What Size Drill Bit for a 1/8” NPT Tap?

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The ⅛” NPT tap is a common tap mainly used for tapping thermocouples and boost gauge fitting holes for gauge sensors. ⅛” NPT taps are generally made of steel and are one of the lightest taps. If you want to know what size drill bit for a ⅛” NPT tap is, read further.

What Size Drill Bit for a 1/8” NPT Tap?

A  ⅛” NPT Tap needs a size “R” drill bit or 0.3390 inches in decimals. NPT taps are also called pipe taps and don’t confuse them with NPTF taps which are entirely different and tapered taps like the 8-32 tap.

Drill, hammer, measuring tape and other tools laid neatly on a wooden table

There’s a general oxide taper pipe tap that is generally used for pipe fittings, cutting internal threads, and anything that requires tight fitting.  It works much like the 1/16″ NPT tap with size D drill bit except that it is bigger in diameter.

A ⅛” NPT high-speed steel pipe tap usually have specification similar to this:

  • Type: NPT tap
  • Size: ⅛”
  • TPI: 27
  • Material: HSS (high-speed steel) HSS-E
  • Ground thread
  • Finish: Bright
  • Drill Size 0.332
  • Straight flute
  • Flute Number: 4

Tips for Using Medium Drill Bits

Wear Protective Eyewear

Since I’m dealing with medium drill bits, debris could fly away and land into your eyes. Accidents related to eyes are one of the most common mistakes people do when handling drills. It’s important to protect your eyes from flying debris, which might lead to serious injuries.

Wear safety glasses as a precaution. There is a lot of protective eyewear you can choose from that has different designs. Choose one that fits snugly in your head so it doesn’t slip off as you are drilling.

You can put it in a checklist before drilling so you won’t forget to wear them. 

Use a Test Board

When drilling wood or metal, your medium drill bits can create a splintered hole on one side. This is called a blowout which most amateur DIY workers get often. Blowouts can damage your workpiece affecting its quality. 

Man drilling a hole in the wooden walls

It can sometimes lead to accidents since the drill bursting through the hole will make you lose control when you’re careless.

To stop this from happening, you can use a test or spare board as a cushion to clamp the piece down. The spare board prevents any blowouts since it acts as a cushion when drilling a hole.

Know Your Drill Speed

For medium drill bits, it’s important to know the speed of the drill. Drill bits get hotter the faster it spins. When drilling metal or bricks, most people would set their drills to low speed and pressure. This is to avoid overheating and damage to the drill.

The drill’s speed should be adequate for the size of the drill bits, which is medium. The bigger your drill bits are, the slower the speed should be. For medium drill bits, moderate speed is generally what you want. It helps you drill through metal and bricks faster and with better precision.

Maintaining the right speed helps prevent the drill from overheating. If you see smoke coming from your drill, it’s a sign that you should stop what you’re doing and let your drill cool down.


 ⅛” NPT taps need a size “R” drill bit and can be used for thermocouples. They’re mostly used for tapping fittings and couplings. If you’re going to use them, you have to have a sealer for the threads.