1984 75hp Merc - Lower Unit Options

[JKTrevecca]

I picked up a motor yesterday that had the lower unit destroyed on it two weeks ago by a rock @ 30MPH.

I can/will post the serial # later today but my question is about finding a replacement lower unit. How would you suggest finding out which years/hp lower units would be direct replacements? Is there an easier way than just finding the OEM part #'s and searching backwards from there to see which other years/HP share the same part #'s?

Thanks in advance.

 

[jimmbo]

Destroyed? Broken Skeg? Case split open and pieces like the propshaft, bearing carrier and reverse gear missing?

The 1984 75hp uses the same gear case housing as the inline 6s but with a 2.3:1 gear ratio. The 2.3 case has been used by the 66.6 cubic inch engines since introduction in 1969 as an 80hp, changing to 85hp in 73 and returning to 80 in 78. As long as the driveshaft(splines and length), and preload pin(if needed)) are the same, they can be used. The pre 1972 65hp also used the same case, with, I think the same ratio, but the driveshaft might be different. Some inline 6 units will fit but the gear ratio, usually 2:1, would require repropping and may still suffer performance wise
Any 1985 or newer gearcase off a 66.6 cubic inch 4 cylinder will work too

 

[JKTrevecca]

Destroyed? Broken Skeg? Case split open and pieces like the propshaft, bearing carrier and reverse gear missing?

The 1984 75hp uses the same gear case housing as the inline 6s but with a 2.3:1 gear ratio. The 2.3 case has been used by the 66.6 cubic inch engines since introduction in 1969 as an 80hp, changing to 85hp in 73 and returning to 80 in 78. As long as the driveshaft(splines and length), and preload pin(if needed)) are the same, they can be used. The pre 1972 65hp also used the same case, with, I think the same ratio, but the driveshaft might be different. Some inline 6 units will fit but the gear ratio, usually 2:1, would require repropping and may still suffer performance wise
Any 1985 or newer gearcase off a 66.6 cubic inch 4 cylinder will work too

Thank you Jimmbo - That is VERY helpful! When I say destroyed, I mean the prop, prop shaft, forward-most gear, bearing carrier, clutch dog, and about 75% of the bullet shape of the housing, including the skeg are in the bottom of the lake somewhere. The drive-shaft is still there but I am pretty sure the teeth are damaged. I'll take a closer look when I get home this evening. Your answer gives me a lot of guidance on what to search for on FleaBay. Thanks again.

 

[jimmbo]

I know a lake about 45 minutes away from me that has many parts like you describe. An old road about 10 inches below the surface of the lake has made lots of gear cases just like yours. Sorry to hear it happened to you

 

[JKTrevecca]

I know a lake about 45 minutes away from me that has many parts like you describe. An old road about 10 inches below the surface of the lake has made lots of gear cases just like yours. Sorry to hear it happened to you

Thank you again. I found a complete gearcase w/ stainless prop locally but it is off of a late 70's model 80hp mariner. Based on your reply, I think it will work. I'm going to take what's left of my old gearcase with me to make sure the casting is the same but if I understand you correctly, the geat ratio's should match between that 80hp and my 75, correct?

 

[jimmbo]

Late in the 1978 model year Merc/Mariner introduced a new gearcase. The earlier case looks a little less sleek, but it does bolt on. All 4 cylinder 75, 80, 85hp have the 2.3:1 gearing

 

[JKTrevecca]

Late in the 1978 model year Merc/Mariner introduced a new gearcase. The earlier case looks a little less sleek, but it does bolt on. All 4 cylinder 75, 80, 85hp have the 2.3:1 gearing

I picked up the used gearcase today and externally, it appears exactly the same. I'm going to bolt it on and test it before I put new seals in it.

I noticed that I am missing a couple vital "clips/retainers/brackets" for the throttle and shift cables. The plasticbracket that holds both cables stationary immediately after they enter the case of the outboard is partially broken and missing. It appears to be a plastic piece that hinges downward locking the two cables in place. Also, the retainers for both the throttle and gear shift that hold each cable, individually, onto their respective studs are missing. I've ebay searched for these and quite frankly, don't know what to call them. I considered rigging something up but I'm not a big fan of the thought of the throttle sticking wide open when whatever I rigged up breaks.

You've been super helpful on the lower unit question. Do you have any good suggestions for the 3 parts I'm looking for? Thanks Jimmbo. I can post pix if needed but I have to run to my son's baseball game right now.

 

[JKTrevecca]

I found the clips/retainers I needed for the control cables at a local merc repair shop. The lower unit worked out great. I had it pressure tested and it passed. I swapped the water pump and installed to lower unit this afternoon/evening.

Thanks for the help! I have a trim question I'll post in a new thread.