Compatible or not?

shaun77

Seaman
Joined
Aug 21, 2010
Messages
62
I recently swapped midsections from a 1990 90hp to my 1989 125 force outboard, I do not know part numbers. Now I have exhaust coming through the shift linkage( i think) into the midsection and into the upper cowling. Are the 125 lower unit and 90 midsection compatible? One diagram shows an exhaust seal #93, which my lower unit does not have. Also there seems to be space between the exhaust tube, see pic in yellow, and a curve cutout, in red, that seems to just be open. Whats normal and whats not?
 

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Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Compatible or not?

Well, first off, I think your years are incorrect: The lower unit has a one piece drive shaft so it should be 1989 or earlier. 1990 lower units had a two piece drive shaft and a larger water pump courtesy of Mercury. However, the casing does have the side "tits" near the front of the water pump which were usually on later lowers. Curious.

The engine has the larger bolt thru transom clamps with four height adjusting holes which would put it as post 1990 as would yout trim/tilt motor and reservoir.

However, the lower unit should be swappable with no problems. There is usually a foam type gasket between the exhaust downtube and the midleg casting, however even if missing this should not cause exhaust problems.

Over the course of years, the manufacturer made a lot of minor changes in the castings but up until about 1994, most parts are swappable.
 

shaun77

Seaman
Joined
Aug 21, 2010
Messages
62
Re: Compatible or not?

Oh yeah...the transom mount and trim pump are off the 90hp also, but the lower unit came with the 125 engine when I bought it unless the previous owner did a swap also. Good to know there compatible but what keeps the exhaust from coming up between the midsection and lower cowling? I thought exhaust only exited the bellow from the shotgun ports or the trim tab but when I pulled the lower unit the midsection all looks "open" like the exhaust can go up through the shift linkage? Maybe i'm not tracing it correctly? Thanks for the help though.
 

RRitt

Captain
Joined
Mar 30, 2006
Messages
3,319
Re: Compatible or not?

Well, first off, I think your years are incorrect: The lower unit has a one piece drive shaft so it should be 1989 or earlier. 1990 lower units had a two piece drive shaft and a larger water pump courtesy of Mercury. However, the casing does have the side "tits" near the front of the water pump which were usually on later lowers. Curious.

The engine has the larger bolt thru transom clamps with four height adjusting holes which would put it as post 1990 as would yout trim/tilt motor and reservoir.

However, the lower unit should be swappable with no problems. There is usually a foam type gasket between the exhaust downtube and the midleg casting, however even if missing this should not cause exhaust problems.

Over the course of years, the manufacturer made a lot of minor changes in the castings but up until about 1994, most parts are swappable.

Was USMarine a division of mercury? I always assumed that Mercury inherited USMarine. Most of the USMarine changes were well thought out improvements. Most of "Force by Mercury" changes were half-baked corner cutting. I wonder what the history is. How could they have gone from good engineering to junior college engineering in the space of 3-4 years?
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Compatible or not?

US Marine was owned by AMF Brunswick as was Bayliner. Then later, whom acquired whom? Mercury or AMF? I know that Mercury and AMF Brunswick were owned by the same corp. SO, Merc got its hands on Force engines. Some of its changes were for the better and the engines did improve in power output and cooling under Merc.

All the legs I have seen will have the exhaust just blowing down. It can travel around the drive shaft and some early Chrysler engines did have an additional shift rod seal at the exit from the leg below the transom clamps.

I can't tell you why, after changing the lower unit, you suddenly have exhaust infiltration issues.
 

RRitt

Captain
Joined
Mar 30, 2006
Messages
3,319
Re: Compatible or not?

US Marine was owned by AMF Brunswick as was Bayliner. Then later, whom acquired whom? Mercury or AMF? I know that Mercury and AMF Brunswick were owned by the same corp. SO, Merc got its hands on Force engines. Some of its changes were for the better and the engines did improve in power output and cooling under Merc.

All the legs I have seen will have the exhaust just blowing down. It can travel around the drive shaft and some early Chrysler engines did have an additional shift rod seal at the exit from the leg below the transom clamps.

I can't tell you why, after changing the lower unit, you suddenly have exhaust infiltration issues.

I think the power curve improvement came at the cost of significant reliability. I would not call it an improvement. They juiced up the power curve by installing inappropriate reed valves. It backfired on them when some of the engines started self-destructing before the warranty was over. I classify that under the half-baked idea column along with the stators that come apart if they get hot.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Compatible or not?

I can't argue with you about that. However, I think you will agree that the Merc type lower unit and water pumps were better and the long rods and higher wrist pin placement in the piston on the later 75 and 90 improved power without sacrificing reliability.

As soon as I can find some of the long rods in a junker, this modification will go into my 140 to see what it does for that engine.
 

RRitt

Captain
Joined
Mar 30, 2006
Messages
3,319
Re: Compatible or not?

I can't argue with you about that. However, I think you will agree that the Merc type lower unit and water pumps were better and the long rods and higher wrist pin placement in the piston on the later 75 and 90 improved power without sacrificing reliability.

As soon as I can find some of the long rods in a junker, this modification will go into my 140 to see what it does for that engine.

I always gave USMarine credit for any changes before 1991 including split shaft, bigger impeller, 2w trim, and crank geometry. There was such a shift in the mentality and attention to detail around 1992. I always just assumed it was a handover from one engineering department to another. From a USMarine engine somebody actually cared about to a Mercury redheaded stepchild.
 
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