Find the perfect drill bit size for tapping threads
Recommended Tap Drill Size
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Decimal Size
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Thread Engagement
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Understanding Tap Drill Sizes
Selecting the correct tap drill size is crucial for creating strong, reliable threaded holes. Use too small a drill and the tap will break; too large and the threads will be weak.
What is Thread Engagement?
Thread engagement refers to how much of the thread profile is cut into the hole:
75% Engagement (Standard): Provides 95% of the thread's full strength - the sweet spot for most applications
70% Engagement: Easier tapping in soft materials, still provides adequate strength
80% Engagement: Maximum strength for critical applications, requires more torque to tap
Metric Tap Drill Formula
Tap Drill Size = Major Diameter - (Pitch à Engagement %)
For example, M6 Ã 1.0 thread at 75% engagement:
Drill Size = 6.0mm - (1.0mm à 0.75) = 5.0mm
Common Metric Tap Drill Sizes
Thread Size
Pitch (mm)
Tap Drill (75%)
M3 Ã 0.5
0.5
2.50mm
M4 Ã 0.7
0.7
3.30mm
M5 Ã 0.8
0.8
4.20mm
M6 Ã 1.0
1.0
5.00mm
M8 Ã 1.25
1.25
6.80mm
M10 Ã 1.5
1.5
8.50mm
M12 Ã 1.75
1.75
10.20mm
Common Imperial Tap Drill Sizes
Thread Size
TPI
Tap Drill (75%)
1/4-20 UNC
20
#7 (0.201")
1/4-28 UNF
28
#3 (0.213")
5/16-18 UNC
18
F (0.257")
5/16-24 UNF
24
I (0.272")
3/8-16 UNC
16
5/16" (0.313")
3/8-24 UNF
24
Q (0.332")
1/2-13 UNC
13
27/64" (0.422")
Tapping Best Practices
Use cutting fluid: Reduces friction and extends tap life
Drill perpendicular: Ensure the hole is straight and at 90° to the surface
Back off frequently: Reverse the tap every 2-3 turns to break chips
Start with a taper tap: For through holes, use taper tap first, then plug tap
Clean the hole: Remove all chips before tapping
Use the right tap: Hand taps for manual work, spiral point for machine tapping
Material Considerations
Soft materials (aluminum, brass): Can use 70% engagement for easier tapping
Hard materials (stainless steel): Consider 75-80% engagement for strength
Cast iron: Tends to break chips easily, standard 75% works well
Plastics: May need slightly larger holes to prevent cracking
Troubleshooting Tapping Issues
Tap breaks: Hole too small, not backing out chips, or tapping too fast
Weak threads: Hole too large, try next size down drill bit
Rough threads: Need more cutting fluid or slower tapping speed
Tap won't start: Hole not perpendicular or chamfer not adequate