Tap Drill Hole Sizes

Page — Tap Drill Hole Sizes (10-32 and 1/4-20)

Before tapping a hole, the correct drill bit diameter must be used. The tap drill is slightly smaller than the final thread size so the tap can cut threads into the material properly.

10-32 Tap Holes

For a 10-32 thread, the correct tap drill size is:

• #21 drill bit (0.159")

This size provides:

• Enough material for strong threads
• Proper engagement for aluminum tapping
• Reduced risk of stripping when used correctly

1/4-20 Tap Holes

For a 1/4-20 thread, the correct tap drill size is:

• #7 drill bit (0.201")

This size provides:

• Strong, coarse threads suitable for higher loads
• Better durability in structural applications
• Lower chance of thread failure compared to tighter fits

Why Correct Hole Size Matters

Using the wrong tap drill size can cause:

• Weak or stripped threads
• Difficulty starting the tap
• Broken taps inside the material
• Poor fastener holding strength

Best Practice

• Always verify drill size before tapping
• Use a center punch before drilling
• Drill perpendicular to the surface
• Use cutting fluid when tapping aluminum

Key Idea

Tap drill size determines thread strength and reliability. In FRC, #21 for 10-32 and #7 for 1/4-20 are standard sizes that ensure strong, consistent threaded holes.