For Sale: 25.4 titanium seat pin £25 - pair of campag hubs £25 - 30.2 tange alloy headset £15

Sheldon says (http://www.sheldonbrown.com/headsets.html) that ISO (for 1" steerer diameter) headsets have 26.4mm crown race inside diameter and 30.2mm frame cup outside diameter.

I had assumed 30.2mm was the stack height- perhaps it is actually one of these: http://www.sjscycles.co.uk/product-Tange-Tange-Levin-1inch-Chrome-Steel-Cup-Threaded-Headset--33.3mm-Stack-Height--Fork-Seat-26.4mm--Frame-30.2mm-10953.htm

Maybe measure the stack height to be sure?

the outer 'tolerance fit' into the head tube
-30.20

you 'may' be confusing yourself hither, as
1 inch and a quater is 31.750mm

Ah ha. These are the same thing.

So it seems the headset is 1" ISO standard. As always Sheldon has the answer :-)