I've read Sheldon's advice on identifying threaded headsets, but the measurements are very close between JIS and ISO, so I want to make sure what I'm measuring.

I popped the fork crown race off, and measured the diameter of the inside edge where it fits on the fork. That was almost exactly 27.0mm, so I thought I was good until I read about mixed standard headsets where builders would use an ISO cup with a JIS cone. Grrr....whoever started that should be beaten senseless.

I haven't taken the headset out of the frame yet, and at first glance I couldn't find anything that looked about 30mm across. Is it the outer lip of the headset where it fits in the frame? I was hoping not to have to remove that before ordering a replacement, but if I have to, I have to.

Is there any other way of distinguishing whether I have a full JIS headset, or a mix/match? Anything else I can measure, or just eliminate based on the type of bike? If it helps, the bike is a 1992 or 1993 Giant Nutra, which was low-end bike shop quality (~$250 then) with components comparable to today's Acera. It's a hybrid (or what passed for that then) if it matters, with mountain dropouts (135mm) but 700c wheels.

Thanks all.

You'll have to pull to measure - either the inside of the headtube or the outside of the inserted cup.

I've used plumbing Teflon tape to shim a slightly too small cup into a frame and so far, so good. I'm sure the beer can shim or aluminum tape shim (auto body repair tape) could work, too.