After all the years in this business I always have to think about pipe sizes, etc. when I am mounting antennas, etc. Here is a very nice sizing chart courtesy of Vogel Tools. http://www.vogeltool.com/ansi.html Chuck Lakaytis Director of Engineering Alaska Public Broadcasting, Inc. 135 Cordova Street Anchorage, AK 99501 907 277 6300 907 277 6359 FAX 907 301 4339 Cell