. The old pipe-thread/fastener-thread confusion I've made many return trips to Home Depot as a result of forgetting the difference (how can that be 1/4 inch thread when it's almost 1/2 inch in diameter ... ). I still have to grab a fitting with the size marked on it and do a trial fit just to make sure I get the correct hardware/drill/etc.

Yep, you've got a 1/4 inch pipe fitting and you can drill the hole for the tap at either 15/32" or 7/16".

 

[boobie]

Re: drill and tap size???

Shouldn't you be using a 1/8 " pipe fitting on that ??

 

[nwcove]

Re: drill and tap size???

Shouldn't you be using a 1/8 " pipe fitting on that ??

after having a good look at everything, i did go with a 1/8" pipe fitting. the 1/4 " was way to big!!