I threw away my bent fork before i knew i needed to measure the diameter. is there a way to determine that without buying one and trying it first?

fork attached to stem diameters? threaded usually 1" briefly 1.125"

threadless? Briefly 1" more common 1.125".

measure the hole in the bearing set it went through. Though I'm assuming you mean the diameter of the steering tube. The stem is the thing between the handlebars and the steering tube.

The steering tube is what goes through the head tube and into the fork. Either way, the above answer in post #2 should work.

Or do you have one of those old style '7' handlebar stems with a quill which might get more questions about what you are really asking for.

The short answer: Yes.

While accurate, the above answer probably isn't too helpful. We can likely tell you exactly what you'd need if you can provide more info.

What kind of bike is it? (Brand? Model? Age?)

Does your stem clamp onto the fork's steerer tube? If so, you've got a threadless headset & fork.

Does your stem have a quill that inserts down into the the fork's steerer tube? If so, you've got a threaded headset & fork.

Once you know whether you need threaded or threadless, you'll need to know the diameter of the steerer tube. 1" and 1 1/8" are common sizes. If you've got a threadless setup, you can measure the inside of the stem clamp.

One other dimension you'll need to be aware of is the length of the steerer tube. It needs to be the right length for your frame. If you've got a threadless setup, this is easy since it can be cut to length. Threaded forks need to have threads for the headset in the proper spot, so you may not be able to trim a threaded fork to length. (Not without tooling to cut more threads.)

If threaded, there are a couple of other considerations: Crown race size can vary, with common ones being 26.4 mm (ISO) or 27.0 mm (Japanese Industrial Standard). And if you've got an old bike from certain countries (namely France), the fork and headset threading may not be compatible with current replacement parts.

Soooo... What kind of bike is it, again?