I needed to cap an open threaded side of a tap after removinhg a heater, so measured it with my (metric) rule, OD was almost exactly 25mm. So I went to my plumbers merchant and asked for a 1" cap. It was too large and measured 32mm. I went back and told him it was too big, he asked what size I wanted, I said I thought I wanted an inch, it seems what I need is 3/4 inch? So what is the measuring of? Is it the intenal diameter of the pipe it is used with?
Yes.
This was done a few days ago on here
See :
Strictly speaking, it is the internal diameter of a particular grade of wrought iron (under 3") or cast iron (3" and over) pipe that results in the wall thickness that gives the correct OD for the thread. In practice, the differences in materials and / or grade will not make enough difference to the ID to be noticeable if you are using a rule to measure it.
Colin Bignell
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