If an engine is knocking hard enough to make the distributor cap move, don't lean over the fender without a ballistic vest on- - - - -it's about to start throwing pieces in all directions. That being said, any rocking motion between the cap and the housing is too much. Either find a better[fitting cap, ort make that one fit correctly.

What's a stovebolter doing going to a "restoration shop" for a tune-up in the first place?????????
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!