What Size Drill Bit for a 1/4" NPT Tap? [Answered] | SawsHub (2024)

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Leveraging my experience as a seasoned DIY hobbyist, I’ll quickly cut through the confusion surrounding drill bits and NPT taps in this guide. With precision at its core, this piece is about mastering the drill, making your projects smoother. Let’s dive in.

How to Use a ¼” NPT Tap

Before using a ¼” NPT Tap, you need to make sure you have the diameter and length measurements correct so that you know you are using the right drill size.

1. Preparation: Ensure the correct diameter and length measurements for the tap. This will help you choose the right drill size.
2. Finding the Diameter: Look for the fraction marking near the shank of the bit. It’s typically stamped above the thread.
3. Measuring Diameter: If you can’t find the stamped measurement, measure it yourself from one side of the outer thread to the other.
4. Using Calipers:
   • Calipers are useful for additional measurements and are available in most hardware or home supply stores.
   • They look like small flat handles with two jaws; the bottom jaw should slide up and down.
   • To measure the tap, hold the body of the screw between the caliper’s jaws.
5. Making a Test Hole:
   • This step is crucial to avoid drilling imperfect holes in your final project.
   • Any incorrectly sized drill bit can result in an imperfect hole, even with a new drill.
   • After drilling the test hole, measure it and compare the size with the drill bit – they should match.
   • Check the fit of the drill bit in the hole; it should be snug, not too loose or tight.

When to Use a ¼” NPT Tap

Once you have the sample hole and you have determined you have the right drill bit for your tap, you are ready to use the tap and start your project. The ¼” NPT tap is a type of tapered pipe as NPT stands for National Piper Thread.

Tapered pipes are most used for sealing materials which is why they are a top choice for home improvement projects and fastening things onto the wall.

NPT threads can also be used for a wide variety of projects because they are unique and useful. Even though they have a large variety of uses though, the sizing does not have a huge variety.

You have to use the correct size drill bit or you will not be able to successfully finish your project.

You can also read what type of drill bit to use for a #10-24 Tap.

Other Considerations for ¼” NPT Tap

A major consideration you need to keep in mind when using a ¼” NPT tap is that leakage is a common problem.

• Leakage: Leakage can occur if you’re using the wrong size drill bit with a ¼” NPT tap. If you notice any leakage, double-check your drill bit size.
• NPT Taps and Pressure: NPT taps, being a type of pipe thread, may leak under high pressure. Avoid using them for high-pressure projects.
• Sealed NPT: Using a sealed NPT can reduce the chances of leakage.
• Thread Gauges: Trying out different thread gauges can help you find the best fit to prevent leaks. Here are some steps to ensure a good fit:
  1. Hold the gauge against the light to check for gaps between the gauge and threads.
  2. Use an outside diameter (OD) caliper to measure the male and female thread calipers.
  3. Use a seat angle gauge on a flared surface. The centerlines should be parallel with the gauge to prevent leaks.
• DIY Tapping: You can tap the NPT thread yourself by hand tapping the threads into an iron block. This involves running the tap into the hole to the correct depth. There are many online video tutorials available if you need guidance.

Also see my guide on what drill bit to use for a 1/8” NPT tap.

Final Thoughts

Now that you have the drill bit perfect size, you can get started drilling. Make sure you are patient with the process and make sure the size is correct before starting.