The threaded steer tubes would be dictated by vintage standards.
https://www.sheldonbrown.com/cribsheet-headsets.html
OD: 25.4
ID/Stem 22.2 (except French 22.0).
So 25.4 - 22.2 = 3.2mm
3.2mm / 2 = 1.6mm
Crown Race: 26.4/26.5/27
It almost sounds like your steer tubes are too thick.
Vintage BMX?
26.4 Crown Race, 25.4 OD, 21.4 Stem?
25.4 - 21.4 = 4mm... / 2 = 2mm.