What Size Drill Bit for a 5/16 Tap?

When it comes to tapping threads in metal or wood, one common question that often arises is, “What size drill bit for a 5/16 tap?” Selecting the right drill bit size is essential to create precise, secure threaded holes. This guide aims to provide a clear and straightforward answer to this question while delving into the world of taps, threads, and the significance of choosing the correct drill bit size.

Understanding Taps and Threads

Tap’s Role:

A tap serves as a cutting tool in metalworking and machining, designed to create threads inside a hole or on the external surface of a cylindrical object. Its primary purpose is to cut uniform, helical or spiral grooves into a material, facilitating the insertion of screws, bolts, or other threaded fasteners. 

Taps are typically constructed from high-speed steel or other durable materials, coming in various shapes and sizes to accommodate different thread profiles and sizes. When rotated into a pre-drilled hole or onto a cylindrical workpiece, a tap removes material incrementally, forming the threaded pattern. This process is vital for achieving secure and reliable fastenings in mechanical and industrial applications.

Thread Size and Pitch:

Understanding thread size and pitch is crucial for selecting compatible fasteners, taps, and drill bits when creating threaded connections. Mismatched lengths or angles can lead to poorly formed threads, weak links, or difficulties in assembly.

  • Thread Size: The thread size represents the nominal diameter of the threaded portion of a fastener or the hole where the pin will be inserted, usually expressed in inches or millimeters. For example, the standard thread size 1/4-20 indicates a nominal diameter of 1/4 inch and 20 threads per inch.
  • Pitch: Pitch refers to the distance between adjacent threads on a fastener or inside a threaded hole, measured in lines per inch (TPI) for imperial units or millimeters for metric units. In the 1/4-20 example, the pitch is 20 threads per inch.

Common Thread Standards

Several thread standards are used globally to ensure uniformity and compatibility in threaded fasteners:

Unified Thread Standard (UTS): Predominantly used in the United States, UTS combines the Unified National Thread (UN) and the Unified National Thread Form (UNF). It employs both imperial and metric measurements for thread size and pitch.

Metric Thread Standard: Metric threads are prevalent in countries following the metric system, such as many European and Asian nations. These threads use millimeters for both thread size and pitch and are often expressed as, for example, M8x1.25, where 8 represents the thread size, and 1.25 is the pitch in millimeters.

British Standard Whitworth (BSW): Historically used in the United Kingdom and some Commonwealth countries, BSW threads have been mostly replaced by metric threads but are still encountered in some older machinery and applications.

National Pipe Thread (NPT): NPT threads are commonly used for sealing pipe connections in the United States, featuring a taper to create a tight seal, designated by pipe size and threads per inch.

ISO Metric Thread Standard: Defined by the International Organization for Standardization (ISO), these metric threads are widely used globally, ensuring compatibility in manufacturing and industry.

5/16 Tap Specifications

A 5/16 tap is a cutting tool designed to create threads with a 5/16-inch nominal diameter. Here are the specifications for a standard 5/16 tap:

  • Thread Size: 5/16 inch
  • Thread Pitch: The thread pitch for a 5/16-18 tap is 18 threads per inch (TPI). This means there are 18 threads in one inch of the threaded portion.
  • Tap Type: The 5/16-18 tap is typically a standard coarse thread tap (UNC – Unified National Coarse). UNC threads are commonly used in applications requiring rapid assembly and disassembly and are less prone to cross-threading.
  • Material: 5/16 taps are often made from high-speed steel (HSS) or other durable materials capable of cutting threads in various metals and materials.
  • Thread Profile: The 5/16-18 tap creates threads with a 60-degree thread angle, a standard angle for many threaded fasteners.
  • Tap Types: Depending on the specific application and material, 5/16 taps may come in various configurations, including plug taps (for general use), bottoming taps (for creating threads near the bottom of blind holes), and taper taps (for starting threads gradually).
  • Usage: 5/16 taps are commonly employed to create threaded holes in various materials, including metal, plastic, and wood, often used with 5/16-18 bolts or screws.
  • Compatibility: When using a 5/16 tap, it’s essential to match it with the appropriate drill bit size for creating a pilot hole before tapping. The recommended drill bit size for a 5/16-18 tap is typically a letter size “U” or 0.368 inches in diameter.
  • Tapping Fluid: Using tapping fluid or cutting oil during tapping helps reduce friction and heat, prolonging the tap’s life and improving thread quality.
  • Safety: When using a 5/16 tap or any tap, always follow safety measures, including wearing appropriate eye protection and securing the workpiece properly to prevent movement during tapping.

Remember that 5/16-18 is a standard thread size and pitch, but variations or special requirements may exist depending on the application. 

Significance of Utilizing the Right Drill Size

Selecting the correct drill bit size when creating threads is of paramount importance for several reasons:

Thread Accuracy: The right drill bit size ensures that the drilled hole matches the intended thread size, crucial for achieving a secure and functional threaded connection. Oversized holes can result in loose threads, while undersized holes can lead to overly tight fits and compromised threads.

Thread Engagement: Proper drill bit sizing allows for optimal thread engagement between the fastener and the material. Threads that are too shallow due to an oversized hole or too deep due to an undersized hole can compromise the integrity of the connection.

Thread Strength: Proper thread depth and engagement directly impact the strength of the threaded joint. A well-matched drill bit size ensures that threads are formed with sufficient material, maximizing the joint’s strength and load-carrying capacity.

Prevention of Stripped Threads: Using the correct drill bit size reduces the risk of stripped or damaged threads. Over-drilling or under-drilling can weaken the material and make the threads susceptible to stripping when inserting or removing fasteners.

Sealing and Tightness: The right drill bit size is essential in applications where threads need to provide sealing or maintain airtight or watertight integrity. Proper thread engagement helps achieve the desired sealing properties.

Ease of Assembly: When the correct drill bit size is used, assembly becomes smoother and more efficient. Fasteners insert and thread more easily into adequately sized holes, reducing the risk of cross-threading and assembly difficulties.

Potential Issues with the Wrong Drill Bit Size

Using the wrong drill bit size when creating threads can lead to a range of issues:

Insufficient Thread Engagement: An oversized hole can result in inadequate thread engagement, causing fasteners not to grip the material securely. This can lead to weak connections and a higher risk of fastener failure.

Excessive Thread Engagement: Conversely, an undersized hole can lead to excessive thread engagement, which may cause the fastener to bottom out prematurely or damage the material.

Difficulty in Assembly: Incorrectly sized holes can make it challenging to insert fasteners or may require excessive force, increasing the likelihood of damaging the pin, the spot, or both.

Reduced Strength: Threads formed in oversized holes may not have sufficient material engagement, compromising the strength and load-bearing capacity of the joint.

Leakage and Sealing Issues: When creating threaded connections that require sealing properties, using the correct drill bit size can result in adequate sealing, preventing leaks or compromised airtightness.

Thread Stripping: Mismatched holes and thread sizes can lead to stripped threads, making it difficult or impossible to secure fasteners properly.

Inconsistent Results: Using varying drill bit sizes for the same thread size can result in inconsistent thread quality and performance, making it challenging to achieve uniform results in manufacturing or assembly.

Recommended Drill Bit Size for a 5/16 Tap

According to industry standards, the recommended drill bit size for a 5/16 tap is typically a letter size “U” drill bit. The U drill bit has a diameter of 0.368 inches or approximately 9.35 millimeters. This drill bit size provides the proper clearance for creating threads using a 5/16-18 tap.

Formula for Determining the Correct Drill Bit Size

If you need to determine the correct drill bit size for tapping threads, you can use the following formula:

Drill Bit size = Major Diameter of Thread / (Number of Threads Per Inch)

In the case of a 5/16-18 tap:

  • Major Diameter of Thread: For a 5/16-18 tap, the major diameter is 5/16 inches, equivalent to 0.3125 inches.
  • Number of Threads Per Inch: The 5/16-18 tap has 18 threads per inch (TPI).

Using the formula:

Drill Bit size = 0.3125 inches / 18 TPI ≈ 0.0174 inches per thread

Therefore, the recommended drill bit size is approximately 0.3125 inches or 5/16 inch.

Practical Tips for Selecting the Drill Bit Size

Use a Drill Bit Gauge: A drill bit gauge or index is a handy tool for quickly identifying drill bit sizes. It contains labeled holes for various standard sizes, allowing you to find the right bit for your needs quickly.

Consult Tap and Drill Charts: Tap and drill charts are readily available online, in machining reference books, or from tool manufacturers. These charts provide recommended drill bit sizes for specific tap sizes and thread pitches. Always refer to these charts to ensure you’re using the correct size.

Measure with Calipers: If you need clarification on the size of a drill bit, you can use digital or vernier calipers to measure its diameter accurately. Ensure that the measured size matches the requirements for your tapping operation.

Check the Tap’s Documentation: If you have the packaging or documentation for the tap you’re using, it may specify the recommended drill bit size for creating pilot holes. This information can be valuable in ensuring compatibility.

Perform Test Fits: Before committing to your primary workpiece, perform test fits on scrap material using different drill bit sizes if needed. This allows you to verify which drill bit size best fits your tap.

Consider Material Properties: The material you’re drilling into can impact the choice of drill bit size. Softer materials may require slightly smaller drill bits, while harder materials may necessitate slightly larger ones. Consult material-specific machining guides for guidance.

Use Dedicated Tap Drill Sets: Tap drill sets often have matched drill bits for specific tap sizes. These sets are designed for compatibility and can simplify the selection process.

Seek Expert Advice: If you’re uncertain about the correct bore size for your particular application, don’t hesitate to consult with experienced machinists, colleagues, or experts who can provide guidance based on their expertise.

Importance of Pilot Holes and How to Create Them

Pilot holes are essential when tapping threads, particularly in more rigid materials. They serve several purposes:

  • Alignment: Pilot holes ensure that the tap starts in the correct location, preventing it from wandering or creating misaligned threads.
  • Reduced Friction: Drilling a pilot hole reduces the initial resistance encountered by the tap, making it easier to start threading.
  • Proper Thread Depth: Pilot holes help control the depth of the threaded hole, ensuring that it’s neither too shallow nor too deep.

To create a pilot hole:

  1. Select the Drill Bit: Refer to a tap and drill chart or use a drill bit gauge to choose the correct drill bit size for the tap you intend to use.
  2. Secure the Workpiece: Clamp or secure the workpiece to prevent movement during drilling.
  3. Mark the Hole Location: Use a center punch or a scribe to mark where you want to create the threaded hole. This mark guides the drill bit.
  4. Drill the Pilot Hole: With the chosen drill bit secured in your drill press or hand drill, drill the pilot hole at the marked location. Ensure that the drill bit is perpendicular to the workpiece.
  5. Use Tapping Fluid: Applying tapping fluid or cutting oil during drilling and tapping reduces friction and heat buildup, extending tool life and improving thread quality.

Step-by-Step Process of Drilling and Tapping with a 5/16 Tap

Materials and Tools Needed

  • Workpiece
  • 5/16 tap
  • Drill press or hand drill
  • Drill bit (correct size for 5/16 tap)
  • Tapping fluid or cutting oil
  • Center punch
  • Safety glasses


1. Prepare Your Work Area:

  • Ensure you have a clean and well-lit workspace.
  • Put on safety glasses to protect your eyes from debris.

2. Mark the Hole Location:

  • Use a center punch to create a slight indentation or mark at the precise location where you want to drill and tap the hole. This mark helps guide the drill bit.

3. Select the Correct Drill Bit:

  • Refer to a tap and drill chart or use a drill bit gauge to choose the correct size drill bit for a 5/16 tap.

4. Secure the Workpiece:

  • Clamp or secure the workpiece firmly to prevent it from moving during drilling and tapping.

5. Drill the Pilot Hole:

  • Insert the selected drill bit into your drill press or hand drill.
  • Align the drill bit with the center punch mark.
  • Apply gentle downward pressure and start drilling at a slow speed.
  • Keep the drill bit perpendicular to the workpiece to ensure a straight hole.
  • Use a moderate speed and consistent pressure, retracting the bit periodically to clear chips and adding tapping fluid or cutting oil to reduce friction and heat.
  • Drill to the desired depth, which should match the length of the threaded section on the tap. You can use a depth stop on your drill press or mark the drill bit to gauge the depth.

6. Prepare the Tap:

  • Ensure the 5/16 tap is clean and free from any debris or burrs.
  • Apply a small amount of tapping fluid or cutting oil to the tap to lubricate it.

7. Tapping the Hole:

  • Insert the 5/16 tap into the drilled hole.
  • Apply even downward pressure while slowly turning the tap clockwise (righty-tighty). This will create threads as the tap cuts into the material.
  • Make sure the tap remains perpendicular to the workpiece to maintain straight threads.
  • Continue turning the tap until it reaches the desired depth, matching the depth of the pilot hole you drilled earlier.
  • Periodically retract the tap to break the chips and add more tapping fluid or cutting oil for lubrication.
  • Be cautious not to overtighten the tap once it reaches the desired depth to avoid damaging the threads.

8. Inspect the Threads:

  • After tapping is complete, remove the tap from the hole.
  • Inspect the threads to ensure they are clean, precise, and without defects.

Best Practices for Achieving Clean, Precise Threads

  • Proper Pilot Hole: Use the correct drill bit size to create a pilot hole. Ensure the pilot hole is at the correct depth, matching the threaded section of the tap.
  • Alignment: Keep the drill bit and tap perpendicular to the workpiece to maintain straight and accurate threads.
  • Lubrication: Use tapping fluid or cutting oil generously during drilling and tapping. This reduces friction, prevents overheating, and prolongs the life of your tools.
  • Chip Clearance: Clear chips and debris regularly during drilling and tapping to prevent clogging and maintain smooth cutting action.
  • Consistent Pressure: Apply constant and moderate pressure during drilling and tapping. Avoid excessive force, which can damage the tap or workpiece.
  • Slow and Steady: Use slow and controlled drilling and tapping speeds, especially when working with hard materials.
  • Inspect Threads: After tapping, inspect the threads to ensure they are clean, well-defined, and without burrs or defects. Make any necessary adjustments or corrections if needed.

Troubleshooting Common Issues

Several common issues may arise during drilling and tapping that can hinder the creation of clean and precise threads. These problems include broken drill bits or taps, misaligned threads, taps getting stuck or binding, inadequate thread engagement, over-tightening or bottoming out, stripped threads, and surface roughness or chatter marks.

Various solutions and troubleshooting tips can be applied to address these issues effectively. To prevent broken drill bits or taps, it’s crucial to use high-quality tools and apply even and controlled pressure during the drilling and tapping. Misaligned threads can be avoided by carefully marking the hole location, ensuring proper alignment, and keeping the drill bit or tap perpendicular to the workpiece.

Taps getting stuck or binding can be rectified using the correct pilot hole size, clearing away chips and debris regularly, and maintaining proper alignment throughout the tapping process. Inadequate thread engagement can be prevented by referring to tap and drill charts to select the correct drill bit size and drilling to the appropriate depth matching the tap’s threaded portion.

Pay attention to the tap’s depth and use a depth stop or marking method for control to avoid over-tightening or bottoming out. Stripped threads can be mitigated by starting the tap straight and accurately and using fixtures in good condition while replacing worn-out ones.

Finally, to reduce surface roughness or chatter marks, minimize vibrations by using stable setups, adjust cutting speeds and feeds appropriately for the material, and ensure the workpiece is securely clamped to reduce vibrations. Following these recommendations, you can troubleshoot standard drilling and tapping issues, guaranteeing clean and precise workpiece threads.


In conclusion, determining the right size drill bit for a 5/16 tap is crucial in achieving successful threading in various projects. It’s essential to match the drill bit size precisely to the tap size to ensure a secure and accurate fit. A drill bit sized at 17/64″ or 8.7313 mm is ideal for preparing the hole before tapping with a 5/16 tap. This careful selection helps prevent issues like stripping threads, misalignment, or overly tight fits, ensuring a smooth and efficient threading process.