What Size Drill Bit for 1/4-20 Tap?

Welcome to the world of DIY and precision machining! If you’ve ever wondered, “What size drill bit for 1/4-20 tap?” you’re in the right place. Understanding the correct drill bit size is crucial when creating threaded holes for bolts or screws with a 1/4-20 thread pattern.

In this guide, we’ll demystify the process, ensuring that you choose the perfect drill bit size for your 1/4-20 tap, ensuring a seamless and secure fit for your fasteners.

Table of Contents

Understanding 1/4-20 Threads

1/4-20 threads refer to a specific type of thread size and pitch commonly used in fasteners, such as screws, bolts, and nuts. Let’s break down what the numbers in “1/4-20” mean:

  • 1/4: This part represents the nominal diameter of the threaded portion in inches. In this case, it’s 1/4 of an inch. So, the fastener’s diameter is 1/4 inch.
  • 20: This part indicates the number of threads per inch, also known as the “thread pitch.” In this case, there are 20 threads per inch.

So, seeing “1/4-20” means you have a fastener with a 1/4-inch diameter and 20 threads per inch.

Here are some key points to understand about 1/4-20 threads:

Common Usage

1/4-20 threads are frequently used in various applications, such as assembling furniture, securing automotive parts, and construction.


To use a 1/4-20 threaded fastener, you need matching nuts, bolts, or screws with the same thread size and pitch. For instance, a 1/4-20 bolt requires a 1/4-20 nut.

Threaded Length

When specifying or selecting fasteners, consider the threaded length. It should be sufficient for your application. The threaded size is the part of the fastener with threads, and it’s essential to ensure it’s long enough to engage with the other components securely.

Torque and Tightening

Be mindful of the torque required to properly tighten 1/4-20 fasteners. Using the appropriate torque settings helps prevent over-tightening, which can damage threads, or under-tightening, which can result in loose connections.

Thread Types

1/4-20 threads can come in different types, such as UNC (Unified National Coarse) or UNF (Unified National Fine). UNC threads have fewer threads per inch, making them coarser, while UNF threads are finer, with more threads per inch. Make sure to select the appropriate thread type for your specific application.


These threads can be found in various materials, including steel, stainless steel, brass, and more. The choice of material depends on the environment and the application’s requirements, such as corrosion resistance or strength.

Factors Affecting Drill Bit Size Selection

The right drill tool size is vital for precise and efficient drilling operations. Several factors influence the choice of drill bit size, including:

The material being drilled

The type of material you’re drilling into significantly impacts the drill bit size selection. More complex materials require more significant and robust drill bits, while softer materials can be prepared with smaller bits. For example, drilling through wood, plastic, and metal will require different bit sizes.

Hole diameter requirement

The hole size you need to create determines the drill bit size. Measure the desired hole diameter accurately to select the appropriate bit.

Screw or fastener size

When drilling pilot holes for screws or anchors, match the bit size to the fastener’s size. A too-large bit can weaken the material and reduce the fastener’s grip.

Drill type and chuck size

Your drill should have a chuck that accommodates the selected bit size. Ensure compatibility between the bit’s shank diameter and the drill’s chuck capacity.

Depth of the hole

The depth of the hole you need to drill can influence bit selection. Longer drill bits or extensions may be necessary for deeper holes.

Drill bit material

Different materials, such as high-speed steel (HSS), cobalt, carbide, and diamond, are used to make drill bits. The material of the drill bit can affect its performance and durability, especially when drilling through hard or abrasive materials.

Workpiece thickness

The thickness of the material you’re drilling through can determine the length of the drill bit required. Ensure the bit is long enough to penetrate the entire workpiece.

Desired finish

If you need a clean and precise finish, choose a bit size that matches your requirements. Smaller bits can produce finer details, while more significant bits may leave rougher edges.

Speed and feed rates

Proper speed and feed rates for the drill bit size are crucial to prevent overheating and premature wear. Consult the manufacturer’s recommendations for the specific bit material and application.

Drilling technique

How you plan to use the drill, such as hand drilling, a drill press, or a hammer drill for masonry, can influence the bit size choice. Some techniques may require specialised bits.

Project specifications

Always consider the requirements of the project, including any blueprints or plans. Follow the specified drill bit sizes if provided.

Calculating the Correct Drill Bit Size

The correct drill bit size for a specific task involves basic mathematical calculations. Here are the steps to determine the appropriate drill bit size:

Determine the Hole Diameter

First, you need to know the hole diameter you want to create. Measure this dimension accurately using a calliper or ruler if possible.

Choose the Material

Identify the material you’ll be drilling into, which will influence the recommended drill bit size. Different materials have varying requirements.

Refer to a Drill Bit Size Chart

There are drill bit size charts available online, in tool catalogues, or from manufacturers that provide recommendations for specific materials and hole sizes. These charts match drill bit sizes to standard fastener sizes or material types. For example, a typical chart might recommend a 1/8-inch bit for a #8 screw in wood.

Calculate the Bit Size (Optional)

If you can’t find a suitable chart or want to calculate the bit size yourself, use the following formula:

javaCopy code

Bit Size = Hole Diameter + (Clearance or Tolerance) 

The “Clearance” or “Tolerance” factor is typically a small amount added to the hole diameter to ensure the desired fit or clearance. It depends on the material and the application. For example, in woodworking, a support of 1/32 inch or 1/16 inch may be added to the hole diameter to allow for wood expansion.

Check the Drill Bit Set

If you have a drill bit set, it may come with labelled bits for standard sizes and materials. Select the closest matching bit to your calculated or recommended size.

Test on Scrap Material

When in doubt or working with critical projects, testing your drill bit size on a scrap piece of the same material is an excellent idea to ensure it’s the right fit before proceeding with the actual work.

Adjust as Needed

Depending on the application, you may need to adjust the bit size up or down slightly to achieve the desired result. Factors like material hardness, fastener type, and personal preference can affect the final choice.

Joint Drill Bit Sizes for 1/4-20 Taps

When tapping holes for 1/4-20 threads, it’s essential to use the appropriate drill bit size to ensure a proper fit and thread engagement. The 1/4-20 thread size is used in various applications. To tap a hole for 1/4-20 threads, you typically use a drill bit size that matches the recommended tap drill size.

The recommended tap drill size for 1/4-20 threads is a #7 drill bit, with a nominal diameter of 0.2010 inches or approximately 5.11 millimetres. However, the nearest standard drill bit size commonly used is a 13/64-inch drill bit, with a diameter of 0.2031 inches or about 5.16 millimetres.

So, when tapping holes for 1/4-20 threads, it’s common to use a 13/64-inch drill bit to create the appropriate pilot hole for threading. This drill bit size ensures a good balance between thread engagement and the strength of the material being tapped.

Importance of testing the drill bit size before tapping

Testing the drill bit size before tapping is a crucial step in the tapping process, and it serves several important purposes:

Ensures Proper Thread Engagement

Using the correct drill bit size creates a pilot hole that allows the tap to engage the material properly. If the gap is too small, the fixture may struggle to cut threads, potentially leading to a damaged tap or poorly formed lines. Conversely, the cables may only hold securely if the hole is manageable, resulting in solid connections.

Prevents Material Damage

Tapping involves cutting threads into the material, which can generate significant stress and friction. If the hole is improperly sized, it can cause excessive heat and friction, leading to material damage, such as overheating, warping, or even breaking of the tap.

Enhances Thread Accuracy

An appropriately sized pilot hole helps ensure the threads are formed accurately and consistently. This is especially critical when working with precision parts or assembling components requiring a tight fit.

Reduces Tap Wear

Using the correct drill bit size minimises the wear and tear on the tap itself. If the hole is too small, the fixture must work harder to cut threads, potentially leading to tap breakage or reduced lifespan. Conversely, if the gap is too large, the tap may only last briefly due to the increased stress on its cutting edges.

Saves Time and Resources

Testing the drill bit size before tapping helps avoid costly mistakes and rework. Correcting improperly tapped holes can be time-consuming and may require additional materials and tools. It’s more efficient to get the sizing right from the start.

Improves Thread Quality

An appropriately sized pilot hole creates clean, well-defined threads. This is especially important in applications where thread integrity is critical, such as aerospace, automotive, or machinery manufacturing.

Enhances Safety

Using the correct drill bit size reduces the risk of accidents or tool failures. A drill bit that matches the tap size ensures more predictable and controlled drilling and tapping operations.

Achieves Consistency

Consistency in hole size and thread formation is essential, particularly in mass production or assembly line settings. Properly sized holes help maintain uniformity across all parts.

Testing the drill bit size before tapping is essential in any tapping operation. It helps ensure the integrity of the threads, prevents material damage, extends tool life, saves time and resources, and contributes to overall safety and quality in manufacturing and assembly processes.

Tips verifying that the chosen drill bit suits the 1/4-20 tap

Verifying that the chosen drill bit is suitable for the 1/4-20 tap is essential to ensure a successful tapping operation. Here are some tips to help you confirm the compatibility of the drill bit:

Check Tap and Drill Bit Charts

Refer to tap and drill bit charts provided by tap manufacturers or machining references. These charts typically specify the recommended drill bit size for various tap sizes. Ensure that the recommended drill bit size for 1/4-20 threads matches the drill bit you intend to use.

Use a Drill Gauge

Drill gauge sets have labelled holes for different drill bit sizes. You can verify the size of your drill bit by inserting it into the corresponding labelled hole. You’re on the right track if it matches the size recommended for 1/4-20 threads.

Measure the Drill Bit Diameter

Use a calliper or micrometre to accurately measure your drill bit’s diameter. For a 1/4-20 tap, the drill bit should have a diameter of approximately 0.2010 inches or 5.11 millimetres. Ensure that your drill bit falls within this range.

Check the Tap’s Documentation

If you have the tap’s packaging or documentation, it may provide information about the recommended drill bit size for 1/4-20 threads. Ensure that the drill bit you intend to use matches this recommendation.

Perform a Test Fit:

  1. Before tapping the actual workpiece, perform a test fit in a scrap piece of the same material.
  2. Insert the tap into the hole created by the drill bit, ensure it fits snugly and starts threading smoothly.
  3. If the fit feels too tight or loose, reevaluate your drill bit size.

Consider Material Properties

Remember that the tapped material can influence the drill bit choice. Softer materials require a slightly smaller drill bit, while stiffer materials need a larger one. Consult material-specific machining guides if necessary.

Inspect the Drill Bit Shank

Ensure that the shank of the drill bit matches the chuck size of your drilling machine. An incompatible shank size may cause slippage or other issues during drilling.

Use Dedicated Tap Drill Sets

Tap drill sets are available, which include both the tap and the corresponding drill bit. These sets are designed to ensure compatibility and are convenient for tapping operations.

Double-Check Measurements

Always double-check your measurements and verify that you’ve selected the right drill bit before proceeding with the tapping operation. Correcting errors at this stage is more accessible than after you’ve started tapping.

Read the Manual

Always read and thoroughly understand the user manual that comes with your power drill. Familiarise yourself with the drill’s features, proper usage, and safety guidelines provided by the manufacturer.

Wear Appropriate Safety Gear

  • Wellbeing glasses or goggles to safeguard your eyes from flying debris.
  • Hearing protection if the drill is loud.
  • Dust mask or respirator when drilling in materials that produce dust or fumes.

Inspect the Drill

Before use, check the drill for any visible damage, loose parts, or frayed cords. If you find any issues, wait to use the tool until it’s repaired or replaced.

Ensure that the drill’s chuck and bits are securely tightened.

Secure Workpiece

Clamp or firmly secure the workpiece to prevent it from moving during drilling. This guarantees exactness and lessens the gamble of the drill bit binding and causing kickback.

Select the Right Bit and Speed

Pick the proper boring apparatus for the material you’re working with.

Set the drill to the correct speed and force settings for your task. High rates suit drilling, while lower speeds are better for driving screws or other fasteners.

Start Slowly

Begin drilling slowly to ensure control and prevent the bit from wandering or snagging on the workpiece.

Maintain a Firm Grip

Hold the drill with both hands for stability and control. Keep one hand on the handle and the other on the auxiliary handle if provided.

Keep the Drill Aligned

Use Proper Technique

Apply steady and even pressure while drilling. Let the drill do the work; excessive force can lead to loss of control.

Avoid leaning on the drill or overexerting yourself, leading to fatigue and mistakes.

Stay Alert

Cord Management

Keep the power cord out of the way to prevent tripping hazards. Use an extension cord if needed, but ensure it suits the tool’s power requirements.

Disconnect Power

Always unplug the drill or turn off the power source when changing bits, making adjustments, or when the tool is not in use.

Properly Store the Drill

Store the drill dry, away from moisture and extreme temperatures.

Keep it out of reach of children and unauthorised users.


When working with materials that produce dust or fumes, ensure adequate ventilation or use a dust collection system to protect against inhalation hazards.

Emergency Preparedness

Know the location of emergency shut-off switches and have a medical aid unit close by in the event of mishaps.

Training and Experience

If you need to become more experienced with power drills or specific tasks, seek training or guidance from a knowledgeable person before attempting it.


During the tapping process, several common issues might compromise the quality of the threads being created. One such issue is a broken tap, which can occur due to excessive force, improper alignment, or a weak tap. To avoid this, use an appropriate drill bit to create a correct pilot hole, ensure proper alignment, and use tapping fluid for reduced friction. In case of a broken tap, consider tap extraction methods.

Taps getting stuck or binding is another problem, often caused by an incorrect pilot hole size or chip buildup. To address this, ensure the pilot hole size is correct, maintain proper alignment, clear away chips during tapping, and use tapping fluid for lubrication.

If the tapped hole is not deep enough, it might be due to insufficient depth in the pilot hole or the tap bottoming out. To resolve this, accurately measure and mark the desired hole depth, use a suitable drill bit length, and apply a stop collar for depth control.

Inconsistent or misaligned threads can be problematic, often caused by a tilted or misaligned tap, uneven pressure, or incorrect pilot hole size. To prevent this, keep the tap perpendicular, apply even pressure, and verify the correct pilot hole size.

A tap breaking through the workpiece can occur due to incorrect depth control or excessive force. Use a drill stop collar for depth control to avoid this, and apply gentle, consistent pressure.

If threads are tight enough, it might result from an oversized pilot hole or an incorrect tap size. Ensure the pilot hole size matches the tap, or consider using a larger tap and fastener size if necessary.

Leakage or spillage of tapping fluid can happen due to improper application or excessive fluid usage. Use tapping fluid conservatively, apply it accurately, and have a container beneath to catch any excess.

Tapping in hard or brittle materials can lead to tool wear, breakage, or poor thread quality. To address this, use high-quality taps suitable for the material, reduce tapping speed, and apply consistent pressure.

Tapping thin materials can cause deformation or tear-out. Use a backer board or sacrificial material beneath the thin workpiece and apply moderate pressure during tapping to prevent this.

Overheating or excessive wear can result from high drilling or tapping speed, insufficient lubrication, or improper tool materials. Choose appropriate cutting speeds, reduce friction with tapping fluid, and select taps made of durable materials such as high-speed steel, cobalt, or carbide for challenging materials.

To minimise these common tapping issues, following proper procedures, selecting the right tools, and maintaining good tool hygiene during the tapping process are crucial. Seeking guidance and training from experienced machinists can also be valuable for improving tapping techniques and troubleshooting issues.


Determining the correct size drill bit for a 1/4-20 tap ensures successful threading and fastener compatibility. The size of the bore relies upon various factors, including the material you’re working with and the desired thread engagement.

Generally, a #7 drill bit is recommended for a 1/4-20 tap in most situations, as it balances thread depth and strength. However, it’s essential to consult manufacturer recommendations and consider specific application requirements to make an informed decision.

Ultimately, selecting the appropriate drill bit size for a 1/4-20 tap is a critical aspect of achieving secure and reliable threaded connections in various projects, and careful consideration should be given to the specific circumstances to ensure the best results.