Why doesn't the thread on a 2 inch pump connection measure 2 inches?
It's a great question and it causes great disbelief when we explain it to customers. In fact if you measure the outside diameter of the thread it will be 78mm. Confusing, eh?
If you've come here after reading the Blog post on pipe sizes, you'll already know how pipes are sized and why. To accommodate a 2 inch pipe (actually measuring more than 2 inches) the pump thread needs to be larger because it goes around the outside of the pipe to allow full water flow through the connection. So, the 2 inch pump union connection measures 78mm (give or take a mm or two) across the outside diameter of the thread.
Take care because the suction connection on the front of the pump is often larger than the discharge connection which points at right angles to the suction. Typically this can be 2.5 inch suction x 2 inch discharge.
| Connection Size | Outside Diameter of Thread |
| 1 inch | 50mm |
| 1.5 inch | 62mm |
| 2 inch | 78mm |
| 2.5 inch | 93mm |
