I dont' remember the title. I thought it was "King Boxer" but all the films with this name that I found are not the one I am looking for.

by SPYROS M ELEFTHERIADES
(NICOSIA, CYPRUS)

This film I saw, if I remember well, at the late sixties.
I remember that the main actor was a young, well build man. His enemy was unbeatable because when he was ready nothing could hurt him. He made his body hard as steel.
The main actor trained his fingers to make them hard. He was punching in hot sand and then on a huge metal bell. He stood an arm's
distance from the bell with his fist closed. He punched at the bell but his closed fist
did not reached it, just one hair away, and then he stretched his fingers and with them he hit the bell. He did this until the bell rang. Then he was ready to face his enemy.
They fought but his punched did nothing to the enemy until he used his secret weapon.
He punched the enemy in the middle of his belly with closed fist. Of course nothing happened because the enemy was prepared, but when he relaxed his muscles the stretched fingers did the trick. Does anybody know the title of this film? I would like to see it again.