What Size Drill Bit For 1 4 20 Tap: Your Ultimate Chart And Guide

You’re standing in front of your drill bit index, a 1/4-20 tap in one hand and a piece of wood or metal in the other. You know that picking the wrong bit can lead to a broken tap, a stripped thread, or a completely ruined workpiece. It’s a moment of hesitation every DIYer and woodworker knows well.

I promise this guide will completely eliminate that guesswork. We’re not just going to give you the answer; we’re going to show you why it’s the right answer, giving you the confidence to tackle any tapping job that comes your way.

In this complete what size drill bit for 1 4 20 tap guide, we’ll cover the precise drill bit you need, explain the crucial concept of thread engagement, walk you through the step-by-step process of drilling and tapping perfectly, and troubleshoot the common problems you might encounter. Let’s get you drilling with precision.

The Straight Answer: What Size Drill Bit for a 1/4-20 Tap?

Let’s cut right to the chase. The industry-standard, go-to drill bit for tapping a 1/4-20 hole is a #7 number drill bit. This isn’t a random choice; it’s calculated to give you the perfect balance of thread strength and ease of tapping.

First, let’s quickly break down what “1/4-20” means. It’s simple:

• 1/4″ refers to the major diameter of the bolt or screw.
• 20 refers to the number of threads per inch (TPI).

The #7 drill bit has a diameter of 0.2010 inches. This specific size creates a pilot hole that allows the 1/4-20 tap to cut strong, well-formed threads, achieving what’s known as approximately 75% thread engagement. This is the sweet spot for most applications in wood, plastic, and metal.

Tap Drill Chart for 1/4-20

Here’s a quick reference chart you can bookmark or print for your workshop wall. It covers the essential info you need at a glance.

Tap Size	Standard Tap Drill	Decimal Equivalent	Common Alternative	Decimal Equivalent	Approx. Thread %
1/4″-20 UNC	#7	0.2010″	13/64″	0.2031″	75% (#7) / 72% (13/64″)

As you can see, the 13/64″ bit is only two-thousandths of an inch larger. In many materials, this difference is negligible and makes it an excellent backup if you don’t own a full number drill bit set.

Why a #7 Bit is the Gold Standard (And When an Alternative is Better)

So, why all the fuss about a few thousandths of an inch? It all comes down to a concept called thread engagement. This refers to how much the threads of the bolt engage with the threads you cut into the material.

Drill a hole that’s too small, and you get close to 100% thread engagement. This sounds good, but it requires a massive amount of force to turn the tap, dramatically increasing the risk of snapping it off in the hole—a truly frustrating experience.

Drill a hole that’s too large, and you get low thread engagement (maybe 50% or less). The threads will be shallow and weak, making them easy to strip out when you tighten the bolt. The benefits of what size drill bit for 1 4 20 tap precision are clear: you get strong, reliable threads without breaking your tools.

Understanding the 75% Thread Engagement Rule

Decades of engineering have shown that around 75% thread engagement provides over 95% of the full holding power with a fraction of the tapping effort and risk. This is the magic number that the #7 drill bit is designed to achieve for a 1/4-20 tap. It’s strong, reliable, and safe for your tools.

When a 13/64″ Drill Bit is Your Best Friend

While the #7 is technically perfect, the 13/64″ bit is a fantastic and practical alternative in several common workshop scenarios:

• You don’t have a #7 bit: Most standard fractional drill bit sets include a 13/64″, making it far more common in a home workshop.
• Working with soft materials: In aluminum, brass, or even some hardwoods, using the slightly larger 13/64″ bit reduces friction and makes the tap easier to turn, preventing material from binding or galling.
• General purpose projects: For a shop jig, a piece of furniture, or a DIY project where you don’t need the absolute maximum tensile strength of a Grade 8 bolt, the ~72% thread engagement from a 13/64″ bit is more than sufficient.

Your Step-by-Step Guide: How to Drill and Tap for a 1/4-20 Bolt

Knowing the right size bit is half the battle. Executing the process correctly is the other half. Follow these steps for perfect threads every time. This is how to what size drill bit for 1 4 20 tap correctly.

Step 1: Gather Your Tools and Materials

Before you start, get everything ready. There’s nothing worse than having to stop mid-process to find a tool.

• Safety First: Always wear safety glasses.
• Drill: A drill press is best for perfectly straight holes, but a handheld drill works too.
• Drill Bit: A sharp #7 or 13/64″ drill bit.
• Marking Tools: A combination square and a sharp pencil or marking knife.
• Center Punch and Hammer: Crucial for preventing the drill bit from “walking.”
• 1/4-20 Tap and Tap Wrench: A “plug” tap is good for most uses.
• Cutting Fluid or Lubricant: Essential for metal, helpful for plastic. (e.g., Tap Magic or even 3-in-1 oil).
• Clamps: To hold your workpiece securely.

Step 2: Mark and Punch Your Hole Location

Accuracy starts here. Use your square and pencil to mark the exact center of your hole. Once marked, place the tip of the center punch on your mark and give it a firm tap with a hammer. This creates a small divot that will guide the drill bit and keep it perfectly on target.

Step 3: Drill the Pilot Hole

Secure your workpiece with clamps. Place the tip of your drill bit into the divot you made with the center punch. Your goal is to drill a hole that is perfectly 90 degrees to the surface.

If using a drill press, this is easy. If using a handheld drill, use a drilling guide or try to sight it from two different angles (front and side) to ensure you’re going in straight. Apply steady, moderate pressure and let the bit do the work. Periodically pull the bit out to clear chips from the hole.

Step 4: Tap the Threads

This is where patience pays off. Secure the 1/4-20 tap in your tap wrench. If you’re tapping metal, apply a few drops of cutting fluid to the tap and into the hole.

Carefully place the tip of the tap into the hole. Ensure it is perfectly aligned and perpendicular to the surface. Start turning the tap wrench clockwise with gentle, even pressure. After the first couple of turns, the tap should bite and begin cutting threads.

Use the classic tapping technique: turn clockwise a half-turn to cut, then reverse counter-clockwise a quarter-turn to break the chip. You will feel a slight release when the chip breaks. Continue this process until you have tapped to the desired depth. Don’t force it; if you feel strong resistance, back the tap all the way out, clear the chips, add more lubricant, and continue.

Common Problems with Tapping a 1/4-20 Hole (And How to Fix Them)

Even with careful work, things can go wrong. Here are some common problems with what size drill bit for 1 4 20 tap and how to handle them.

Problem: The Tap Broke Off in the Hole!

This is the most dreaded failure. It’s almost always caused by using a dull tap, not clearing chips, or applying too much force. Prevention is the best cure.

Solution: If a piece of the tap is sticking out, you may be able to grab it with locking pliers. If it’s broken flush or below the surface, you’ll need a specialized tool called a tap extractor. These tools have small “fingers” that slide into the flutes of the broken tap, allowing you to turn it out.

Problem: The Threads Feel Loose or Stripped

This happens when the pilot hole was drilled too large, maybe because you accidentally grabbed a 7/32″ bit instead of a 13/64″. The tap doesn’t have enough material to cut into, resulting in weak, shallow threads.

Solution: The best fix is to use a thread repair kit, like a Helicoil. This involves drilling the hole out to a larger specific size, tapping it with a special tap from the kit, and then inserting a coiled wire insert that brings the thread size back down to a strong and reliable 1/4-20.

Problem: The Bolt Goes in Crooked

This is a clear sign that either the pilot hole was drilled at an angle or the tap was started crooked. An angled bolt won’t seat properly and creates a weak connection.

Solution: Unfortunately, this is very difficult to fix once the threads are cut. The best approach is prevention. Use a drill press or a drill block to ensure your hole is straight, and take extra care to align your tap perfectly before you start turning.

Best Practices for Tapping Different Materials

The material you’re working with can change your approach slightly. Following these what size drill bit for 1 4 20 tap best practices will ensure success across different media.

Tapping in Wood (Hardwood vs. Softwood)

Tapping wood is common for making jigs or installing hardware. You generally don’t need cutting fluid, though a bit of beeswax on the tap can make it turn more smoothly in dense hardwoods like maple or oak.

For hardwoods, the #7 or 13/64″ bit is perfect. For softwoods like pine, the wood fibers can compress, sometimes leading to weaker threads. For critical connections in softwood, consider using a threaded insert, which provides durable machine threads.

Tapping in Metal (Steel, Aluminum, Brass)

This is where precision and proper technique are paramount.

• Steel: Use a sharp #7 bit and high-quality cutting fluid—it is not optional. Go slow and clear chips often.
• Aluminum: This metal is soft but “gummy” and can bind to the tap. Using a 13/64″ bit can help, and a lubricant designed for aluminum (like A9 or even WD-40 in a pinch) is essential to prevent galling.
• Brass: This copper alloy is a joy to machine. It cuts cleanly and easily. A #7 bit is ideal, and while lubricant helps, it’s less critical than with steel or aluminum.

Eco-Friendly and Sustainable Tapping Practices

Being a good craftsperson also means being a good steward of your workshop and environment. Consider these sustainable what size drill bit for 1 4 20 tap tips. Instead of petroleum-based cutting oils, look for plant-based or biodegradable lubricants. When working, collect your metal shavings for proper recycling rather than letting them mix with general waste. A good what size drill bit for 1 4 20 tap care guide also involves sustainability: invest in high-quality taps and drill bits from reputable brands. They will last for years with proper care, reducing waste and saving you money in the long run.

Frequently Asked Questions About What Size Drill Bit for a 1/4-20 Tap

Can I use a 3/16″ drill bit for a 1/4-20 tap?

No, you should absolutely not use a 3/16″ (0.1875″) drill bit. It is far too small for a 1/4-20 tap. The tap will have to remove too much material, which will almost certainly cause it to bind and snap off in the hole.

What’s the difference between a taper, plug, and bottoming tap?

These refer to the amount of chamfer at the tip of the tap. A taper tap has a long, tapered end (8-10 threads) and is the easiest to start in a new hole. A plug tap has a medium taper (3-5 threads) and is the most common general-purpose tap. A bottoming tap has almost no taper (1-2 threads) and is used to cut threads all the way to the bottom of a blind hole (a hole that doesn’t go all the way through).

Do I need to use cutting fluid when tapping wood?

Generally, no. Wood doesn’t generate the same kind of heat and friction as metal. However, for very dense, oily hardwoods like Ipe or ebony, a little bit of paste wax or beeswax on the tap threads can reduce friction and make for a smoother cut.

How do I know when my tap is dull?

A dull tap will be noticeably harder to turn than a sharp one. It might also produce a squeaking or chattering sound as it cuts. When you inspect the threads, they may look rough, torn, or poorly formed. A sharp tap should cut smoothly and produce clean, crisp threads with minimal effort.

Mastering the small details, like choosing the correct tap drill size, is what separates frustrating projects from successful ones. With the knowledge that a #7 or 13/64″ drill bit is your key to perfect 1/4-20 threads, you’re now equipped to add strong, reliable connections to all your future builds.

Remember the fundamentals: mark carefully, drill straight, use the right lubricant for your material, and let the tools do the work. Now you can approach your next project with the confidence of a pro.

Happy building, and stay safe in the workshop!