British Standard Pipe Threads BSP threads are fairly uncommon in the workshop but widely used in the home where most U.K. and European plumbing fittings use these threads. The threads are sized not on diameter but on the nominal bore of the pipe to which they are applied. Just to be really odd a ½" pipe doesn′t necessarily have a ½" bore because wall thickness varies with the pressure rating of the pipe. Angle of thread = 55 deg
Size Tpi Pitch mm Major Dia Inch Major Dia Mm Core Dia Inch Core Dia Mm Guage Length Inch Guage Length Mm Tapping Drill Size Mm R95% Tapping Drill Size Mm R80%
​1 7⁄8 11 2.31 2.24 57.00 2.13 54.04 5⁄8 15.90 54.2 54.6
2 11 2.31 2.35 59.61 2.23 56.66 5⁄8 15.90 56.8 57.2
​2 1⁄4 11 2.31 2.59 65.71 2.47 62.75 11⁄16 17.50 62.9 63.3
​2 1⁄2 11 2.31 2.96 75.18 2.84 72.23 11⁄16 17.50 72.4 72.8
​2 3⁄4 11 2.31 3.21 81.53 3.09 78.58 13⁄16 20.60 78.7 79.2
3 11 2.31 3.46 87.88 3.34 84.93 13⁄16 20.60 85.1 85.5
​3 1⁄4 11 2.31 3.70 93.98 3.58 91.02 7⁄8 22.20 91.2 91.6
​3 1⁄2 11 2.31 3.95 100.33 3.83 97.37 7⁄8 22.20 97.5 98.0
​3 3⁄4 11 2.31 4.20 106.68 4.08 103.72 7⁄8 22.20 103.9 104.3
4 11 2.31 4.45 113.03 4.33 110.07 1 25.40 110.2 110.7
​4 1⁄2 11 2.31 4.95 125.73 4.83 122.77 1 25.40 122.9 123.4
5 11 2.31 5.45 138.43 5.33 135.47 1 1⁄8 28.60 135.6 136.1
​5 1⁄2 11 2.31 5.95 151.13 5.83 148.17 1 1⁄8 28.60 148.3 148.8
6 11 2.31 6.45 163.83 6.33 160.87 1 1⁄8 28.60 161.0 161.5
7 10 2.54 7.45 189.23 7.32 185.98 1 3⁄8 34.90 186.1 186.6