I had a strange experience with a headset on my 80's Raleigh.
Basically, the headset that was on the bike wasn't compatible with the threading of the fork. I'm not sure if Raleigh used a weird, non-standard thread at some point, but none of the cones that I had would thread on reliably. Instead, what would happen is that I could get everything snugged up, but as soon as I tightened the lock nut down, it would push the cone off half of its thread. Basically, the thread on the fork was too small for the cone to fit and when I took the fork off the bike, you could see the way that the cone would slip if you put force on it.
So I ended up with a new fork and a new headset.
YMMV.