Which threaded headset size do I use? I have a frame that I want to use. It is an old early 80's Cannondale frame. How do I tell what size threaded headset to use? What diameter should the tube be?
If you have the fork in hand, it's easy enough to directly measure the outer diameter of the steerer tube using a set of calipers or a ruler across the top. The common threaded headset size is 1" (25.4mm), but 1 1/8" (28.6mm) is not unheard of.

If you have only the frame and are trying to measure from the inner diameter of the head tube, then you are measuring for the outer diameter of the headset cups. For a 1" headset this should be 30.1mm, for a 1 1/8" headset this should be 33.9mm, give or take at most 0.2mm.

1980's should be too early, but Cannondale also experimented with oversize head tubes at several points - if the fork or frame is larger than that specified for the 1 1/8" size, you'll probably want to look into a different fork, and probably a reducing headset as parts are going to be almost impossible to find.