I measured across the thread of my new taps to go in and they measured 3/4 inch. I went off and bought 3/4 inch tap connectors only to find they were too big! I bought 1/2 inch tap connectors later on and they fitted perfectly! I have since measured the 3/4 inch tap connector and it measures an inch across! In fact, nothing on the fitting measures 3/4 inch! Could anybody explain this?

There are industry standards that dictate the nominal inside and outside daimeters of pipes. In general the size quoted is approximately the inside diameter of the pipe, although it varies depending on wall thickness. The outside diameter is therefore larger than the quoted size. A 3/4" pipe for example has OD of 1.05 inches, and an inside diameter that could be between 0.74 to 0.88 inches. See the attached link for a chart of the different standard sizes.

By the way - there is no need to use exclamation points on every sentence.

http://i65.photobucket.com/albums/h215/jaredef/AmericanStandardPipeDiameters.jpg

Figure it this way, The OD of a 1/2" Id fitting is 5/8ths and the OD of a 3/4" fitting is 7/8ths, Does that make it a little simpler> Cheers, Tom

Sounds like the IRS came up with how to size pipes. (<:

I think the simple answer here is that pipe is measured by its INSIDE diameter.

It's the same reason that a 38 and 357 magnum have the same diameter, 0.357. Why is it a 38?