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Drills

Use the charts below to determine the correct drill bit size for your taps

British Standard Cycle & Cycle Engineer Institute

BRITISH STD CYCLE (CEI)
BSCy (60°)
Sizes in inchesTPIDrill
1/8 40 2.65
5/32 32 3.3
3/16 32 4.1
1/4 26 5.6
5/16 26 7.2
3/8 26 8.7
7/16 26 10.3
1/2 26 11.9
9/16 20 13.1
9/16 26 13.5
5/8 20 14.5
5/8 26 15.0
3/4 20 17.8
3/4 26 18.2

British Standard Whitworth and BS Fine

BRITISH STANDARD WHITWORTH FORM THREADS (55° angle)
DIA.BSWBSFBSBBSP & BSPTME/WHITFORMBSCon
SizesTPIDrillTPIDrillTPIDrillTPIDrillTPIDrillTPIDrill
1/16 60 1.2                    
3/32 48 1.9                    
1/8 40 2.55         28 8.8 40 2.55    
5/32 32 3.2             40 3.3    
3/16 24 3.7 32 4.0         32, 40 4.0, 4.0    
7/32 24 4.5 28 4.6         40 4.8    
1/4 20 5.1 26 5.3 26 5.3 19 11.8 24,28,32,40 5.3,5.4,5.5,5.5    
9/32 20 5.8 26 6.0 26 5.8     32 6.1    
5/16 18 6.5 22 6.8 26 5.8     24, 32, 40 6.75, 7.0, 7.3    
3/8 16 7.9 20 8.3 26 8.4 19 15.25 24, 32, 40 8.4, 8.6, 8.9    
7/16 14 9.3 18 9.7 26 10.0     20,24,32,40 9.8,10,10.3,10.5    
1/2 12 10.5 16 11.1 26 11.5 14 19.0 20,24,32,40 11.5,11.9,11.9,12 18 11.5
9/16 12 12.1 16 12.7 26 13.1     20 13.1    
5/8 11 13.5 14 14 26 14.7 14 21.0 20 14.5 18 14.2
11/16 11 15.1 14 15.5 26 16.5     20 16.2    
3/4 10 16.25 12 16.75 26 17.8 14 24.5 14, 20 17.1, 17.8 16 17.5
7/8 9 19.25 11 19.75 26 21.0 14 28.25 14,16, 20 20.2, 20.6, 21.0 16 20.6
15/16 9 20.6 11 21.5                
1 8 22.0 10 22.75 26 24.2 11 30.75 12, 20 23.0, 24.0 16 23.8
1.1/8 7 24.75 9 25.5 26 27.5            
1.1/4 7 28.0 9 28.5 26 30.5 11 39.5     16 30.1
1.3/8 6 30.1 8 31.5 26 33.7            
1.1/2 6 33.5 8 34.5 26 36.9 11 45.00     14 36.1
1.5/8 5 35.7 8 37.7 26 40.0            
1.3/4 5 39 7 41.0 26 43.5 11 51.0        
1.7/8 4.1/2 41.3 7 43.7 26 46.5            
2 4.1/2 44.5 7 47.0 26 49.6 11 57.0        

Model Engineer, British Standard Brass & British Association

MODEL ENGINEER THREADS
(BA + ME + BSB)
DIABA (47.1/2°) DIAME (55°) BSB (55°)
SizesDiaPitchTPIDrill   Sizes in inchesTPIDrill   TPIDrill
0 6.0 1.0 25.4 5.1   1/8 40 2.55      
1 5.3 0.9 28.2 4.5   5/32 32 3.2      
2 4.7 0.81 31.4 4.0   5/32 40 3.3      
3 4.1 0.73 34.8 3.4   3/16 32 4.0      
4 3.6 0.66 38.5 3.0   3/16 40 4.0      
5 3.2 0.59 43.0 2.65   7/32 32        
6 2.8 0.53 47.9 2.3   7/32 40 4.8      
7 2.5 0.48 52.9 2.05   1/4 32 5.5   26 5.3
8 2.2 0.43 59.1 1.8   1/4 40 5.5      
9 1.9 0.39 65.1 1.55   9/32 32 6.1   26 6.1
10 1.7 0.35 72.6 1.4   9/32 40        
11 1.5 0.31 81.9 1.2   5/16 32 7.0   26 5.8
12 1.3 0.28 90.9 1.05   5/16 40 7.3      
13 1.2 0.25 102.0 0.98   3/8 32 8.6   26 8.4
14 1.0 0.23 109.9 0.80   3/8 40 8.9      
15 0.9 0.21 120.5 0.70   7/16 32 10.3   26 10
16 0.79 0.19 133.3 0.60   7/16 40 10.5      
            1/2 32 11.9   26 11.5
            9/16       26 13.1
            5/8       26 14.7
            3/4       26 17.8

Unified Coarse & Unified Fine, National Pipe Thread

AMERICAN NATIONAL FORM THREADS (60° angle)
DIA.UNCUNFNPT & NPSUNSDIAUNCUNF
Sizes in inchesTPIDrillTPIDrillTPIDrillTPIDrillSize NoTPIDrillTPIDrill
1/16         27 6.3 64 1.2 0     80 1.25
3/32             48 1.85 1 64 1.55 72 1.55
1/8         27 8.7 40 2.6 2 56 1.85 64 1.9
5/32             32 & 40 3.2 & 3.3 3 48 2.1 56 2.15
3/16             32 & 40 4.0 & 4.0 4 40 2.35 48 2.4
7/32             24 & 32 4.5 & 4.8 5 40 2.65 44 2.7
1/4 20 5.1 28 5.5 18 11.1 24 & 32 5.30 & 5.6 6 32 2.85 40 2.95
5/16 18 6.6 24 6.9     20 & 32 6.7 & 7.2 8 32 3.5 36 3.5
3/8 16 8.0 24 8.5 18 14.5 20 8.30 10 24 3.9 32 4.1
7/16 14 9.4 20 9.9     24 10.0 12 24 4.5 28 4.70
1/2 13 10.8 20 11.5 14 18.0 12 & 24 10.5 & 11.5          
9/16 12 12.2 18 12.9                  
5/8 11 13.5 18 14.5     12 13.9          
11/16             11 & 16 15.0 & 15.9          
3/4 10 16.5 16 17.5 14 23.25 12 16.7          
7/8 9 19.5 14 20.4     12 20.2          
1 8 22.25 12 23.25 11.5 29.0 14 23.5          
1.1/8 7 25.0 12 26.5                  
1.1/4 7 28.0 12 29.5 11.5 38.0              
1.3/8 6 30.75 12 32.75                  
1.1/2 6 34.0 12 36.0                  
1.3/4 5 39.5