Re: Can 1955 mark 25 lower drive unit fit 1956 mark 25

[20mercman]

As for oil in the Mk-25’s, These are all full roller bearing motors, and Mercury years ago stated that they can be run on 50:1 ratio with no harm. I use 48:1 usually, easier math, but the idea is the same. There is no major difference in the internals of this motor than the 1981 Mercury 20hp that ended the line for this fine family of motors. Too rich of a mixture can cause loading up at idle and poor idle quality. Please, not trying to open a can of worms on an oil debate. I use 24:1 on after a rebuild and break-in, but after that, they don’t need the rich mixtures.

Steve