Re: Is it worth it? 1955 Mark 55
Hi Ron,<br /> Actually, the Mark 58A is a 1959 model. The 'A' designation on any Mark-series motor means 1959. That was Mercury's 25-th Anniversary model year, 'A' designating anniversary.<br /> The Mark 58A and Mark 55 are quite similar with some significant differences. Most significant is the displacement, as the Mark 55 is 39.6 cubic inches, and the Mark 58A is about 44-cubic inches. The increase was gained via an increase in bore size, with all else remaining the same (rods, crankshaft, bearings, etc.). The Mark 58A also had larger intkae openings on the crankcase, which accomodated larger carburetors. The Mark 55 put out a solid 40hp, while the 58A was rated at 45hp. It is possible to sneak a Mark 58A powerhead under a Mark 55 cowling and have a bit of a sleeper motor, although you likely won't notice the difference on anything but the lightest of runabouts. Pulling power might be a bit better, too. Where you'll notice it the most is at the gas pump, as the bigger carbs will drink more fuel.<br /> The Mark 58A will also have the better starting system that uses a traditional style spin-up strter bendix, rather than the side-arm one used on the 1955 model.<br /> The lower units are interchanceable, and except for the earliest 1955 Mark 55s, the props should be interchangeable, too. Early 1955 models had a 14-spline propshaft, while all other 40/44 cube Mercs up to 1970 (or so) had 11-spline propshafts.<br /> The "interface" as you call it was updated to the more common rubber boot sometime in 1956. The earlier metal plugs work fine, but are getting harder to replace if needed.<br /> Hope that helps...<br />- Scott
