120 lower unit compatibility

pelletman

Cadet
Joined
May 25, 2010
Messages
26
I just found out that my overheating and restarting problem was caused by a fried (nearly dry) lower gearing unit. I'm now looking at a used lower unit from a 1991 90HP to put on my 1990 120 HP engine. Both units appear to be dual exhaust and compatable. As long as I have the shaft and coupling
from my engine, does it matter whether the replacement is coming from a long or short shaft engine ?

From a reply Frank Acampora previously posted, it appears that my unit might be a 20" short. However, the shaft actually measures 24" not including coupling. - So I'm not positive what I really have.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: 120 lower unit compatibility

As long as you have the shaft from your engine, It does not matter what length the donor engine was. Couplings and shafts are all splined the same.
 

pelletman

Cadet
Joined
May 25, 2010
Messages
26
Re: 120 lower unit compatibility

I love this forum - you guys are the best. Thanks Frank - AGAIN.
I am new to boating (at least as an adult) and am convinced that without this forum I would have had to spend half a salary to get started. Local mechanics seem very limited, only focused on local market.

How would you go about inspecting a used lower unit?
How much can you really do without tearing it down?
 

brianvolt7

Chief Petty Officer
Joined
Jun 7, 2008
Messages
423
Re: 120 lower unit compatibility

I didn?t inspect mine; I bought it used on line for 325 and 45 shipping.
The questions I asked the sellers of these units where:
When the shift rod is pushed in does the prop shaft lock?
When the shift lever is in the middle does the prop spin freely?
When the shift lever is pulled out does the prop lock?
Does it leak?
Is the anode in good shape?
Most of them dont come with props, mine did though.
 
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