Re: will 650 drive lower unit fit on my 850?
Depends on what year and # of cylinders your motor has. They made 850's in 4-cyl and 6-cyl flavors.
If the donor motor is a 4-cyl 65 it will likely fit. 3-cyl 65's have a different lower unit.
What you want to do is make sure the styles match up, i.e. if your motor is a 4-cyl it'll have an exhaust tube, the 650 needs to have fitment for one also. You'll be able to tell by the presence of a rubber exhaust tube gasket in the lower unit.
If you have an older 850-6, you can adapt a newer-style lower unit, with mods to the shift shaft and reverse lockout assy. But you can't adapt an older, non-exhaust-tube lower unit to a motor with exhaust tube.
Other than that, the bolt patterns, driveshaft diameters, and spline counts are the same.
Gear ratio's may be different. Later model ('70, '71 if I recall) 650's had a 2.3:1 gear ratio, same as the 4-cyl 800's, 850's, 80's and 75's.
If you end up with a different ratio than your present lower unit (such as 2:1), it'll affect your prop selection. For example, going from 2.3:1 to 2:1 makes an effective pitch increase in the prop of approx 2".
HTH..........ed
