What Size Drill Bit for a 3-1/2” NPT Tap?

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The 3-½” NPT tap is a carbon steel pipe tap mainly used for cleaning or re-threading damaged or jammed pipes. 3-½” is one of the biggest NPT taps around. If you want to know what size drill bit for a 3-½” NPT tap is, I got you covered.

What Size Drill Bit for a 3-1/2” NPT Tap?

A 3-½” NPT Tap needs a size drill bit of 3 ¾ inches or 3.75 inches in decimals. People often confuse NPT taps with NPTF taps and Tapcons like the 3-16 Tapcon which are entirely different as NPT taps are considered pipe taps.

Stainless drill bits stored upright in an opened box

The 3-½” NPT tap is generally used to clean or re-thread damaged or jammed pipe threads like the 2-1/2″ NPT with bit size of 2 5/8″. If you plan to use NPT threads, do know they require a sealer like Teflon tape or pipe compound.

To know if you are using a 3-½” NPT tap, check the following specifications if it matches:

  • Size: 3-½-8 NPT
  • Material: Carbon Steel
  • Finish: Uncoated (bright)
  • Style: NPT
  • Cutting direction: Right hand
  • Tap size: 3-½ NPT8
  • Number of flutes: 8
  • Overall length: 6-½
  • Matching drill bit: 3-39/64
  • Flute type: NPT

Tips for Using Large Drill Bits

Monitor Your Progress When Drilling

As you are drilling, check your work to see if there are any adjustments you need to do. If there are lugs or jams, remove it and clean them up. You can use the bit to clear the hole from debris.

Make it a habit to pull out the drill bit every now and then and blow it off. This is important when you are drilling wood. Any flakes or chips of wood can get stuck in the bit.

The flakes and chips can make your drill bit heat up faster. This may be unnoticed by you. When your drill bit gets hot, it will function effectively less.

Learn the Proper Safety Precautions

When handling large drill bits, it’s important to practice safety to avoid accidents. You don’t know what might happen when drilling, or you might have overlooked something.

Green and black cordless drill placed in a table

That’s why it’s not bad to learn about safety practices when drilling. Practice them and make it a habit when working on a drilling project.

One thing you can start with is wearing protective gear such as:

  • Drilling hand gloves
  • Masks
  • Safety glasses or goggles
  • Tight clothing

Starting with these and making it a habit to wear one will minimize the chance of accidents. Reading the manual before you start using the drill is also a great habit.

Keep A Spare Set of Drill Bits

You never know when your drill bit dulls or breaks. So having spare sets help replace them immediately. Another benefit of using two sets of drill bits is you can alternate them, so you have a sharp one at the ready.

Some professional drillers even use them at the same time. They use the new set of drills to drill a hole. The premise is that it keeps the sharper set for longer. After starting the hole, they switch to the old and sharpened set to finish the drilling making it less prone to dulling.


3-½” NPT taps require a larger drill bit size since they are large too. They can be used for a wide range of ferrous and non-ferrous materials. If you plan on using one, make sure to follow the correct procedure and sizing.