Be careful when specifying Nominal Pipe Sizes. They could either be USA sizes, or BSP (British Standard Pipe) sizes which are similar to but NOT the same. Generally, when a thread specification is called out on a pipe, it will determine which system is needed. However, this is not an absolute. Many of the pipe sizes will support either the BSP or USA (NPT) threads. Nominal Pipe Size BSP pipe O.D. in mm BSP pipe O.D. in inches USA pipe O.D. in mm USA pipe O.D. in inches 1/8 10.2 0.402 10.3 0.405 1/4 13.5 0.531 13.7 0.540 3/8 17.2 0.677 17.1 0.675 1/2 21.3 0.839 21.3 0.840 3/4 26.9 1.059 26.7 1.050 1 33.7 1.327 33.4 1.315 1 1/4 42.4 1.669 42.2 1.660 1 1/2 48.3 1.902 48.3 1.900 2 60.3 2.375 60.3 2.375 2 1/2 76.1 2.996 73 2.875 3 88.9 3.500 88.9 3.500 3 1/2 101.6 4.000 101.6 4.000 4 114.3 4.500 114.3 4.500 5 139.7 5.500 141.3 5.563 6 165.1** 6.500 168.3 6.625 8 219.1 8.626 219.1 8.625 10 273 10.75 273 10.75 12 323.9 12.75 323.8 12.75 14 355.6 14.000 355.6 14.000 16 406.4 16.000 406.4 16.000 18 457 17.992 457.2 18.000 20 508 20.000 508 20.000 **Depending upon the standard number, this may also be 168.3 mm THINK ! - MARYLAND METRICS - The One-Stop Source For Metric & British Sized Fasteners, Wrenches, Cutting, & Measuring Tools, Metal Shapes, Oil Seals, O-Rings, Mechanical Power Transmission Equipment, Bearings, Hydraulic and Pneumatic Fittings & Tubing, Workholding Components, Plumbing Fittings, & Some Electrical & Electronic Components. Click: http://mdmetric.com   for the MARYLAND METRICS   P.O. Box 261   Owings Mills, MD 21117 USA website.