Immersion Heaters FAQ

Range BSP Threads on Brass Heads used in immersion heaters.

It should be simple.

You measure something, and that is the size.

Not if you are measuring a BSP (Bristish Standard Pipe) screwplug, sadly.

The size was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended. This often causes confusion, as "normal" people think the size refers to the outside diameter of the male thread.

For example, the most common BSP Thread Size for UK Immersion Heaters is 2¼” BSP. If you measure roughly across the threads to check the diameter, it looks closer to 2½”, 2.58″ or 65.7mm, to be precise. Best be using a vernier gauge to be that accurate!

If you ask for a 2½” BSP immersion heater to be made, it will arrive with a thread size of almost 2.96″ or 75.18mm, which will be really annoying as there is no way it will fit, it will be the thread size you asked for but not the size you need. It is the same for all other BSP threads so caution is required.

Here at IHUK we believe prevention is better than cure, so here you go -

    1. Cut a strip of paper that you can wrap around the thread you want to measure.

    1. Draw a Line across it with a pen or pencil.

    1. Wrap the paper strip once around the thread and make a second mark where the paper overlaps the first mark.

    1. Unwrap the paper and measure the distance between the marks. This should correspond to one of the measurements on C=Circumference (C)  mm  above.