U bolts are sized to suit cast iron british standard pipe (BSP) which is measured by NB (nominal bore), you may use them with steel pipe or scaffold tube but you need to measure it correctly Nominal bore is the internal size of the pipe itself and not the external measured size, although pipe sizes are widely understood by industry we have detailed the internal size of the U bolt to assist you The NB (nominal bore) size is measured like this So if you are using BSP pipe then the NB size is the correct size to order. If you are using steel pipe or scaffold tube you need to look at the ‘between the leg’ size to order the correct size 1 inch 25mm NB = 33.7mm between the legs of the U bolt. Leg diameter is 8mm, the length of thread is 35mm 1.1/4 inch 32mm NB = 45mm between the legs of the U bolt.
eBay