To create a 1/4-20 threaded hole, the standard drill bit is #7. If you don’t have this drill bit, you can also use a 5.1mm or 13/64-inch drill bit as an alternative, with similar results.
1/4-20 Tap Drill Size Chart
| Drill Bit Type | Size | Notes |
|---|---|---|
| Standard (Number) | #7 | Most recommended, provides about 75% thread engagement |
| Metric Alternative | 5.1 mm | Works very well, very close to #7 |
| Imperial (Fractional) | 13/64″ | Easily available, slightly larger hole, easier tapping |
| Slightly Smaller | 5.0 mm | Suitable for softer materials (e.g., aluminum), fuller threads |
Selection Recommendations:
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General Use: Simply use a #7 drill bit.
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If the Exact Bit Is Not Available: A common 5mm drill bit can also work, but the tapping will be tighter. Be sure to apply plenty of lubricant and frequently back out the tap to remove chips to prevent tap breakage.
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For Hard Materials: Using a 13/64″ drill bit will make tapping smoother and reduce the risk of breaking the tap.
What is a 1/4-20 Tap?
A 1/4-20 tap is a tool used to cut Imperial threads inside a hole. The “1/4” indicates the nominal diameter of the thread is 1/4 inch (approximately 6.35 mm), and the “20” means there are 20 threads per inch (TPI). It is one of the most common coarse (UNC) thread standards in the United States and is typically used with 1/4-20 bolts or screws.
Quick Overview of Key Parameters:
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1/4: Major diameter (outer diameter), which is 1/4 inch ≈ 6.35 mm.
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20: Number of threads per inch (TPI), meaning there are 20 threads in one inch.
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UNC: Stands for Unified National Coarse thread.
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Corresponding Tap Drill: The most commonly used drill bit is #7 (0.201 inch ≈ 5.1 mm).
Applications of 1/4-20 Tap
The 1/4-20 tap has a wide range of applications and is one of the most common fastener sizes in both North American and North African standards. Because it balances strength and size, it can be found in everything from precision photography equipment to heavy machinery modifications.
Main Applications of the 1/4-20 Tap:
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Photography and Film Equipment: This is the most well-known use of this size. Nearly all tripod mounts, camera base screws, ball heads, cold shoe adapters, lighting fixtures, and monitor mounting holes use 1/4-20 threads.
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DIY and Home Repairs: It is one of the most readily available imperial bolt sizes in hardware stores. Commonly used for assembling furniture, wall mounts, small power tools, and various handcrafted projects.
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Automotive and Mechanical Maintenance: 1/4-20 bolts are widely used in interior panels, small engine bay accessories, dashboard brackets, and cover plates for agricultural machinery.
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General Manufacturing and Assembly:
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Enclosure Mounting: Metal housings for electronic devices or distribution boxes.
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Motor Installation: Securing the base of small to medium-sized motors.
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Jigs and Fixtures: Used in factories to create temporary positioning or workpiece-holding molds.
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Aerospace and Electronics: Used to secure non-structural avionics components and brackets, offering a balance of lightweight design and sufficient stability.
Why Choosing the Right Drill Bit Size Matters？
When selecting a drill bit size, accuracy is the key factor that determines the quality of the threaded hole.
If the drill bit diameter is too small, the tap will encounter excessive resistance during tapping, which can cause it to break and render the tool unusable, while the bolt may not fit properly due to the undersized hole.
Conversely, if the drill bit diameter is too large, the thread depth and contact area will be insufficient, leading to loose bolts or thread stripping, directly reducing the mechanical strength and safety of the connection.
Therefore, it is essential to strictly match the drill bit to the specified thread size. This ensures smooth assembly, maintains thread integrity, minimizes tool wear, and mitigates machining risks.”
Step-by-Step Guide to Drilling and Tapping 1/4-20 Threads
When cutting a 1/4-20 thread, start by drilling the pilot hole with a #7 drill bit. After drilling, make sure to clean out any chips from the hole and add a few drops of cutting oil for lubrication. When tapping, keep the tap perfectly vertical and follow a “three turns forward, one turn back” rhythm to clear the broken chips. By working this way, you’ll create precise, smooth threads while also protecting the tap from breaking.
1. Prepare the Tools
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Drill Bit: Use a #7 drill bit (approximately 5.1 mm in diameter). This is the standard size for cutting 1/4-20 threads.
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Tap: 1/4-20 tap.
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Other Tools: Tap wrench, cutting oil, and bench vise.
2. Drill the Hole
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Secure the workpiece in a bench vise.
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Center-punch the drilling location to prevent the bit from wandering.
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Drill vertically through the workpiece or to the required depth using the #7 drill bit.
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Apply a few drops of cutting oil during drilling.
3. Clean and Lubricate
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Remove all metal chips from the hole and the surface.
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Apply cutting oil to both the tap and the drilled hole to reduce friction and protect the tap.
4. Tap the Hole
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Insert the tap into the tap wrench and align it vertically with the drilled hole.
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Rotate the wrench clockwise while applying downward pressure to start cutting the threads.
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Key Technique: After turning ½ to 1 full turn clockwise, rotate ¼ turn counterclockwise to break and clear the metal chips. This prevents the tap from binding or breaking.
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Repeat this process until the threads reach the desired depth.
5. Clean and Inspect
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Remove the tap from the hole.
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Clean out any remaining chips and oil residue.
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Test the threads by screwing in a 1/4-20 bolt to ensure smooth engagement.
Common Mistakes When Drilling for 1/4-20 Taps
If the tap is not used properly, it can easily break inside the hole. Because the tap itself is extremely hard, once it jams, it is very difficult to remove, and the entire workpiece could be ruined.
Additionally, if the pilot hole is drilled crooked or to the wrong size, the resulting threads will not reach the proper depth, causing bolts to loosen or strip during use and compromising the safety of the mechanical connection.
1. Choosing the Wrong Drill Bit Size
This is the most common mistake. Using a drill bit smaller than #7 (5.1 mm) dramatically increases the resistance on the tap, making it very easy to break inside the hole. Conversely, using a drill bit that is too large (e.g., 1/4 inch) results in shallow threads, causing bolts to loosen or strip.
2. Drilling or Tapping at an Angle
If the pilot hole or tapping is not vertical, the bolt will not sit flush against the surface. For high-precision workpieces, it is recommended to use a drill press or guide block to ensure proper vertical alignment.
3. Ignoring Chip Removal
If you continuously rotate the tap clockwise without backing off, metal chips will accumulate in the tap flutes. This leads to rough threads or even a jammed tap that can break. Always reverse the tap about 1/4 turn after every half turn forward to break and clear the chips.
4. Lack of Lubrication
Dry tapping generates excessive heat and friction. Not using cutting oil results in poor thread quality and shortens the lifespan of both the tap and the drill bit.
5. Forcing a Jammed Tap
If you encounter sudden resistance and try to force the tap, it will usually break inside the hole. Due to the extreme hardness of the tap, a broken piece is very difficult to remove and can ruin the entire workpiece.
6. Insufficient Pilot Hole Depth
For blind holes, if the pilot hole is not deep enough, the guiding end of the tap will hit the bottom of the hole. Continuing to force the tap at this point can damage the bottom threads or break the tap.
Tips for Smooth and Accurate Tapping
To cut smooth and precise threads, the first step is to choose the correct drill bit for a 1/4-20 tap—that is, the commonly used #7 drill bit—and make sure the hole is drilled straight and steady.
During the process, apply plenty of cutting oil, as it significantly reduces resistance. Don’t try to turn the tap all the way in at once; keep a steady rhythm: advance a little clockwise, then back it off slightly. This backing-off motion breaks and clears the chips, preventing the tap from jamming in the hole. As long as the pilot hole is the correct size and your hand is steady, the resulting threads will be smooth and of high quality.
Conclusion: Achieving Perfect 1/4-20 Threads Every Time
Cutting perfect 1/4-20 threads isn’t that complicated—the key is choosing the right tools and keeping a steady hand.
First, the pilot hole is the foundation. Use a #7 drill bit—that is, the drill bit for a 1/4-20 tap—which is the best match for the 1/4-20 specification. After drilling, be sure to apply a few drops of cutting oil before tapping. This step is essential, as it greatly reduces friction and makes threading much easier.
During the operation, patience is more important than force:
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Keep it vertical: At the start, repeatedly check that the tap is perfectly perpendicular to the surface. If it tilts, the threads will be ruined.
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Advance three, back one: Turn the tap forward a few turns, then reverse about half a turn. This breaks and clears the metal chips, preventing the tap from jamming.
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Avoid forcing the tap: If you feel a sudden increase in resistance, do not push harder. Back the tap out, clear the chips, and continue.
As long as you follow this rhythm and don’t rush, the threads you cut will be strong, tight-fitting, and your tap will be well protected. Whether you’re repairing a camera mount or making small DIY parts, mastering these steps will allow you to achieve professional-quality threads every time.
If you encounter any issues during the process, or if you want to learn about pilot hole sizes for other thread specifications, such as 10-24 or 5/16-18, feel free to contact ZC Tools!
