The fork is supposed to engage a slot on the throwout bearing, (which slides back and forth on the front bearing retainer) and a ball-shaped pivot that is bolted into the bell housing. The ball socket on the fork has a retainer spring that's supposed to snap over the ball, but they get broken or lost pretty regularly. Most of the resto-ripoff vendors have replacements for the retainer springs available.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!