Re: 75hp Mariner Gearcase Swap
'79 and later 80hp/75hp are all the same thru end run of the 66 C.I. Inline Four and all have a gear ratio of 2.3:1.
Earlier 85 & 80 HP Four models all have a 2.3:1 gear ratio and will fit, but have the "unimproved", old gearcase design.
Also minor diff's on the method of driveshaft seal, very early models have an O-ring fitted to the end of the driveshaft where it seals into the crank.
Yours wouldn't have the O-ring as it seals the driveshaft with an O-ring carrier pressed into the end of the crankshaft.
You can use a lower unit with O-ring grooved driveshaft, but I'd recommend filling in the groove with Marine Tex (or equivalent) and sanding flush, so the groove would slide past the seal in the end of the crankshaft without catching.
A lower unit off an Inline Six will also bolt up, but most of those have 2:1 gears and you'd have to "pitch down" in prop size to make up for the "overdrive" gears.
Note that the lower units from 75+hp Big Threes (the motor that replaced your 4-cylinder) are not the same as Inline Fours or Sixes and won't bolt up to Four or Six.
HTH.............ed
