I'm looking for a replacement headset for the no-name one that originally came with my (mid-1980's) Panasonic DX-2000 (Japanese) road bike. I know that it's a "standard" 1-inch headset, but I don't know for certain if it's ISO or JIS. Sheldon notes a difference of 30.2 vs. 30.0 for the "cup outside diameter" but I'm a little uncertain where to measure this. Is this the part that fits inside the headtube? Or does someone know with a high degree of certainty that it is ISO?

Also, I've done my best to measure stack height without re-assembling. So without the ball bearings, the lower cup, upper cup, upper race, washer, locknut add up to about 38 mm, only counting the parts of the cups that are exposed. Seems to me, if anything, adding the bearings would make it a little bigger. So am I OK with anything that has a stack height of 38 or less?

And if looking on eBay, how would one know the stack height of a particular headset if it's not stated? Just a matter of searching the web for a reference??

Thanks.

Ye u are right is the Internal diameter of the headtube. What I would do is this, take the fork out, look at the crownrace area, measure right above and flushed to the crownrace it should be 26.4, if something else like 27 at least you will know what crownrace headset to look for. U can put a jis in an ISO frame and backwards too as long u are not playing in and out every single day with those cups. the .2 in my opinion is not that critical. The crownrace is the critical part.

Good luck.

Are you talking about measuring the diameter of the "shoulder" that the crown race is seated on (i.e. the part slightly larger than the tube itself)?

You're starting off the hard way.
I'm assuming you have everything disassembled, so to measure the required stack height:
1. Measure the length of your steerer to the bottom of the crown race seat.
2. Measure the length of the headtube on your frame.
3. Subtract #2 from #1. The result is the MAXIMUM stack height headset you want. Anything shorter you can make up with spacers.
4. Measure your fork crown [as you indicated in the post above]. If it's 26.4 then you have an ISO headset. Skip step 5. If it's 27.0, you have a JIS crown - but it's possible you have either a JIS or ISO headtube. You can buy mixed headsets - I've had to buy 'em. Proceed to step 5.
5. Measure the ID of your headtube [where the lower cup/upper race seat] or the insertable OD of your headset cup/race to determine the proper headset standard.

You will need a digital caliper to make these measurements. Harbor Freight currently has their 6" model on sale for $9.99. I own one - it's perfectly fine for a bike mechanic. I probably wouldn't use it on my Biturbo crankshaft.

The crown race seat (dimension "L" below) is 27.0mm for JIS and 26.4mm for ISO.

The head race seats (dimension "K") are 30.0mm for JIS and 30.2mm for ISO.

It is not difficult to mill a JIS frame to accept ISO parts if finding a JIS headset is a problem.

Thanks, all. Will be doing more measurements this evening, I guess.

The steerer - head tube was right around 40mm.

Measure twice, cut once.

Good thing I checked -- looks pretty clearly like 27mm and 30mm, spot on with my $5 analog caliper. Looks like I'll be shopping for a JIS headset.

YEs

Yes too.

I ordered this Velo Orange headset. The stack height is a little iffy, but hopefully it'll work out.