NSP (National Standard Pipe thread) is the standard most commonly used for plumbing connections in the United States. Unlike many European systems, where thread sizes are often specified differently, NSP threads are defined by the number of threads per inch (also known as the "pitch") and the angle of the thread. NSP threads feature rounded peaks and valleys, following the Whitworth thread form.
There are two main types of NSP threads:
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NSPT – Tapered threads
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NSPP – Parallel threads
NSP sizes are always specified in imperial measurements. For example, our cast iron radiators are fitted with ½" NSP reducing bushings.
The table below provides a useful reference for standard NSP sizes, showing both metric (mm) and imperial (inch) equivalents:
| NSP Size | Actual Diameter |
|---|---|
| ⅛" | 0.35 inch, 9 mm |
| ¼" | ½ inch, 12 mm |
| ⅜" | 0.6 inch, 15 mm |
| ½" | 0.75 inch, 19 mm |
| ¾" | 1 inch, 25 mm |
| 1" | 1.25 inch, 31 mm |
| 1¼" | 1.67 inch, 40 mm |
| 1½" | 1.75 inch, 45 mm |
| 2" | 2.25 inch, 57 mm |
| 2½" | 2.87 inch, 73 mm |
| 3" | 3.33 inch, 85 mm |
If you have any questions about your cast iron radiator installation, we recommend consulting a licensed plumber or heating contractor for professional guidance. You can also contact our team directly at 212-814-6888 — we’ll be happy to discuss your requirements and provide expert support.