Force 150 hp lower unit

[Gerard1970]

Just seized up my lower unit this morning and debating if I should rebuild or look for a used one. It's a 1992 and I believe it is a mercury lower unit. If I go for a used one, is a 125 hp interchangeable? Maybe someone can steer me in the right direction on which ones will fit. Thanks a bunch!

 

[pnwboat]

You have several options. I'm assuming you have a 20" leg which is the most common. You do have to pay attention to the gear ratio. Your lower unit originally had a 1.93:1 gear ratio. Some of the lower unit that will fit your motor may have a 1.73:1 ratio, which in my opinion is not as desirable as the 1.93:1 ration. Basically any lower unit from a 1990-1994 90HP or 120HP motor should fit. To check the gear ratio, shift the lower unit into gear and turn the driveshaft. If you turn the driveshaft almost 2 revolutions to get the prop shaft to make 1 revolution, then it's probably a 1.93:1 ratio. If it only takes 1 3/4 revolutions of the drive shaft to make the prop shaft turn one revolution, then it's probably the 1.73:1 ration.

 

[Gerard1970]

Good to know.As always, you are a big help. Thank you!

 

[jerryjerry05]

The 150's all came with a 2 piece drive shaft. No worry about the shaft size.

 

[Gerard1970]

Is there a way to cross reference which motors will fit. Finding all kinds of merc lower units, all year ranges, but without seeing it in person, I do not know if they will fit. Some are hours away and would like to know if there is a chart available. Thanks again guys.

 

[Gerard1970]

Just a question...found a lower unit but the drive shaft is a onepiece. The guy told me it will fit mine even though it was a two piece. There are different types of lower units c,d,l drive, is there one I should avoid? Thanks.

 

[Frank Acampora]

The Force 150 was only made for a couple of years and discontinued in 1992 I think). It used the Force designed one piece lower unit. NO Mercury designed lower unit will fit. Thus: You need to find a pre-1992 lower unit. Since all these lower units were 1.95 ratio, you can use a lower from an 85, 90, 120, 125, or 150.

These one piece lower units were made since 1979 and all have the same bolt pattern. HOWEVER: The earlier Chrysler and Force lower units, while they will fit, used a different gear ratio--most were 1.76 and a couple were 2 to 1. Again, while they will fit, The higher gear ratio will hurt performance. Additionally, The later Force lower units had more robust gears than the earlier ones. Your strong 150 may be able to strip out the gears of earlier units.

Were it my engine, I would wait for a lower unit from a later Force--anything with a 2 piece drive shaft. If you are desperate and the price is right, then gamble on an earlier one piece ASSUMING the drive shaft is the correct length.

 

[SkiDad]

lower unit for 1992 was not mercury version. I just did a lower swap myself this week and you need to know if you have a 20" shaft like most have said - a 125 lower will reduce the hole shot some but will fit if you have also a 20" motor. Anyhow the other thing to be concerned about is the props you have - if they are closed props you that means your water pickups are on the round part of the lower unit and not on the flat part. If you get a lower unit that has the water pickups on the flat areas (dual exhaust) that means you will have to get different props. Well unless your props are already open hub. Pictures of what you have would really help the guys here (lower unit and 1 of whole engine)

 

[Gerard1970]

Ok guys...here is the scoop. I decided to take an internal look of my lower unit. My drive shaft bearing was destroyed. By the looks of it the unit was not getting oil on that bearing and fused it together. I removed the prop shaft and inspected the gears and there is no damage to any of them. It looks like I got lucky. I have rebuilt smaller gearboxes in the past and am going to give it a shot. I have a service manual here so that is a plus. Any words of advice?

 

[Frank Acampora]

Yeah! To remove the old bearing and shaft, you must remove the threaded ring under the water pump. You must then get some sort of wrench into the gearcase and remove the pinion nut. NOW, you must use a couple of wedges inside the gear case and against the drive shaft to pop off the pinion. It will be difficult because it is both splined and on a locking taper.

As far as I know, that bearing is not available as a single part. I believe it comes as an assembly with the drive shaft. HOWEVER: I also believe it is a standard part available in most bearing supply stores. It is a tapered roller bearing and you must buy both the cup and cone.

It is a pressed-on or shrunk-on bearing. Removing the old cone from the drive shaft will be difficult without the correct tools but certainly not impossible. Couple of curse words will help ease things LOL.

Putting on the new one is way easier. Heat the cone over a 100 watt incandescent bulb for about 10-15 minutes. This expands the bearing without any danger of overheating and ruining the bearing. Put the drive shaft into the freezer for the same period of time. NOW, with a heavy pair of gloves take the cone and quickly slide on the shaft, NARROW side up. The cone goes into the case facing down with shims underneath it. The load is up and the shims set the gear lash when the engine is running. To check gear lash, after installing both bearing cone and cup and installing the threaded ring to tighten the outer race against the shims, you must pull up on the drive shaft.

Now, bearing technology is relatively precise as far as tolerances so you should be able to use the original shims. however, if you destroy a shim or need more they are NLA so you would probably need to make your own. Moral: Be careful!.

To prevent future occurences, fill the gearcase with oil with the engine in a vertical position and the drain plug in place and tight. Fill through the fill until oil comes out the vent hole.. Let sit for 15-30 minutes to let air rise out of the oil then top off and replace both plugs. The bearing is below the fill and vent holes so if filled correctly, it will have an adequate supply of oil.

Oh, yeah--- I believe the pinion nut is tightened to 80 or 90 foot pounds. Not quite sure and I don't have the manual in front of me.

 

[Gerard1970]

Update...found a complete lower unit that is pressure tested and ready to go. It was a reasonable price and will get me back on the water. Thinking of parting out the old one . The gears and bearings are all in excellent shape. It was hard to find the lower unit, so I am guessing someone could use the gears.

 

[pnwboat]

Sent you a PM

 

[Gerard1970]

sent you one...let me know if u got it. Thanks.

 

[pnwboat]

Looks like I'm not receiving any PM's. I noticed my PM message IN box is all messed up too.

 

[Gerard1970]

E mail me at jerryisbored@Hotmail.com...