Tap Drill Size for NPT and NPTF Taper Threads

You got the job. You ordered and received material. You ordered and received taps and gages from North American Tool. The job is now out in the shop to be manufactured and the first operation is to produce a hole before tapping. How do you determine what tooling to do that with?

Holes before tapping pipe threads are produced either straight or tapered. The choice of method is based on one’s experiences and preferences. I’ve heard all kinds of pros and cons to doing it one way or another, from the added cost of a taper reamer and it’s additional operation, to a tapered hole resulting in better finish and reduction in tapping torque, to a straight hole eliminating tap starting issues as well as avoids creating a work hardening hole surface in some materials. There are many more reasons to support this one way or the other and as I said, it’s based on experiences and preferences.

In either case, the actual hole size, tapered or straight, needs to be small enough so that the tap produces 100% of the product thread form. This includes the minor diameter crest flats and ensures the minor diameter to pitch diameter concentricity.

In addition to the proper diameter, the straight drilled hole, whether prior to or without taper reaming, needs to be a length that contains enough fully formed threads to meet the requirements.

The actual hole size prior to tapping is dependent on a number of variables but should approximate the diameter Ko, (Basic Minor Diameter at Small End of the External Pipe), listed in ASME B1.20.1. You may need to vary your hole sizes from Ko slightly smaller or larger so the tap can cut an acceptable pipe thread with the required thread form. This can be dependent on the material being cut, the machining operation, the style of drill used, or any combination of these. Therefore, it is advisable, that if you are going to run a large quantity of parts, to run a trial in the same material and machine that you intend to use to produce the final parts.

The table below shows, the suggested drill sizes for use with or without a taper reamer, how deep to drill to accommodate standard taper pipe taps, and reference diameters at the top of a reamed hole. If your application requires a hole length shorter than the length listed, a short projection pipe taps must be used. All you have to do is give us a call and we can design a tap with the proper projection.

The best size hole to achieve best tap performance, and product threads within specifications, is usually determined by trial. Some drill sizes listed in the table may not be available as stocked drills. Larger sizes are generally bored or reamed, and a good starting point is to bore to Ko, (Basic Minor Diameter at Small End of the External Pipe).