Loose Headset

.....having recently built up a late 1970's Benotto I find that the headset keeps loosening after riding approx. 20 miles or thereabouts, it is fitted with a Campag Super Record headset complete with the correct size and amount of 3/16th bearings in their retainers also with the stem out I can still see about 3mm of top locknut thread so it's not too low on the stack height, can anyone advise?
Griff

Is the tab washer missing/not fitted correctly?

the original tab washer is in place, it's just getting frustrating having to keep tightening every day.....

The top of the headset should be working like a locknut. Tightened against each other using tools, they shouldn't be coming undone.

When I've experienced this sort of thing, it was because I had tightened the headset by hand, and whilst my grip is incredibly strong, it is not quite the same as using big spanners.

I'm tightening the top lock nut by using two headset spanners....one to hold the the upper race and the other to hold the lock nut, when done all feels fine but an hour into a ride and both are loose again!

is the crown race cracked?

When you tighten the headset do the top cup and locknut lock together solidly?

Whenever I have had experienced threaded headsets coming loose its been because something* has had substandard threads and this can usually be felt when locking the top cup and locknut.

*Could be the thread on the top cup, the locknut or the fork.

everything seems fine with the headset once installed but being an alloy one I'm wondering if the threads are ok after 35 years (although they seem to still tighten up ok)......guess I'll need to swap it over to another headset to find out!

You could try some sort of threadlock compound. I'd be interested in how you get on.

Give Loctite a try, though it is unusual for them to loosen when tightened with two spanners as you have done.