Tap Drill Size for 5/16-24 UNF

Thread Specifications for 5/16-24 UNF

Here are the complete specifications for the 5/16-24 UNF thread:

• Major Diameter: 0.3125″
• Threads Per Inch: 24
• Tap Drill Size: I (0.272″ / 6.91mm)
• 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 5/16-24 UNF:

Tap Drill = 0.3125″ – (1 / 24) = 0.3125″ – 0.0417″ = 0.2708″

The nearest standard drill size to 0.2708″ is I (0.272″), 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 I drill provides for 5/16-24 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 #12-28 UNF — 14 (0.182″)
• Tap Drill for 1/4-28 UNF — #3 (0.213″)
• Tap Drill for 3/8-24 UNF — Q (0.332″)
• Tap Drill for 7/16-20 UNF — 25/64 (0.3906″)