120hp Force lower unit compatability

[Boat-ryder]

Trying to find what years and HP had lower units I can interchange with on my 1990 120hp. Model #- H1208F90B. Is there any other info I need? Thanks

 

[SkiDad]

anything 1984-1994 should work 85-150 hp Force. Variables - some are 20" and some are 25" shaft - MEASURE before buying. Some are dual exhaust and might require a different prop style. 1984-1987 where all single exhaust - 1988-1990 could have been sold either way. Dual exhaust requires prop with open hub.

 

[Boat-ryder]

which part of my shaft needs measuring? when I pulled the lower the shaft seems to be a 2 piece with a splined coupler.

 

[jerryjerry05]

The 84-94 isn't right.
I don't think the exhaust come out the prop on any up until after 94
It came out the snout on the bottom.
Then they used Mercury lowers and the exhaust came out the prop.
The 80-90 used a regular drive shaft. In 20 or 25"\
​1990 they started using a 2 piece shaft and some 1 piece shafts..
​The L=drive used the same short shafted drive.
Then after the L=drive they went back to the regular 20 or 25" shaft.

You particular model could have 3 different drives????

Take it off and post pics. Lots of pics.

 

[Boat-ryder]

I definitely have the 2 piece shaft and if I measure it with the splined coupler its 20" exactly not including the part if the shaft that goes into the gearcase. Just the shaft itself is 18". On the exhaust I am positive its a 1990 and my exhaust comes out the prop and the snout. I'm assuming I have a 20" shaft. Can anyone tell me how to measure the 2 piece shaft?

 

[Boat-ryder]

Tried to upload pics but it won't work.

 

[Nordin]

jerryjerry I think in 1989-90 when the 2 piece drivshaft was in, all of them were dual exhaust.

Boat-ryder Put your old gearcase back and measure the distance between the transom clamp that rest on the transom to the cavatitionplate.
Should be about 20 in if 20 in leg, if 25 in then it is a 25 in leg.

 

[jerryjerry05]

Measure the transom. From the top to the bottom in the center.

When running on the hose, does water come out of the prop and the snout?

The 89/90 dual exhaust meant nothing,
​Just a different style prop.
My 88 has the new style prop.
The exhaust still goes through the snout until 1995.

 

[Boat-ryder]

Gearcase is at shop and they can't remove prop shaft carrier nut which is why I'm searching for lower unit housing in case it cracks. I will measure transom after work, what measurements will tell me which shaft I have? I believe you are correct Jerry Jerry I have duel exhaust but it come out of 2 holes halfway up leg and at the snout on the bottom not out the prop.

 

[SkiDad]

on my 20" if you measure from the bottom of the water pump to the top of the shaft it's around 23". If it measures 28" then it's a 25" shaft.

if you get another lower unit that is 2 piece it doesn't matter just use your old shaft. If you go 1 piece it matters.

see this post: http://forums.iboats.com/forum/engi...tboards/624116-swapping-lower-units-one-piece

 

[pnwboat]

If your lower unit has a prop carrier nut, (which is known to be very difficult to remove sometimes) then the exhaust comes out by the prop, and also out the snout. This is the reason they refer to it as "Dual Exhaust".

Both single and dual exhaust units have the two low speed exhaust relief holes half way up on the back of the lower unit. This is only functional at idle and just above idle speeds when the lower unit is sitting deeper in the water which increases exhaust back pressure. Once speed increases, back pressure is reduced as the lower unit is not as deep in the water and exhaust exits through the snout (single exhaust) or through the snout and prop hub (dual exhaust).

I'm doing some experimentation switching between single exhaust and dual exhaust units right now. The dual exhaust unit is a tad quieter. I suspect because a portion of the exhaust gasses are exiting through the prop hub which is deeper under water than the snout. Also looks like the dual exhaust units are built more ruggedly than the single exhaust. Drive shaft is bigger in diameter, and internal gears/bearings are slightly bigger.

Oh I forgot to mention that the dual exhaust unit I have been playing with is a 20" version and the drive shaft is 18 1/2" long from the end of the splines on each end. 19" if you include the small little nipple on the end that goes into the coupler to the lower unit.

 

[Boat-ryder]

If your lower unit has a prop carrier nut, (which is known to be very difficult to remove sometimes) then the exhaust comes out by the prop, and also out the snout. This is the reason they refer to it as "Dual Exhaust".

Both single and dual exhaust units have the two low speed exhaust relief holes half way up on the back of the lower unit. This is only functional at idle and just above idle speeds when the lower unit is sitting deeper in the water which increases exhaust back pressure. Once speed increases, back pressure is reduced as the lower unit is not as deep in the water and exhaust exits through the snout (single exhaust) or through the snout and prop hub (dual exhaust).

I'm doing some experimentation switching between single exhaust and dual exhaust units right now. The dual exhaust unit is a tad quieter. I suspect because a portion of the exhaust gasses are exiting through the prop hub which is deeper under water than the snout. Also looks like the dual exhaust units are built more ruggedly than the single exhaust. Drive shaft is bigger in diameter, and internal gears/bearings are slightly bigger.

Oh I forgot to mention that the dual exhaust unit I have been playing with is a 20" version and the drive shaft is 18 1/2" long from the end of the splines on each end. 19" if you include the small little nipple on the end that goes into the coupler to the lower unit.

These are my exact measurements of the shaft so I have a 20". Found one on eBay and now I can tell the guy to do what he has to do to get it out. If it cracks it cracks. Thank you

 

[Nordin]

Insist with you pnwboat about dual exhaust.
I have a Force 120 Hp 1991 and it is a dual exhaust. Parts of the exhaust passes through the prop and parts through the snout.
Have had a Force 90 Hp 1991 and same design, both have 2 piece drivshaft both have the prop carrier nut.

Mercury that started to design gearcases with exhaust through prop as early as in the begining of 1960 did not have the snout.
I have one 1970 Merc 500 four cylinder and one Merc 800 1972 also four cylinder and none of them have a snout.
All exhaust passes through the prop.