Is there an expert in the house?

[brianvolt7]

Is there a boat mechanic in the house that knows Force Lower Units?
Okay- I am confused.
I have a stubby shaft lower unit (NO it?s not genetics). It?s a two piece system. It?s for my 1990 Force 120 forward gear A and driveshaft O. I have been told by my mechanic that he is not sure if these units are directly interchangeable with any stubby shaft with different forward gears and driveshaft letters. One previous post I received on this subject simply implied that all of them are interchangeable.

Is there an expert in the house that can truly clarify for me so I don?t blow a grand on a lower unit that won?t work?

GOOD FISHING TO YOU ALL

 

[Frank Acampora]

Re: Is there an expert in the house?

A GRAND? Are you buying rebuilt from a dealer? Spend a lot less on a used one at the auctions.

Anyway: Given the shaft length is the same and the splines are the same, they are all interchangeable from any three and four cylinder Chrysler and Force from about 1980 to about 1992, maybe as late as 1994. (I forget the exact year Merc changed to a Merc lower unit) There were only a few gear ratios: Older units had 2 to 1 and 1.76 to 1 while the Mercury designed ones up to about 1994 has about 1.80 to 1--I forget the exact ratio. Your 120 with the stubby shaft probably had the 1.80 to 1. Now, if you use a 1.76 to 1 ratio off a 4 cylinder engine, you will lose a small bit of performance while if you use the 2 to 1 from a three cylinder 90, you should gain a bit with a prop change.

While the complete lower units are swappable, the internals are not. The two piece drive shaft lowers have gears cut in the opposite direction. While they (the gears and dog clutch) will fit in a one piece drive shaft case, the load on the bearings are different and the stub drive shaft will not fit in the case.

The letters on the drive shaft and forward gear were used by the factory to standardise and speed up assembly. They had a chart that listed forward gear letters, case letters or numbers, and drive shaft letters. This chart showed how much shimming was necessary for various combinations without the necessity for direct measuring, thus saving a step and assembly time.

So, if you buy a complete lower with different letters, as long as they are matched and shimmed correctly it will work on your engine.

 

[brianvolt7]

Re: Is there an expert in the house?

Thank you very much for the info.

Found a nice one remanufactured with warrantee for 750 in the Sacramento area. Forward gear A same as mine driveshaft I. I figure 100 for them to add a water pump and probably 100 or so for install onto my boat.

I?ll let you know when I am back on the water.

Thanks again.

GOOD FISHING