NPT" Thread Sizes (National Pipe Taper) the I.D. of the fitting, and the AN fitting size with the closest-matching I.D.(inside dimension). Pipe Thread Size: Threads Per Inch: App. Interior Diameter: Closest AN Size: 1/16" 27: 1/16" 2: 1/8" 27: 1/8" 4: 1/4" 18: 1/4" 6: 3/8" mm: 1/64: 0.016: 0.397: 1/32: 0.031: 0.794: 3/64: 0.047: 1.191 TAPDRILL SIZE CHART FOR NPT THREADS (National Pipe Threads) NPT Size: Tap Drill Size (in.) (Decimal) (in.) 1/16 - 27 "C" (0.242) 1/8 - 27 "Q" (0.332) 1/4 - 18: 7/16 (0.438) 3/8 - 18: 9/16 (0.562) (1.484) 1•1/2 - 11•1/2: 1•23/32 (1.719) 2.0 - 4wQ0gOx.

1 2 npt thread dimensions in mm