Hello, first time posting in this forum.
I have a 1981 Miyata Seven Ten I bought new in '82 that I'm trying to get it in decent form so my son can ride it.
I believe the headset is shot and in need of a replacemnet because of strong indexed steering.
The HS is stamped "Tange Falcon No FL250" and has not been removed yet.
I am looking to replace it with a same or similar Tange HS.
From what I can measure on the bike, the stem diameter is around 22.2mm and the HS stack height is a combined ~42mm.
Can I assume this is a JIS spec and buy a 27mm inside diameter Crown race (and not the 26.4mm) or do I need to take it apart and measure it?

Thanks!

First, welcome to the classic & vintage bike forum.

If you can somehow tell that the steer tube is 22.2mm dia, why not use the same way to measure the crown race diameter?

If it's much over 37mm stack height you'll get the final result with the lock washer and spacer shims, even if you get a headset with a short stack height.

I think most of the 80's Miyatas are ISO 26.4mm crown races, and the 700 series you have is from a better model line, which makes that more likely. I think you can get Tange Levin from online retailers for ~ $19 +shipping.

The tools are another story - the crown race extractor can be $60-150 and the installer/setter $25-75. LBS might be the way to go to pop the old one off the fork and put a new one on.

Sometimes you can buy some time by removing the caged bearings and installing more loose bearings and the indexing goes away as the bearings are riding in different positions

Take it apart and measure. I'd guess it's 26.4 but who knows? Even if it's 27.0 a good shop could cut it down to 26.4 if you want more headset choices. I have no trouble removing all the headset pieces with a punch and hammer but YMMV.

Welcome to BikeForums! Sounds like a nice bike and I'm sure it will be a great rider for your son. While you're at it I assume you're going to overhaul the rest of the bike, too. Oh....we'll need pics once it's done

JIS and ISO headset were available with both 27.0 and 26.4 fork races. The only sure fire was to tell what your bike has is to measure. Sometines the inside of the cups are stamped as well as the underside of the fork race.

Thank you for the welcome and advice!
I measured the stem to be 22.2mm just above the HS, not sure if that's the same as steer tube measurement; being a bike novice, I may be misusing some of the terms I picked up browsing this great forum.

I guess I should take the HS apart and measure, and also look for additional lettering.
I read about swapping the caged bearings for loose bearings but I'm leaning toward r/r the entire HS so I won't have to touch that part again for some time.

Is there much difference between the Tange Levin and Falcon HS(~$40 +s/h)? I am hoping to replace parts as close to original if possible.

I would like to overhaul everything but it's going to take years with all my other projects needing attention around the house (per my beautiful wife) so what I will do is post current pics because I don't know when I'll be able to post finished pics.