Re: HP difference
First, I think you are talking Chrysler because to the best of my knowledge, Force never made a 55 or 60.
NOW: The Chrysler 50 was old school, 2 cylinder, single carb, 3.187 bore, 44 cubic inch, two piece lower unit with a 5/8 (I think) prop shaft, 11 splines? and a 9/16 inch drive shaft of the 35, 45, 50, and 55 series.
The 55 was made in two versions: One the same as the 50 with a spacer in the exhaust chest for more volume and easier breathing, slightly different porting and a different carb.
The second version was the two cylinder 55, 60, and 65 horse engine series. These engines were completely different, having been engineered from scratch. They featured big bores (3.375), 50 cubic inch, long rods, packed crankcase, massive crankshaft, larger bearings, through (piston skirt) induction, separate exhaust runners, and two carbs. A real monster of an engine with gobs of low end torque. Later models featured a one piece lower unit just like the bigger 3 and 4 cylinder brothers, and while the prop was still 10 3/8 diameter, the prop shaft was 3/4 (I think 13 spline) and the drive shaft was also either 5/8 or 3/4 inch.
