Re: 1970 Merc 650 Lower Unit Cross Reference
Any of the 75-80-85HP 66 cubic-inch 4-cylinder motors are a good fit and will have a favorable gear ratio (2.3:1). Note that none of the 3-cylinder models (even up to 90 HP), or later model 100/115 4-cyls will work.
Any Inline Six from '66 on up will also fit but may have a different gear ratio (such as 2:1 or 1.78:1). I wouldn't use a 1.78:1 on a 65 HP motor as the gearing is a bit "tall" and you'd have to go to a very small-pitched prop.
The 2:1 ratio will work but if your L/U has 2.3:1 gears you'll need a prop with 2" less pitch to retain the same performance.
One small caveat, Merc went to a different design for the driveshaft-to-crankshaft sealing on later models. Yours has an O-ring groove machined into the driveshaft; later models have no groove but do have a seal assy pressed into the bottom of the crank.
If you use a later-model L/U you'll need to grind a groove in the driveshaft to take an O-ring. I've done this before, it's not that bad if you measure carefully and take your time. If you don't install an O-ring, water will get in to the crank splines and corrode them away. Or worse, lock the crank splines and driveshaft together so you can't remove the lower unit.
You could also press one of those seal assy's onto the bottom of your motor's crankshaft, but you'd need to pull the powerhead to do that. Grinding the O-ring groove is much easier.
HTH............ed
