I was led to believe that the shorter engines were developed for the glow headed versions to maintain the compression in light of the removal of a contra piston. Just a shorter cylinder with same rod and stroke. Sounds possible .This was later adopted for the sparkie too.. Then to standardize production it was adopted for the diesel version too.with a shorter rod. How much of this is correct I cannot say with any degree of certainty. It was related to me by a former machinist of E.D. so i'm at least half inclined to accept it.