1975 Mercury 7.5 Lower Unit Removal...IT's STUCK!

[dimlight85]

I'm hoping to find some suggestions on how to get the lower unit off of this Merc I just purchased. It was a great deal...good compression, pumps water and shifts easy. The only problem, I got it home and cracked upen the lower unit screws to change the fluid and no oil came out, just fresh, clean water. The owner is someone I know and he had not used it in several years and with the easy that it shifts, I'm not worried that the lower has any gear issues.

Anyhow, the lower needs to come off to get to the seals and to check the impeller but I can't get the stinking thing off. What I've done so far:

Removed the two nuts; under the cavitation plate near water intake and the other above the cavitation plate on the front (in neutral). After removing those, I pulled with no joy and then tried using a rubber mallet and a piece of wood to tap it loose. I then tried getting some flat head screwdrivers between the lower and the upper all the way around and prying it free but did nothing but crack and damage the aluminum. Fortunately, it's nothing that some JB Weld can't fix. I left the nuts off and ran the motor on a in a barrel, shifting it heavily as someone suggested to break the shaft splines free. No joy there. Finally, I tried someone else's suggestion of loosening the six nuts that attach the powerhead and dropping it onto a padded surface on the floor (about a foot by a suggestion on another forum) to break it free...still no joy.

Am I missing something here? Any other suggestions on how to get this thing off? If I can get it off, I can change the seals and may end up taking it to a guy I know that's a pro at welding aluminum instead of the JB Weld fixes.

 

[tpenfield]

I have a 1969, which I removed the lower unit . . . I believe the 1975 is essentially the same . . .

http://home.comcast.net/~tpenfield/W...ard_Motor.html

Beyond the 2 bolts that you mentioned, the only things that could be 'hanging on' are the water intake tube, the drive shaft, or the shift shaft. I suspect that one (or more) of these things is rusted solid.

I cannot think what else you could be missing . . .

I wonder if someone used a permanent adhesive on the water tube mating during a previous set of maintenance :noidea:

 

[dimlight85]

Yeah, mine looks almost identical. Word has it that the drive shaft or the shift shaft splines can rust together in the powerhead. When I get home today I'm going to do what some others have suggeted and put as much PB blaster or WD-40 in there that I can get and let it sit upside down for a day or so and see if that helps. Someone also suggested filling the up portion with the engine upside down with diesel fuel and letting it sit to see if it breaks apart the rust. If it's melded together, I won't be able to get the powerhead off either but I will probably try that too. This little motor is pretty simple so I'm not real worried that I can screw it up by pulling the powerhead.

 

[dimlight85]

Drilled a 1/2" hole in the upper and put it what an old expert I work with suggested to break up the rust; a mix of 50/50 transmission fluid and acetone. In the picture, it looks like you cut the water tube. Were you able to find a replacement? I haven't had a lot of luck when looking into replacement parts. I'm also curious, how far does the shift shaft go up? I can't really figure out what the hard plastic link rod between the mounting bracket and the part just under that metal cowling is about halfway down. Is there a shift linkage that needs to be disconnecting like in some of the newer Mercs?

 

[SeaKaye12]

Well....you are doing all the correct things. You've been reading the forums so I suppose you know that this is caused by failure to grease the driveshaft splines...and/or leaving out the o-ring at the top of the driveshaft. They can get stuck from rust so bad....sometimes it's just about impossible to free things up.

At least yours is pumping water.....so it is usable as is for a little while....as long as the impeller holds out. Of course you would have to be keeping a close watch on the gear oil and changing it out frequently. Could be worse. Get the cheap 90 generic gear lube at Walmart. Some people change it after every outing if need be. What else can you do?

Being a 1975....that might even be in back in the days when Mercury was still using carbon steel driveshafts ....no Stainless. But even the stainless ones can get seized to the crank.

 

[dimlight85]

I checked it this morning but still no change. If this acetone/transmission fluid doesn't work I'll empty it and attempt to remove the powerhead. I'm trying to figure out where the shift shaft might be binding. About halfway up is the safety cam that is supposed to keep the motor from kicking up in reverse and it looks like the shift shaft goes into there somehow since it turns when I manipulate the shift lever. I just can't seem to find a diagram that pieces it all together.

 

[dimlight85]

I checked it this morning but still no change. If this acetone/transmission fluid doesn't work I'll empty it and attempt to remove the powerhead. I'm trying to figure out where the shift shaft might be binding. About halfway up is the safety cam that is supposed to keep the motor from kicking up in reverse and it looks like the shift shaft goes into there somehow since it turns when I manipulate the shift lever. I just can't seem to find a diagram that pieces it all together.

 

[SeaKaye12]

I had the same scenario recently with the 4.5 HP. It's still stuck. If the driveshaft is rusted to the crank...efforts to remove the powerhead will be futile as well.

I just re-read your original post and something caught my attention. You say that only clear water drained from the lower when you removed the screw. That's not usually what happens. Usually you get a milky mix of oil and water.

Perhaps the PO drained the oil at some point and ran it empty for some period? I don't see how else you would end up with just clear water in there. Maybe the leak is not so bad after-all? I would re-fill it with the correct oil and try it for a while. You may indeed be able to run for a while.

The other thing that you might find with that vintage Merc.....the drive shaft and shift shaft might be so badly rusted that even a new seal won't stop the leak.....the shafts need to be sleeved or machined with new metal....expensive .....

As good as these engines are (were); issues such as these with the lower units land many of them in the parts pile.