Tap Drill Size for #4-40 UNC

Thread Specifications for #4-40 UNC

Here are the complete specifications for the #4-40 UNC thread:

• Major Diameter: 0.112″
• Threads Per Inch: 40
• Tap Drill Size: 43 (0.089″ / 2.26mm)
• Thread Engagement: ~75% (standard)

How Is the Tap Drill Size Calculated?

The tap drill size for unified threads is calculated using the basic formula:

Tap Drill Diameter = Major Diameter – (1 / TPI)

For #4-40 UNC:

Tap Drill = 0.112″ – (1 / 40) = 0.112″ – 0.025″ = 0.087″

The nearest standard drill size to 0.087″ is 43 (0.089″), which gives approximately 75% thread engagement — the recommended percentage for most applications.

Why 75% Thread Engagement?

Thread engagement percentage determines how deep the threads are cut into the material. While it might seem like more is better, there’s a practical limit:

• 50% engagement provides about 90% of the thread’s full strength — use a slightly larger drill for softer materials or where tap breakage is a concern.
• 75% engagement is the industry standard — provides excellent strength with reasonable tapping effort. This is what the 43 drill gives you for #4-40 UNC.
• Beyond 83% engagement adds almost no additional strength but significantly increases tapping torque and the risk of breaking the tap.

Engineering tip: In aluminum, brass, or other soft materials, 50-60% thread engagement is often preferred. Save the 75%+ for steel and other hard materials where you need maximum holding force.

Related Thread Sizes

• Tap Drill for #2-56 UNC — 50 (0.07″)
• Tap Drill for #3-48 UNC — 47 (0.0785″)
• Tap Drill for #5-40 UNC — 38 (0.1015″)
• Tap Drill for #6-32 UNC — 36 (0.1065″)