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Some hopefully non-dumb answers (but watch out, I may be just another uninformed chopper).
1) I've seen some ISO headsets advertised as NJS. Not seen them in the flesh, though. I had thought they would only be certified if they were JIS. Headsets should be stamped JIS (27/30) or ENG/ISO (26.4/30.2).
Stacks should be similar, but not necessarily identical. My two NJS frames have about 0.3mm difference in the amount of stack needed.
Someone had bodged ISO cups and a JIS crown race into my Makino. The dodgy interface chewed the crown and took some fixing.
2) Yes, 68mm shell, BSC/ISO thread. But always check if someone has messed your threads.
lesterama-
Cool, so it's just going to be a measure and see job on the headset.
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I may be talking out of order and not helpful but I've owned a bunch of NJS frames and all had Hatta or Shimano headsets except for my Samson with a Chris King.
I bought the Samson used long time ago and thought it was maybe a hackjob seeing the King headset but it all appears kosher. No grinds and have inspected all of the headtube/fork races.
Long time ago I had one of those pedalroom accounts and would get constant direct messages asking how I got a Chris King on a NJS bike and the whole ISO/JIS deal.
Also have a 3Rensho with a rare Shimano headset that's not supposed to fit. But it does.
Now that I've typed all of this out, I expect every headset of mine to explode in a loose ball bearing head tube pipe bomb on the next ride.
Jonny69
user86310
Couple of dumb questions:
1) Are NJS headsets all the same, in terms of crown race diameter, cup diameters and stack heights? Or are they all different?
2) Are bottom brackets conventional threading and 68mm wide? Will something like a UN-55 go straight in?