Ralph Macchio -- The Crane Kick Is BS ... Would Never Work In a Real Fight

Mr. Miyagi's gotta be rollin' in his grave ... because his finest pupil just TALKED SMACK about the legendary crane kick ... telling TMZ Sports the move would never work in a real fight.

Daniel LaRusso himself -- Ralph Macchio -- was out in NYC yesterday ... almost 30 YEARS after the "Karate Kid" was released in theaters ... when he dropped a truth bomb about the most iconic fighting move of all time.

Note to Cobra Kai -- sweeping the leg ... also generally ineffective.