Stuck lower unit

H

Harb

Cadet
Joined
Mar 11, 2021
Messages
11
I have a 1986 Mercury 75hp and cannot get the lower unit off.....All of the bolts are removed.....deadblow hammer used (not TOO aggressively) but barely an 1/8th inch gap where lower unit meets main part.


A few guys on YouTube show the exact issue with similar Merc's and advise to turn the motor upside down, spray a lot of degreaser and penetrating oil in, the wait until it loosens the cooked in grease and oil that have the driveshaft spline "welded" tight.

Do you agree with that analysis and solution?

Thank you
 
R

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
37,840
How many nuts / bolts on your work bench ?----Trim tab is out of the way ?
 
H

Harb

Cadet
Joined
Mar 11, 2021
Messages
11
Total 4 bolts.....under the trim tab, front center, and 1 each side
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,860
Seized spline can and does happen. See if the shift rod is willing to separate, as that can contribute.

People use wooden wedges to help it separate. I would expect hanging it upside down 1 inch off the floor and spraying the driveshaft with penetrating oil would help.

After it separates, just clean up and inspect the splines. They were probably just stuck, as the driveshaft should be stainless steel on that motor.
 
R

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
37,840
Make sure ALL THE FASTENERS are removed.----Should be 6 I think.
 
Last edited:
H

Harb

Cadet
Joined
Mar 11, 2021
Messages
11
Chris1956 said:
Seized spline can and does happen. See if the shift rod is willing to separate, as that can contribute.

People use wooden wedges to help it separate. I would expect hanging it upside down 1 inch off the floor and spraying the driveshaft with penetrating oil would help.

After it separates, just clean up and inspect the splines. They were probably just stuck, as the driveshaft should be stainless steel on that motor.
Click to expand...
Thanks. I have heard about using wedges or putty knives to force separation but also heard some pretty gruesome tales doing major damage by using too much pressure to pry them apart.....how much is too much?
Also, question about the shift rod.....
I had 70hp Johnson with access to a shift rod disconnect about midway down the front of the midsection. Easy to see and separate when dropping the lower unit.... this Merc does not have anything like that. Nothing that I can see that looks even remotely like a shift rod.....could there be something higher up that disconnects?....maybe even under the cowling?
I wondered about the shift rod, from past experience w the Johnson, but dismissed it when there was no similar place to separate.
Thanks again
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,669
racerone said:
Shift rod is splined , nothing to disconnect.
Click to expand...
Ahhhh? My recollection is yes but the female is part of the shift shaft from above and the LU male stub, are both made of metal. Yes they are a slip connection but that doesn't take corrosion into account. If the drive shaft is stuck in the power head, why wouldn't one expect the shift shaft to have the same fate? Seems to me it's a test of endurance....will the shift shaft come off the stub before the stub comes out of the ShifterCam ?

Along those lines, I went with a friend to a Merc repair facility back in the late '60's to check on the progress of his 90 HP Tower of Power. Walking into the shop area, I came face to face with a lower unit in a vise...may have been his, don't remember. What I do remember is what appeared to be a Carbon Blue Steel drive shaft sticking up out of the LU with Blobs of rust scale on it.....when OMC had been using SS drive components for 10's of years.....In retrospect, how on earth, did any mechanic, in a salt water area, ever get anything done having to fight rust of that sort?
 
Last edited:
R

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
37,840
That is another topic but no it does not matter.-----You just have to make sure lower unit and control box are in sync on assembly.-----How many fasteners on your work bench now ?
 
H

Harb

Cadet
Joined
Mar 11, 2021
Messages
11
2 nuts are removed......the one under the trim tab and the on in front.
There is one on each side, sandwiched between two metal "lips" or flanges on the upper and lower units, that are loose but the lower unit has to separate some to make space to loosen it more and eventually remove it.
 
H

Harb

Cadet
Joined
Mar 11, 2021
Messages
11
I am pretty sure there are only 4 bolts holding this LU....as much time as I have spent looking, tapping, and fretting.....I think I would have more by now if there were more to find
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,860
Texasmark, Merc did use mild steel driveshafts in the early 70s and before. However, that motor, if it is original gearcase will have a SS driveshaft.

The shiftshaft coupler is splined to mate with the shiftshaft. One could reach under the lower carb and move the shiftshaft up/down to see if it is corroded together at the junction.

My inline 4 850 and inline 6 1500 motors always had 5 nuts and a bolt holding the gearcase to the midsection. Not sure what the OP has.
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,669
From Chris1956: "The shiftshaft coupler is splined to mate with the shiftshaft. One could reach under the lower carb and move the shiftshaft up/down to see if it is corroded together at the junction."

That's a great idea and I would think a candidate for a "Sticky" considering the outcome if the joint is frozen and the shaft is pulled out of the shifter cam in the LU.
 
You must log in or register to reply here.
Top