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NPT Thread Size Guides

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Use our tables and free downloads to ensure you're ordering the correct parts when replacing bushings and parts on old radiators.

National Pipe Thread: The North American standard

National Pipe Thread is the standard thread used on plumbing fittings in North America (but not in most of the rest of the world).

- Download our NPT Thread Size Template here -

The nominal size refers not to the outside of the pipe, nor even to the outside of the thread, but to its internal bore - the inside of the tube where the fluid flows. It makes sense when sizing pipes for flow rate but it makes things confusing when measuring for replacement parts.

Below is a summary of the common NPT thread sizes used with cast iron radiators and their corresponding actual outside diameters. The actual outside diameter is what you can measure when looking at a valve spud, a bushing or an opening in the radiator.

Nominal pipe size (NPT) Threads per inch Actual outside diameter, OD
inch mm
18" 27 0.405 10.3
14" 18 0.540 13.7
38" 18 0.675 17.1
12" 14 0.840 21.3
34" 14 1.050 26.7
1" 11-12 1.315 33.4
1-14" 11-12 1.660 42.2
1-12 11-12 1.900 48.3
2" 11-12 2.375 60.3
2-12" 8 2.875 73.0

Right first time

We've made a handy PDF that you can print out at home to use as a gauge when measuring threads. Download our NPT Thread Size Template, print it onto US Letter paper at 100% scale and follow the instructions. It includes both circles that are sized for various NPT threads as well as a handy cut-off tape measure that wraps around the pipe if the ends aren't accessible.

Diagram: Adding a plug to a cast iron radiator

- Download our NPT Thread Size Template here -

The difference between NPT and BSP

NPT - National Pipe Thread - is the US standard. Most of the rest of the world uses BSP - British Standard Pipe.

NPT has flattened peaks and troughs where BSP is rounded. NPT has a thread angle of 60º where BSP is 55º.

At certain sizes, NPT and BSP share the same number of threads per inch - at ½" and ¾" - but at all other sizes the two are quite different. The table below lists the TPI for both NPT and BSP at sizes commonly used with domestic radiators.

Nominal pipe size NPT BSP
Threads per inch Threads per inch
18" 27 28
14" 18 19
38" 18 19
12" 14 14
34" 14 14
1" 11-12 11
1-14" 11-12 11
1-12 11-12 11
2" 11-12 11
2-12" 8 11

The threads we use in our radiators

In brief: products for sale in North America ship with NPT threads; products for sale in Europe ship with BSP threads.

Our radiators are fitted with steel NPT bushings for the American market. 

Our valves, for the US market, use NPT spuds and, where it's a compression fitting, are made for ½" US-standard pipe, with a ⅝" ferrule. Where the valve uses threaded connections, those are US-standard FIP (Female Iron Pipe).

Two-pipe steam radiator in Park Slope Brownstone