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.