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NPT Threads - Taps and Drill Sizes

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The NPT - National Pipe Taper Threads - is based on the Briggs Standard Pipe Thread.

Drill sizes for Briggs Standard Pipe taps for direct tapping without reaming are indicated below:

NPT Threads - Taps and Drill Sizes
Size Taps
TPISize Drill
1/8 27 21/64
1/4 18 27/64
3/8 18 9/16
1/2 14 11/16
3/4 14 29/32
1 11.5 1 1/8
1 1/4 11.5 1 15/32
1 1/2 11.5 1 23/32
2 11.5 2 3/16
2 1/2 8 2 9/16
3 8 3 3/16
3 1/2 8 3 11/16
4 8 4 3/16
  • TPI = threads per inch
  • 1 inch = 25.4 mm
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