What Size Drill Bit for a ⅜” Tap?

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If you are new to tapping threads into metal, you might wonder what size drill bit for ⅜” tap. Since it is not an activity often performed in the house, many people are not aware of the technicalities involved in the process. Thus here we are explaining to you every single detail you need to know.

What Size Drill Bit for ⅜” Tap?

The best size drill bit for the ⅜″-16 tap is 5/16″ drill bit. For the ⅜”-24 tap, the 21/64″ drill bit is best, which corresponds to an Q drill bit.

A ⅜” cap comes in two different thread counts: 16 and 24. Thus, the size of the drill bit is also different for each one of them.

Before moving forward, make sure to test on scrap materials before drilling on actual metal.

A selective focus of a drill bit tip


The concept behind tapping the threads is not that hard, but the practical approach might be a bit tricky to handle. To get the best results, you need to be careful about:

  • In the case of tiny holes, the tapping will extend to the center; thus, screwing the bolt into the hole is a challenging task.
  • In case of large holes, the threads will not mate with the bolt.
  • In the case of a soft or average material, the tap drill is 75%, but it will be reduced to 50% for tighter connections if the material is hard.
  • In the case of hard materials and tighter connections, the recommended drill size is 5/16″ inches.

Here is a table with different sizes of drill bits depending on the number of threads per inch.

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Here are some considerations that you need to take before drilling a ⅜ tap. It is necessary as the slightest mistake can result in bad accidents.

  • Do not forget to pre-decide about what material you are working with, like the size for the drill bit changes depending upon the hardness of the material.
  • Try drilling through a scrap piece in the beginning to get a better idea about what you are dealing with.

The Process of Drilling And Tapping

Since you are deciding the size of the drill bit, it is time for some action. First, let’s talk about the process of drilling and tapping.

  1. Begin with fixing the material between two clamps to avoid any spinning or slipping while you drill. If you miss this step, you might end up injuring yourself.
  2. Draw a mark where you want to drill and wear your safety goggles and gloves. It is advised to mask the area with painter’s tape to keep the drill bit in one place.
  3. Ensure the drill is perpendicular, as any different angle will create problems in tapping and screwing the bolts.
  4. Keep an eye on your drill bit, and it should not get too hot. Do not forget to sharpen the drill bit and use cutting fluid.
  5. In the case of hand-held drills, use a drill press or a guide.

Here is the link to a video guide that explains the slightest details about drilling and tapping. This will give a practical understanding of the whole concept.

Using A ⅜” Tap

When you are done drilling a single hole, move on to testing by using a ⅜” tap. It will give you a practical comparison, and it will be apparent if you are headed on the right path.

  1. After the drilling is done, start with deburring the edge and removing the sharp edges around the hole.
  2. Now add chamfer by using either specialized or normal drill bits to ease out the tapping.
  3. Make the extra dust and debris inside or around the hole is thoroughly cleaned.
  4. Use a guide while applying pressure on the hole and the material you are working on; otherwise, you can end up with a crooked tap. Make sure you take your time during this critical part.

Here is a link to the study that indicates drilling threaded holes and the whole history behind it.

Person holding a hand drill for tapping metal


Pecking is defined as one of the methods of tapping threads. This is how it works:

  • The principal concept behind pecking is leaning forward to a short distance and then coming back up.
  • This process is excellent in protecting your drill bits from overheating, and it makes sure there is no extra torque that triggers the breaking of the material.
  • The key point behind this process is that a full turn is used to tap in, but then it is turned half around back.
  • Another way to go about pecking is by removing the whole tapping, cleaning the hole, and then going in two turns. The process is supposed to be repeated until the hole is completely tapped.

This is a long process, but it ensures the quality of the tapping.

Frequently Asked Questions

How Long Are Drill Bits Expected To Last?

Each drill bit is expected to drill about 150-250 holes on average. However, the life of a drill bit depends on several factors, including the amount of steel or rebar present in the concrete and what type of sharpness the drill bit has. Life is also dependent upon the age of the concrete we are working with.

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Why Is My Drill Not Able To Penetrate A Hole In The Wall?

Usually, the reason behind the whole not getting penetrated is the rotation of the drill bit. If it rotates in the wrong direction, no hole will be penetrated, but if the resistance happens after the bit has entered the wall, the reason is probably the metal plate.


Even the slightest mistakes can make you end up with crooked tapping, and nobody wants that. Therefore, while tapping, it is essential to be cautious at every step, choose the size of the drill bit carefully depending on the hardness of the material, and never forget to test first and then move forward with the final drilling. 

An expert at home repair, remodel, and DIY projects for nearly 40 years. His first experience came in completely restoring an antique home. Completely redone from the inside out, and restored to its original form, the home is a featured design by renowned Southern California Architect Cliff May, considered to be the father of the California Ranch Home. Now Dennis spends his time on fine woodworking projects and tool comparisons.