Mr. Diethorn, of Brookline, was the coach at the highly successful St. James and St. Canice schools in the Catholic B League.
His teams won 10 section titles, six city titles and four state titles. But he is best known for compiling a 66-game winning streak at St. James from 1949 to 1952, a Catholic League record.
"He was quite a man," said Rocco Julian, who played for those undefeated teams. "He not only taught us basketball, he taught us how to live. There's no 'I' in team, and we did it together. I'll miss him. He was like a father to us. He was one in a million."
Mr. Julian said one of the secrets to the winning streak was that Mr. Diethorn was able to keep his players focused by keeping their egos in check.
"He kept telling us, 'You're good, but there's always someone out there who's better than you, so don't get a big head.' We did it with his help and support."
The winning streak was snapped by St. Adalbert on the South Side.
It was a one-point loss. Undaunted by the setback, St. James would win its next 23 games.
During his 30 years in coaching, Mr. Diethorn compiled a 547-202 record. For his accomplishments, he was twice a recipient of Dapper Dan awards for coaching, in 1951 and 1964. His 1964 team at St. Canice was 33-1 and won the state championship in its classification.
According to his son, Richard, one of his father's proudest moments came two years earlier when he led a team without a player over 6 feet to the state championship game.
That team, with an average height of 5-10, lost the state title game to South Scranton and finished with a 25-2 record.
Above and beyond the wins and losses, Mr. Diethorn enjoyed shaping the lives of young people through his coaching.
"Dad was most proud of the players he had and the hardships they had to overcome," said Richard Diethorn, of Baden.
"Some of the players he had could have ended up on the wrong side of the tracks.
"He got just about every one of them into college and they all kept in touch with him. He always said he was more of a teacher than a coach."
Mr. Diethorn worked in the auditing department at The Pittsburgh Press and Post-Gazette for 45 years before retiring in the late 1970s.
He was inducted to the western chapter of the Pennsylvania Sports Hall of Fame in 1986.
Mr. Diethorn is survived by four other sons, Thomas of Delray Beach, Fla.; William and James of Bethel Park; and John of Murrysville; two daughters, Diane Bogatay of Brookline and Kathleen Vallier of Hampton, N.H.; a brother, Elmer Diethorn of Whitehall; sisters Marie Boyle of Bethel Park and Dorothy Lewis of Brentwood; 17 grandchildren; and seven great-grandchildren.
Visitation is today and tomorrow from 2 to 4 and 6 to 8 p.m. at Frank F. De Bor Funeral Home, 1065 Brookline Blvd.
A Mass will be celebrated at 10 a.m. Thursday at Church of the Resurrection.