Why Speedex made the engines slide back and forth to tension the drive belt is beyond me lol. IMO that is an overly complicated, bad design with many components that will wear out.
I had a Speedex tractor that the belt was tensioned by sliding the engine back and forth. When I got it the engine cradle was worn significantly and the rollers on the bottom had worn into the frame and caused the engine to violently jump around and randomly throw the belt off. It was a square hood tractor with that funky round transmission. Pretty much the same as an S14 but with the mid 70s hood and squared off fenders.
Very neat little tractors though. They were quite unique from any other machine of the time.