What Size Drill Bit For a 1/2″ NPT Tap? [Answered]

Unraveling the intricacies of DIY work can be a daunting task, but not when you’ve got the right guide. As a seasoned DIY enthusiast with numerous successful projects under my belt, I understand how vital it is to choose the correct drill bit for each task.

In this concise guide, I delve into a common, yet often perplexing scenario — selecting the perfect drill bit for a ½” NPT tap.

Here are some common similar sizes and their drill bit sizes to give you an idea of what you might need for your project:  

How to Use a ½” NPT Tap?

The first thing you need to do is find your drill bit diameter to make sure you are using all the correct tools. They are usually stamped near the markings you will find on the bit’s shank. They are always written as a fraction, so they are pretty to spot once you know what you’re looking for.  

The diameter of the screw is measured by looking at the outside thread of one side to the out thread of the other side.  

• If you need to take the measurements yourself, you will need to find calipers. They can be found in most home supply stores and are usually budget-friendly. It will look like a small flat handle with two jaws on the top. The bottom jaw should be able to slide up and down.  
• To measure the screw, place it between the jaws and then press the jaws down to hold the screw in place. Don’t place the thread portion though.  
• The last step is to make a test hole. Using the wrong drill almost always results in an imperfect hole. This can happen even if the drill you are using is new, so don’t be tempted to not measure and possibly use the wrong drill bit size.  

Here are the steps for making a test hole:  

1. Measure the depth of the sample holes. Make sure to tally them with the size of the drill bit.
2. Put the bit inside the sample home and check the fit. The fit needs to be tight and not too loose where the drill can move around.

Knowing the size of drill bits and how to figure them out is important, but knowing more about NPT is also important so you know exactly what tools you are using and how to use them. NPT stands for National Piper Thread and might also be referred to as American National Standard Pipe Thread.  

NPTs have tapered and thread series. Both are used to test pressure-tight sealing and rigidity, and some even test both.  

When to Use ½” NPT Tap?

These tapered pipes are usually used for sealing materials. The tapered qualities of the NPT threads allow them to seal and generate torque when they are pressed and drilled onto flanks. In some cases, they might also be used for transporting materials like gases, liquids, and solids.  

While the unique characteristics of the ½” NPT Tap thread make it useful and extremely durable, it also means you have to take special care when sizing it to make sure you are not making any mistakes.  

All NPT threads should have a taper and center axis that is at an angle of one degree 47 feet 24 inches with each other.  They also have roots and crests to have a flat truncation. Thread per inch is the unit for measuring the pitch of threads and the thread of the angle should always be exactly sixty degrees.

Other Considerations for ½” NPT Tap

The size of the drill bit can be found by subtracting the pitch of the thread from its diameter. This is the easiest way to find the size of the drill bit and you can do it yourself easily at home. If you are having issues, you can always take your drill into the local hardware or home supply store and ask them for advice.  

In general, ½” taps have many different types and the type you have will depend on what size bit you need. For example, a ½”-13 NC Tap needs a 27/64” size. A ½”-24 NS and a ½”-20 NF need a 29/64” size.

You also need to keep in mind that ½” NPT taps can often have leakage issues that you need to bear in mind or you might end up with a messy and incomplete project because of a drill that is leaking and not performing correctly.  

If you do not use the right thread and drill bit size, you will find that the drill leaks more. Make sure you are always checking for leaks between the thread and gauge. This can be easily done by holding it against a light source to more easily detect the leak and where it’s coming from.

If you have an OD caliper, you can also hold it at a slight angle and get a reading to see if the diameters of both sides match. If they don’t, you might have a leak. You can also use a seat angle gauge if you have one. Hold the gauge against the surface of the ceiling and make sure the centerlines are parallel with the gauge.  

Final Thoughts

If you are thinking of using a drilling machine for your next home improvement project, you need to make sure you are always using the right drill bit size or you can find leaks or imperfect holes in your whole. The process of finding the right drill size can be difficult and needs patience, but it will make sure you get don’t create problems with your walls or project.