Measuring a headset compression ring?
 

Sort of destroyed mine, 1988 Fuji Del Rey. How would I go about measuring or finding the size of one so I can get a replacement? Thank you!

Hm, I'd guess many are fairly generic to the steerer size, eg 9/8". Some might not play well with other brands, but I'm tipping that'd be rare.

You should be able to score one for $0 at your LBS.

Come to think of it, '88? If it's a road bike, that could be a 1" one... threadless 1" setups are a bit on the rare side.

Compression rings don't conform to any standards. They're an internal part of the headset, and so are brand specific. That said, there are many different headsets that share parts. Start by searching based on the specific brand and if you don't find one, look for similar looking headsets from the same country, which hopefully will be made by the same people, or at least ones who share vendors.

I don't know that you'll find one new, so the best place to look is in the scavenge boxes that many old time mechanic keep of parts saved from repairs. Also you might find something at a co-op where it might have come off a relic scrapped for parts.

BTW- how badly did you "sort of" destroy the original one. There's plenty of latitude in condition where they'll still work. As long as you have 3 OK areas spaced roughly equally, they'll properly center and support the cup (think of a Tri-pod). Before you go nuts looking for a replacement, see if you can file off any high spots, so then headset can rest on the original surfaces.

It was cracked and split, and now in about 10 different pieces. Its a threaded setup, could I just get away without it? It fits into a conical groove in the upper race ring.

If the section with the upper bearing isn't threaded then it depends on the compression ring for centering and stability against the steerer. In a pinch you can make do with an O-ring. You need one that is a bit too big to squeeze in there, and sticks out over the top a bit so the next part can compress it into the conical space. Go to the hardware store and bring back a few in the right diameter, but varying thicknesses, and try them trial and error until you come up with a winner.

You'll notice that I started by saying that if the upper bearing unit wasn't threaded... That's because compression rings are usually used in threadless headsets, and hybrid headsets that use a compression ring combined with a threaded locknut are rare birds.