Re: will a earlier merc 65 hp lower unit bolt up to later 65 hp redband?
Nope, won't work properly due to differences in exhaust and reverse lockout mechanism/shift shaft. The best compatibility for your later-model 650 would be either another 650 ('65-'71), or a 4-cyl 66-c.i. "800", "850", 85hp, 80hp, 75hp as they all have the 2.3:1 gear ratio which works much better on smaller inlines.
Note that later 650-4's use a driveshaft seal assy pressed into the crankshaft and do not have an O-ring groove below the driveshaft splines. Older motors have a shaft with a groove for the sealing O-ring which rides in the i.d. of the crank.
Whichever way your motor is, and whatever way the donor lower unit is, make sure they match up; in other words, if yours needs the O-ring groove in the driveshaft and the donor doesn't have one, take a grinder and make a groove.
And if your motor has the seal assy at the end of the crankshaft, and the donor lower unit has an O-ring groove, clean the groove and fill it with JB Weld, Marine Tex, or similar material that'll stick and get hard. Smooth the filler so it's flush with the driveshaft. If you don't fill in the O-ring groove, the groove will catch on the O-ring seal in the crankshaft and might damage it.
It's very important to get this small detail correct, since if the crankshaft and driveshaft splines aren't sealed correctly, water will get in and rust-out the splines. Worse case, they'll rust away until they strip out, then the crankshaft has to be replaced. I just saw this happen on a 65 hp a few weeks ago, when the crankshaft stripped (on the water), it wasn't pretty!!!! :0
HTH...........ed
