You might try rethreading the top of the shaft with a slightly smaller thread, maybe a metric one a few thousandths smaller in diameter. As long as the shaft thread and the nut used on it match, a different thread pitch and/or diameter won't cause any problems. Otherwise, you'll need to change the entire shaft and worm gear assembly.
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!