Here is a few quotes from a very knowledgeable man from another site about what you seek.
QUOTE:
"Hi Bill, Here goes. The first "Big Twin", as they are commonly referred to, came out in 1951 as a 25hp. They were approximately 35.7 cui. This model ran through the 1954 model year, more or less unchanged.1955 saw a fairly substantial change toward quieting the motors, as they sealed the powerhead and made changes to the exhaust and lower unit sections. Horsepower remained 25 and displacement remained 35.7. They also changed the throttle arrangement from cables to gears where it entered the powerhead area.1956 saw a change in the cylinder head and piston dome to go from a 5:1 compression ratio to 6:1. Displacement remained 35.7, but hp bumped to 30.1957 they bumped the displacement up to 40.5. Same stroke, larger bore. Horsepower was bumped to 35. 1958 marked the introduction of the "Super Quiet" motors with the double wall mid-sections and better sealing for sound. Evinrude's was the Lark, and Johnson's was the Super Seahorse. You could get either the Super Quiet version or the standard Big Twin model in 1958 (similar to the 1957s without the quiet package).In 1959 the quiet models were the only 35s available.1960 saw a bump up in displacement once more to 43.9 cui and horsepower went to 40. Unfortunately, they were still using essentially the same crankshaft that they started with in 1951 and the 40s began breaking cranks. 1961 saw a beefier crankshaft introduced, retaining the same stroke as 1960 and prior.Hope that helps!- Scott"
QUOTE:
"Mostly... I had to double check on the displacement for the 40 horse.. Forgot to handle point three of Bill's query...Lots of interchangeability between years. Lower units from 1951 to 1954 interchange. Units from '55 to '57 interchange. '58 units get a little interesting as they had the two versions offered. Silent units interchange up 'til they went to electric shift in the 1960s. Base (non silent package) model units interchange with earlier models without the silent pkg. Units generally interchange as an assembly between years, within a style, as there was sometimes variations in the innards themselves between years. Magneto parts are fairly standardized throughout the run, but flywheels changed periodically. Confused yet?It's interesting that the Big Twin design remained in the OMC lineup well into the 1980s as the WorkTwin 40. Made for a 30+ year run using the same basic design....- Scott"
This is not my info but I do know this man knows his stuff..... hope it helps you. He recited almost all this info off the top of his head :shock:
