What Size Drill Bit for 10-24 Tap: A Practical Guide
Learn the exact drill bit size for a 10-24 tap, including tolerance guidance, material considerations, and practical steps to verify hole size for clean, reliable threads in common metals.
The recommended tap drill size for a 10-24 thread is typically 0.190 inches (4.84 mm). In practice, many technicians use a narrow range around 0.186 to 0.193 inches to accommodate variations in alloy, heat treatment, and tolerance classes. For softer metals, you might lean toward the larger end of the range to ensure clean threading; for harder alloys, the smaller end can help prevent binding during tapping.
Understanding the 10-24 Thread and Tap Drill Size
When you see a designation like 10-24, you are looking at a national form UNC threading standard where the first number (10) represents the major diameter size and the second (24) represents the thread pitch. The drill size you pick is called the tap drill and is generally smaller than the final threaded hole. For a 10-24 thread, a common starting point is around 0.190 inches (4.84 mm). The exact size depends on the material, tolerance class, and whether you are aiming for a tight fit or easier assembly. Manufacturers often publish a chart, and many DIYers keep a small range (0.186–0.193 in) as their default.
This section explains why that starting point matters: the tap will cut threads in the hole, and the hole size directly impacts thread geometry, strength, and the chance of binding or breaking the tap. A hole that's too small can cause the tap to seize or produce distorted threads; one that's too large may yield sloppy threads and reduced pull-out strength. Your goal is a clean, well-formed internal thread that matches the screw or bolt you plan to use.
Why Drill Size Matters for Tap Quality
The drill size sets the critical baseline for thread engagement. A precise tap drill keeps the thread depth consistent along the hole and helps the taps cut evenly. Even a small deviation from the recommended size can lead to under- or oversized threads, which may cause cross-threading or poor grip. Material type affects the optimal size: softer metals often tolerate a slightly larger hole, while harder alloys benefit from a slightly smaller hole to maintain thread integrity. Lubricants also influence the result by reducing heat and wear during cutting, which in turn helps keep the hole dimensions true.
To minimize risk, always prepare the hole with deburring and ensure the workpiece is clamped securely. If you’re unsure, start slightly smaller and test-tap a sacrificial piece to confirm the feel and thread quality before committing to the final part.
How to Choose the Right Drill Bit: Material and Coatings
For most hand-tap work, High-Speed Steel (HSS) bits are sufficient, but for harder metals or high-volume production, cobalt or carbide-tipped bits can extend tool life and maintain precision. The coating is also a factor: black oxide coatings reduce friction, while titanium nitride coatings can extend wear life. When selecting the bit, consider:
- Material hardness of the workpiece
- Access and space for drilling at the desired depth
- Availability of lubrication and coolant
Always pair the chosen bit with a sharp, properly sized drill and a steady feed rate. A dull bit or uneven feed will alter the hole size and degrade thread quality, regardless of the initial size.
Step-by-Step: Pilot Hole, Debur, and Tapping
Follow a disciplined process to achieve optimal results:
- Mark the hole location and drill a pilot hole if needed (especially on thicker materials).
- Use a drill bit slightly larger than the pilot hole to reach the final tap drill size (approximately 0.190 in for 10-24 in common steels/aluminum).
- Deburr the hole to remove sharp edges that can hinder thread cutting.
- Apply a suitable cutting fluid to reduce heat.
- Start the tap straight and apply steady torque to cut threads evenly. Back off frequently to clear chips.
- Check the thread with a matching screw or gauge to ensure proper engagement and alignment.
This sequence helps maintain accuracy and reduces the risk of tap breakage.
Tap drill size comparisons for common thread standards
| Thread Size | Tap Drill (in) | Tap Drill (mm) | Notes |
|---|---|---|---|
| 10-24 | 0.186–0.193 | 4.72–4.90 | Standard practice; varies by material |
| 8-32 | varies | varies | Smaller fasteners; refer to chart |
| 1/4-20 | varies | varies | Common for medium screws |
Got Questions?
What size drill bit should I use for a 10-24 tap?
The standard drill size is 0.190 inches (4.84 mm), with a common range of 0.186–0.193 inches depending on material and tolerance. Start there and adjust based on fit.
Use 0.190 inches as your starting point, then test with a scrap piece to confirm the feel before final drilling.
Can I tap aluminum without a pilot hole?
A pilot hole is still necessary; use the recommended tap drill size for aluminum, typically within the standard range to ensure clean threads. Lubrication helps prevent galling.
Yes, you still need a hole. Use the recommended size and proper lubrication.
What happens if the hole is too small for tapping?
If the hole is too small, the tap can bind or break, and threads may become distorted. Re-drill to the correct size and re-tap carefully.
If it’s too small, the tap will bind or break. Re-drill to the correct size and try again.
Is lubrication important when tapping?
Lubrication reduces friction, heat, and wear, promoting smoother thread cutting and longer tap life. Use an appropriate cutting fluid for the material.
Lubrication matters; it keeps the tap cooler and exit threads cleaner.
Should I ream after tapping?
Reaming after tapping is not always required. It can be used to achieve a precise hole size beyond the thread, but for most standard tapped holes, tapping alone suffices.
Reaming after tapping isn’t usually necessary unless you need a precise hole size beyond the thread.
“Using the correct tap drill size is essential for clean, accurate threads and to prevent binding during tapping.”
Top Takeaways
- Start with the standard 0.190 in (4.84 mm) tap drill for 10-24 threads.
- Adjust within the 0.186–0.193 in range based on material and tolerance.
- Use lubrication to improve finish and tool life.
- Deburr and verify hole size before tapping.
- If in doubt, test with a sacrificial piece first.
