Drill size to use for 1/8 NPT tap?

[GrimJack]

The logical choice is that you use a 1/8 drillbit for the initial hole if you're using a 1/8 NPT tap.

Is the logic correct?

 

[dav_dman]

The logical choice is that you use a 1/8 drillbit for the initial hole if you're using a 1/8 NPT tap.

Is the logic correct?

google drill size tap size.

drill size R, which is 0.339. 5/16 and hold it steady will get you there, don't waller it out.

 

[GrimJack]

Thanks for the info. Is it just me, or does this seem overly complicated to everyone?

 

[wingman]

haha NPT sizes were always confusing to people...why they just can't do the regular thread size and a "taper" note or something always bugged me.

 

[GrimJack]

Is that what NPT means? Tapered? I was really wondering why the hell there was Metric, Imperial, AND NPT.

 

[dbsupra90]

npt = national pipe thread. it is 1/8 x 27 and yes it is tapered.

on a side note, bspt/pt = 1/8 x 28 and not interchangeable w/ npt.

 

[frank]

It's not so complicated, just seems that way because the 1/8 refered to in the NPT (National Pipe Taper) designation is the nominal inside diameter of the pipe, no reference at all to the thread diameter. (Pipe is measured ID to 12" diameter, then it switches to OD measurement.) Note I said nominal ID. The OD is constant, but a schedule 80 (heavy wall) pipe will have a smaller ID than a schedule 40 pipe, due to it's thicker walls.
I use a 21/64 bit for a 1/8 NPT tap.

 

[aljordan]

Vermont American has always listed drill sizes on the pacakging of their taps. They figure you're going to get both items if you need a tap.

 

[dbsupra90]

It's not so complicated, just seems that way because the 1/8 refered to in the NPT (National Pipe Taper)

npt = national pipe THREAD not taper