What Size Drill Bit for 1/8 NPT Tap?

In this article, I’ll help you to figure out the right type and size of the drill bit for your 1/8-inch NPT Tap.

NPT taps have tapered threads and are available in various sizes. As an experienced handyman, I know the importance of picking the right size drill bit to make a perfect hole for the 1/8 NPT taps. Otherwise, the depth and diameter of the hole will not allow the NPT tap to fit well.

Check the correct metric table to establish the right size drill bit for a 1/8 NPT tap. The “R” drill bit with size 11/32″ is the ideal drill bit size for 1/8″ NPT taps installation.

I will cover more details below. Let’s go in-depth.

The Correct Size Drill Bit for NPT Taps

correct size drill bit for NPT taps chart

Using a correct size drill bit to install a 1/8 NPT Tap is vital. Otherwise, the 1/8 NPT tap won’t hold. It may even detach from a material and cause damage.

So, what is the right size drill bit to use?

You must go for the 11/32, “R” drill bit when using your 1/8 NPT tap.

1/8 NPT Taps

NPT (National Pipe Thread) taps often re-create damaged threads or make new ones in pipes.

For NPT, the actual threaded section is tapered. The numbers don’t correspond to size on inches because they (NPT Taps) are based on pie sizes and pipes. (1)

In that vein, a 1/8-inch NPT tap’s diameter is not 0.125 inches. It is based on the thread on the end of a piece of a 1/8-inch pipe. Hence it is approximately 0.405 inches in “big” diameter.

The threads, pipe ends, and holes of 1/8 NPT Taps are tapered. Tapering makes them leak less when they are put together.

So, if you put some tape or pipe dope on your 1/8 NPT tap items and tighten them up firmly, you won’t see any leakage. (2)

Important Notes: Do not tap a 1/8-inch NPT tap like you would do a normal tap. It should not be tapped down or through in a material. Just tap it enough to obtain a nice taper for a fitting.

I recommend tapping it to 3/4 of an inch. Tapping the whole length will enlarge the taper, making it too big.

How to Drill a 1/8 NPT Tap

Drilling a 1/8 NPT tap is not difficult:

Step 1: Mark the Spot You Want to Drill

mark the spot for drilling
Video | DIY KAI

You can use any applicable tool like a marker to indicate the spot.

Step 2: Get the Right Size Drill Bit

As discussed, get an “R” drill bit – i.e. an 11/32-inch drill bit.

getting the right size of the drill bit
Video | DIY KAI

Step 3: Drill a Hole

Gently drill the spot you marked in step one. Since you are using an 11/32 drill bit, the depth and diameter of the hole will be ideal for the 1/8 NPT tap, making it fit snugly into the hole.

Step 4: Install the 1/8 NPT Tap

Lastly, fit the NPT tap on a drill bit and install it on the material. While installing, tap it to 3/4 of the threaded portion as aforementioned.

1/8 NPT tap
Video | DIY KAI

Wrapping Up

Having the right drill bit is crucial when using a 1/8 NPT tap or another NPT tap size. Doing so enables a perfect hole and installation of the tap. You can always refer to the metric table if you forget the drill bit size to use for a given size of NPT tap.

Take a look at some of our related articles below.

(1) pie sizes – https://www.kingarthurbaking.com/blog/2019/05/28/the-best-pie-pan-youll-ever-own
(2) leakage – https://www.sciencedirect.com/topics/earth-and-planetary-sciences/leakage

Video Reference

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.