Tap Drill Size for 1-8 UNC

Thread Specifications for 1-8 UNC

Here are the complete specifications for the 1-8 UNC thread:

• Major Diameter: 1.0″
• Threads Per Inch: 8
• Tap Drill Size: 7/8 (0.875″ / 22.23mm)
• 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 1-8 UNC:

Tap Drill = 1.0″ – (1 / 8) = 1.0″ – 0.125″ = 0.875″

The nearest standard drill size to 0.875″ is 7/8 (0.875″), 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 7/8 drill gives you for 1-8 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 3/4-10 UNC — 21/32 (0.6562″)
• Tap Drill for 7/8-9 UNC — 49/64 (0.7656″)
• Tap Drill for 1-1/8-7 UNC — 63/64 (0.9844″)
• Tap Drill for 1-1/4-7 UNC — 1-7/64 (1.1094″)