What Size Drill Bit For a 1/4″ 20 Tap? [Answered]

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Ever wonder about the right drill bit for a 1/4″ 20 tap in your DIY project? Precision is key for this, especially when it comes to tapping new threads into metal, where using the right size drill bit makes all the difference. I’ll share everything you need to know below:

What Size Drill Bit for 1/4″ 20 Tap?

Use a #7 drill bit with a diameter of 13/64″ (0.203″) as the best choice for 1/4′′ 20 UNC tap threads. Use lubricant to avoid overheating and potential damage to either the bit or the material you’re working with.

Screw SizeDrill BitDrill Bit Size
1/4″ 20 Tap#713/64″
1/4″ 28 Tap#37/32″

Tapping threads is a fundamental aspect of many DIY projects, but it’s not as straightforward as it seems. A precise balance must be struck when selecting the hole size for the tap to work effectively.

  • A small hole might cause the tap to protrude too much, making the bolt difficult to screw in.
  • Conversely, a hole that’s too large won’t secure the bolt properly due to poor thread engagement.

The material hardness significantly influences the tap drill size. Typically, a standard tap drill of 75% is adequate for most materials, but harder substances like steel, iron, or stainless steel might require a 50% thread engagement for a tighter connection.

Cordless drill

When dealing with softer materials such as aluminum, plastic, or brass, a standard tap drill is usually sufficient. However, always refer to the manufacturer’s instructions to ensure the correct size tap drill for your particular scenario.

In the case of harder materials, finer threading of 50% might be desirable. This adjustment can impact the drill bit size. For instance, when using a 50% tap drill for a 1/4″ 20 tap, a 7/32” drill bit is advisable.

Remember, the required drill bit size varies based on the threads per inch for 1/4″ screws, as follows:

Screw SizeThreads Per InchDrill Bit Size
75% Thread
Drill Bit Size
50% Thread
1/420#77/32
1/428#3#1
1/4327/32#1

As you can see, there are some commonalities, but also some differences in what size drill bit to use when taking threads per inch into account. In general, the higher threads per inch, and the more fine the threads are, the larger (slightly) your drilled hole that you will be tapping should be.

Selecting what size drill bit for 1/4 20 tap

Other Considerations for Drilling 1/4″ 20 Tap

When you drill any type of material for tapping, be sure to decide beforehand whether you will be tapping with 50% or 75%. This may change depending on the hardness of the material, so if you are able to drill through a scrap piece first, that may give you a better idea of how you should tap.

How to Drill for Tapping

When you have decided on which size drill bit to use, you will then need to move on to the drilling process. When drilling into metal you will want to mount that material with at least two clamps to reduce the chance of the metal spinning and possibly injuring you. Wear safety goggles and consider thick gloves as well.

You will first need to mark the area to be drilled. Use masking or painters tape not only to leave a nice area to mark, but also to help keep the step drill bit in one place. Using a center punch with a hammer will help as well by leaving a divot for the bit to rest in. If drilling into a thin or flexible material, be sure it has solid backing so you do not deform it.

Make sure your drill is completely perpendicular at 90 degrees to the material you are drilling. Any angle created may create problems with both tapping and screwing in bolts. If possible, use a drill press or a guide for your handheld drill. Use cutting fluid and regularly sharpen your drill bits, make sure to do your best to keep the drill bit from getting too hot.

How to Use a 1/4″ 20 Tap

The purpose of tapping is to create a threaded hole for a screw or bolt to enter, so always have the bolt ready for a visual comparison that you are headed in the right direction.

Close up of a drill bit

After the initial hole is drilled, you will first want to de-burr the edge of the hole and remove any sharp edges. Additionally, adding a chamfer at this point will make the tapping go easier. It is easy to chamfer with specialized drill bits, but you can use normal drill bits as well.

Be sure to clean out the chips, dust, and other debris that may have accumulated in the hole or around it.

When setting up the tap, it helps to use a 90° tap guide. When I add so much downward pressure to the hole and tool that I am working on, it is easy to lean to the side, ending up with a crooked tap. Use the guide and take your time.

Pecking

One manner of tapping threads into a hole is called “pecking.” This process involves going forward a short distance, and then backing off. It helps keep your tap or drill bit from overheating or even breaking under the extreme torque used to get through metal.

The most common form of pecking involves turning a full turn of the tap in, and then a half turn out. To be even more careful, you could go a full turn, remove the tap completely, clean out the hole, go in two turns, and repeat until the hole is fully tapped. This takes quite a long time but will create the most even thread tapping.

See my article on the right size bit for a 10-24 bit.

How to Choose and Use Drill Bits | Ask This Old House

Conclusion

Selecting the correct size drill bit for a 1/4″ 20 tap depends on the coarseness of the threads you will be tapping, which is in turn based on the hardness of the material you will be drilling through. Make a decision beforehand and test with scrap material if available so you are able to have the right tools on hand.

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.