Swapping lower units (one piece)

[Frank Acampora]

This question frequently comes up so here is the answer: You can swap ANY lower unit from any 3 or 4 cylinder Chrysler or Force between 1979 and early 19900s --When mercury changed to a Mercury lower unit. The exception is the Force 70 and 75.

However, if you want to swap a later unit with a Mercury water pump onto an earlier midleg--like I show here-- You must do some minor modifications.

In order to clear the rear bolt and boss on the Mercury water pump, you must remove some metal from the web. In order to clear the front two gearcase cover bolts, you must also sometimes remove a little metal from the front two lower unit bolt bosses.

The mounting bolt pattern is all the same. With earlier lower units with a one piece drive shaft, you need to be certain that the drive shaft is the correct length for your particular engine--long shaft or extra-long shaft. With two piece drive shaft lower units, It doesn't matter--all the lower units are the same and you merely need to get the correct upper drive shaft to fit your engine. Unfortunately, the first two photos show that I lost the gears in my older lower unit and needed to replace it with a later model Force lower.

 

[Frank Acampora]

Re: Swapping lower units (one piece)



here you see the interfering boss on the water pump and the metal I removed from the midleg web. In this case the factory has removed enough metal from the front two bosses so I did not need to grind and away from them. However, in the first photo in the above post, I did need to remove some metal.

 

[SkiDad]

Re: Swapping lower units (one piece)

Hi Frank, is there anything you have to do before installing a dual exhaust lower on an engine that previously had a single exhaust ?

also, if i got one with a 2 piece drive shaft does the top (larger) piece of the drive shaft drop down when taking off or is it attached to engine ?

 

[Frank Acampora]

Re: Swapping lower units (one piece)



It depends upon the year of the single exhaust engine. If it has the complete web, you will need to relieve it as in the above photos. The two piece drive shaft is not captive either in the crankshaft or in the coupler on the lower unit. Thus depending upon its whim It may stick up inside the engine midleg or come down with the lower unit.

Other that that, it is a straight swap with no special preparation.

I would suggest trying the two piece without the drive shaft before actually installing it to see any areas of interference. Usually, the interference happens the last 1/8 to 1/4 inch

The dual exhaust lower unit is supposed to allow the engine to produce more horsepower. My experience has been that it does not appear so. On the same engine on my 15 foot Glastron hull, top speed was exactly the same and I felt that it tended to ventilate a bit more (subjective feeling)

SO: If you are swapping because the current lower unit is trashed and you happen to have a dual exhaust---good. If you are swapping for more performance---don't be disappointed.

Remember, the dual exhaust is 1.95 to 1 ratio. If the current lower is 2 to 1 no prop change will be necessary. If the current lower is 1.76 to 1, then an increase in pitch MAY be necessary, depending upon where full throttle RPM is now.