Below is the National Pipe Thread – NPT Tap Drill Size Chart. It tells us what size hole must be drilled to begin tapping the tapered NPT thread.
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Standard NPT Tap Drill Sizes |
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NPT Size
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Tap Drill Size (in.)
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Decimal Equivalent (in.)
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1/16 – 27
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“C”
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0.242
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1/8 – 27
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“Q”
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0.332
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1/4 – 18
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7/16
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0.438
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3/8 – 18
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9/16
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0.562
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1/2 – 14
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45/64
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0.703
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3/4 – 14
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29/32
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0.906
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1.0 – 11•1/2
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1•9/64
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1.141
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1•1/4 – 11•1/2
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1•31/64
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1.484
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1•1/2 – 11•1/2
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1•23/32
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1.719
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2.0 – 11•1/2
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2•3/16
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2.188
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2•1/2 – 8
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2•39/64
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2.609
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3.0 – 8
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3•15/64
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3.234
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Metric Tap Drill Sizes
Standard UNC Tap Drill Sizes
Standard UNF Tap Drill Sizes