Swaping Bottom Ends

[Rippin_54]

I have a 1968 650 (65) Merc Serial number 2044532 (not positive on it), now I need a new bottom end, it has a small crack in casing. Been dealing with it, but i have a buddy of mine who has the bottom end from a 1978 402 (40) Merc and they look identical. Serial number to the 402 is #5041484.
Now the only trouble is i keep this boat 10 hours away up north from me, and would really like to know if i could just drop the bottom & be sure they fit before i went for vacation. They look the same,,, but i havent been up there for a year also.. Any help would be greatly appriciated!

I just wanna do a simple bottom end swap if they match

 

[emckelvy]

Re: Swapping Bottom Ends

Re: Swapping Bottom Ends

Lower units from a 650 4-cylinder and a 402 are not interchangeable. Completely different family line.

A lower unit from any 66 c.i. Four or any Inline Six from approx '66-up will bolt right up to your motor. You'll probably want to stay with a 65 HP gearcase, it has a favorable gear ratio, 2.3:1. I can't say off the top of my head that yours has the same but I think so.

You can confirm this by turning the driveshaft 2 revolutions and if the propshaft turns exactly one rev, you have 2:1 gears. If less than 1 rev, they're 2.3:1.

You'll have to re-prop if you don't get a gearcase with the same gear ratio as yours.

One other difference, later model lower units did not have a groove for the driveshaft O-ring where it "plugs into" the crankshaft.

You can either grind a groove in the later-style lower unit's driveshaft to fit an O-ring, or remove your powerhead and install the later-style seal carrier/O-ring assembly on the end of the crankshaft.........ed

 

[Rippin_54]

Re: Swaping Bottom Ends

ok well thanx for the reply. Saves me alot of monkeying around.

Would i be able to use the prop off the 40, and use it as a spare for the 65?

 

[Rippin_54]

Re: Swaping Bottom Ends

the prop on my 65 has 2 blades, the one on the 40 has 3 blades

 

[emckelvy]

Re: Swaping Bottom Ends

The prop off the 40 will not fit the propshaft of the 65hp, the 40's hole would be too small. The body of the 40hp prop is smaller also.

Both the driveshaft and propshaft of the 65hp are bigger, as well as the gearcase.......ed