Tap Drill Size for #6-40 UNF

Thread Specifications for #6-40 UNF

Here are the complete specifications for the #6-40 UNF thread:

• Major Diameter: 0.138″
• Threads Per Inch: 40
• Tap Drill Size: 33 (0.113″ / 2.87mm)
• Thread Engagement: ~75% (standard)

How Is the Tap Drill Size Calculated?

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

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

For #6-40 UNF:

Tap Drill = 0.138″ – (1 / 40) = 0.138″ – 0.025″ = 0.113″

The nearest standard drill size to 0.113″ is 33 (0.113″), 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. While more might seem better, there is a practical limit:

• 50% engagement provides about 90% of full thread strength — ideal for softer materials or when tap breakage is a concern.
• 75% engagement is the industry standard — excellent strength with reasonable tapping effort. This is what the 33 drill provides for #6-40 UNF.
• Beyond 83% adds almost no additional strength but significantly increases tapping torque and tap breakage risk.

Pro tip: In aluminum, brass, or other soft materials, 50-60% thread engagement is often preferred to reduce tap breakage. Save 75%+ for steel and harder materials.

Related Thread Sizes

• Tap Drill for #4-48 UNF — 42 (0.0935″)
• Tap Drill for #5-44 UNF — 37 (0.104″)
• Tap Drill for #8-36 UNF — 29 (0.136″)
• Tap Drill for #10-32 UNF — 21 (0.159″)