1986 Force 125 powerhead swap with 1996 Force 120 Question.

[db6261]

I have a US marine 1986 Force 125 with a severely scored #1 cylinder and have located a 1996 force 120 engine block on Craigslist. The bore on the 125 is 0.070" smaller than the bore of the 120 and the stroke of the 125 is 0.70" more than the 120. Are the blocks interchangeable with the carburetor half of the block? Will the 125 force crank fit with the 120 block. What can I interchange if at all? If I got the complete 120 powerhead would it mount to the 125 lower half? Thank you for any info.
Daniel

 

[pnwboat]

If the 1996 power head uses the larger diameter crankshaft (which I'm pretty sure it does), then none of the internal components will inter-change. The connecting rods might, but that's about it. If you got the complete 1996 power head with flywheel and ignition system, you can drop it onto the 1986 lower unit. You might have to use the adapter plate off of the 1986 motor. The ignition system used on the 1996 motors (Mercury CDM Thunder Bolt) is much simpler to trouble-shoot than the 1986 (Prestolite) from my experience.

 

[flyingscott]

If the 1996 power head uses the larger diameter crankshaft (which I'm pretty sure it does), then none of the internal components will inter-change. The connecting rods might, but that's about it. If you got the complete 1996 power head with flywheel and ignition system, you can drop it onto the 1986 lower unit. You might have to use the adapter plate off of the 1986 motor. The ignition system used on the 1996 motors (Mercury CDM Thunder Bolt) is much simpler to trouble-shoot than the 1986 (Prestolite) from my experience.

Question doesn't the 1996 model also use the mercury lower unit and driveshaft as well.

 

[pnwboat]

It may or may not use the mercury lower unit and driveshaft. Either way, the power head itself has the same bolt pattern on the bottom. 1996 - 1996 is kind of a cross over year when the factory was transitioning over to the Mercury lower units. The adapter plate that is bolted to the bottom of the power head will allow you to swap mid-1990 and earlier power heads. The true Mercury lower units have a "twist" shift shaft to engage the gears, the earlier units have a shift shaft that goes up and down to go into Forward, Reverse etc.

 

[flyingscott]

It may or may not use the mercury lower unit and driveshaft. Either way, the power head itself has the same bolt pattern on the bottom. 1996 - 1996 is kind of a cross over year when the factory was transitioning over to the Mercury lower units. The adapter plate that is bolted to the bottom of the power head will allow you to swap mid-1990 and earlier power heads. The true Mercury lower units have a "twist" shift shaft to engage the gears, the earlier units have a shift shaft that goes up and down to go into Forward, Reverse etc.

thank you I was wondering if the drive shaft would have the same spline between the 2

 

[jerryjerry05]

I don't think the drive shaft splines are the same.
​The 96 is pretty much all Mercury except the block.
Crank and electronics, Mercury

 

[pnwboat]

Like Jerry said, to be 100% sure, you have to check the splines. Your old motor should have a 15 spline hole in the end of the crankshaft. I've swapped out several mid-1990 120HP dual exhaust, two piece drive shaft lower units onto older 1989 125HP and earlier power heads and they fit fine (with a few minor modifications). The splines matched up.

If the 1996 motor had a two piece drive shaft, it'll probably match up. If it had a single piece drive shaft with the "twist" type splined gear lever, then the splines may not be correct.