What Size Drill Bit for a 4” NPT Tap?

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The 4” NPT tap is one of the biggest pipe taps you can use. 4” NPT taps are generally made with carbon steel or high-speed steel. If you want to know what size drill bit a 4” NPT tap is, read further.

What Size Drill Bit for a 4” NPT Tap?

A 4” NPT Tap needs a size drill bit of 4-¼ inches or 4.25 inches in decimals. NPT taps are commonly called pipe taps. They are different from your NPTF taps and Tapcons like the 1/4 Tapcon.

Drill and other repair tools like screws and nails stored in a container

The 4” NPT tap is generally used for heavy-duty threading and cleaning much like the 3-1/2″ NPT. They’re rarely used for cutting or threading into plastic. NPT threads require a sealer like Teflon tape or pipe compound in order for them to work properly.

4” NPT tap (carbon steel) are generally identified by the following specifications:

  • Size: 4-8 NPT
  • Material: carbon steel
  • Finish: Uncoated (bright)
  • Style: NPT
  • Cutting direction: right hand
  • Tap size: 4 NPT8
  • Number of flutes: 8
  • Thread length: 2-¾ inches
  • Overall length: 7 inches
  • Shank size: 3 inches
  • Matching drill bit: 4-¼ inches
  • Flute type: NPT

Tips for Using Large Drill Bits

Make Use of the Hammer Action

Some drills have the hammer action feature. This allows you to “punch” on a hard surface repeatedly. For example, if you’re drilling through concrete, you might want to turn on the hammer action to make the drilling efficient.

Hammer action is a great feature, especially when using large drill bits for concrete. Concrete is hard to drill without the right equipment.

The hammer action makes a vibration that transfers to the drill bit that, gives it better penetration. This will lessen the time for you to drill on concrete and be less taxing on the drill bit since it’s supported by the hammering motion to penetrate the surface.

X Marks the Spot

When you’re drilling a pilot hole, it’s recommended to tape an X over the spot. You can use painter’s tape to mark it. The tape helps reinforce the wood or other surface to prevent cracking.

Man wearing black overalls doing a thumbs up while holding a cordless drill

Cracking often happens with large drill bits since I’m handling big drilling projects. Reinforcing the surface isn’t bad when drilling since you don’t want cracks forming within your workpiece.

Aside from reinforcing with tape, you can also avoid cracking by drilling at the right depth. Drilling too much may cause cracking, so you must measure how deep you are drilling.

Avoid Jamming Your Drill

One common thing when using large drill bits is jamming. It’s a common mistake amateur drillers make since they usually apply more pressure when drilling, which leads to jamming. You should learn how to avoid jamming your drill.

When you jam your drill, you might have to replace the tip, and your drill’s lifespan will shorten. You want to avoid spending more than you should when it’s avoidable. One thing you can do is not apply too much pressure when drilling.

Also, feel the drill and spot when you think it is slowing down. Pull it out when you think it’s slowing down and let it cool for a while, cleaning off any debris stuck in the drill.


4” NPT taps are among the largest drill bit size. They can be used for a wide range of ferrous and non-ferrous materials. They’re mostly used for cleaning and re-threading heavy-duty materials because of their large size.