Mercury 70hp stuck lower unit

At the end of last season I picked up a good running 1983 Mercury 70hp.
The motor ran great, has good compression but was overdue for a water pump.

At that time I tried to remove the lower unit with no luck, it will drop down about 1/8" or so and stop.
Figuring I had all winter to mess with it I went ahead and put the motor on a stand with some weight hanging from the lower unit in hopes that it would drop free, but no luck. I even turned the motor over on its flywheel for a few months with penetrating oil dripped down into the mid section in hopes of freeing the stuck shaft, but again, no luck, it's now been almost 6 months and it's still stuck, it's been sitting with about 50lbs of weight hanging from the lower unit now for two months but it won't budge.
At this point I've finished everything else I wanted to do to the motor, and even ran it on the water for a day in hopes that some torque would help dislodge the driveshaft but again, no luck.
The water pump is still working, but it spits a bit at low RPM, the motor hasn't run hot or anything yet but its no doubt just a matter of time.
The motor has been run mostly only in freshwater, even when in saltwater, it was launched in freshwater up the river a ways. The rest of the motor is super clean.
Any suggestions as to how to free a stuck lower unit or driveshaft?

Re: Mercury 70hp stuck lower unit

I had this happen to a smaller outboard engine as well. I eventually got it seperated but I don't know if I would recommend my method. I just kept hitting down on the cavitation plate on each side with a rubber mallet. After almost giving up the lower unit dropped out. As I suspected the drive shaft was rusted to where it meets the bottom of the motor. If you get it loose be shure to use some marine grade grease on the splines when you put it back together. Good luck.

Re: Mercury 70hp stuck lower unit

Don't bang on the cavitation plate. Only use wedges fitted between the LU and the leg.

Have you got all of the hardware undone? Have you removed the trim tab and the stud nut located under the tab as well as the stud nut under the caviatation plate?

Gerry

Re: Mercury 70hp stuck lower unit

All of the hardware is undone, the lower unit will move up and down about 1/4" or so. I've tried careful tapping on it but no luck. I really figured that letting it sit upside down with a load of Kroil sprayed inside that it would have sooner or later loosened it's grip but no luck.
It really surprised me that this was stuck since the motor looks so clean. Usually it's the ones that were used in saltwater that are stuck.
I've had stuck drive shafts before but never one this persistent. One shop here said that the only way to get it apart is to sacrifice either the mid section or driveshaft, or both. Right now, it's sitting on a stand with the lower unit unbolted with almost 100 lbs hanging from it in hopes it will sooner or later let go.

I think I'd sacrifice the lower unit before any other part, its far easier to just find another lower than to tear down the whole motor to swap out a mid section but I really hate to destroy any parts that are in good shape. I even made up several small 1/4" jacks to expand inbetween the case halves using several 1/4" studs and nuts cut to just fit so I could back off the nuts and apply pressure downward.

Re: Mercury 70hp stuck lower unit

Wood wedges work every time.....are you sure you don't have one nut left.....has happened to the best of us.....you need the wedges and keep pounding them in as they will force the two apart(if you got that last nut lol)

Re: Mercury 70hp stuck lower unit

We all want to be sure you got all the hardware...two 5/8" nuts (one on each side of the LU), one 5/8" (IIRC) nut on the bottom of the cavitation plate, the 1/2" (IIRC) bolt that holds the anode/fin, and then a 9/16" (IIRC) nut under the andoe/fin.

If it really is the drive shaft stuck into the crank, try a few hits of the starter to see if you can break them apart.

Re: Mercury 70hp stuck lower unit

sure it's not the shift shaft where it joins to LU ?

Re: Mercury 70hp stuck lower unit

The lower unit drops just far enough to disengage the shifter linkage. With the lower unbolted and hanging the shifter is disconnected.
I doubt bumping the starter will help since running it don't seem to do anything either?
For the record, I removed the two nuts on each side towards the front, I removed the anode fin, removed the nut above it, and removed the bolt at the very front.
It won't drop low enough to allow the two nuts on each side to come out, at this point the shifter is just barely free of the shaft.
I've got the lower unit secured to the bottom of the engine hoist right now while lifting the motor by the flywheel tool.
I've tapped, lubed, and run the engine and gained nothing so far. I can lift and drop the lower unit for a slide hammer effect but there's just not enough of a gap open to really apply any force.
I'm afraid of damaging the lower unit housing if I just force it too hard. I can certainly secure the lower unit to the hoist and just keep pumping but no doubt something will give, either the threads in the flywheel, or the lower unit bearing retainer and water pump housing.
What surprised me was that the prop wasn't even the slightest bit stuck, it came right off. I find it hard to believe that the crank splines are so badly stuck. I hate to do any damage as the motor has perfect 145 psi compression on all three cylinders and runs like a top.
I all but filled the mid section with Kroil the first time and PB Blaster the second time and let it sit, I can't believe that dumping nearly a gallon of penetrating oil into that area didn't get where it needed to be.
If I apply too much force what is the most likely point of failure? (Power Head or lower unit?)

I thought about drilling a 1" hole into the mid section and using an air chisel hammer to shock the driveshaft but I'm not sure how much else inside I'd be destroying, as in lower exhaust housing and inner tubes. Without having one of these here already apart its hard to guess where to drill if that becomes the only option.

I've also unbolted the power head and get the same movement up top. I have the lower loosely attached right now with the power head unbolted and hanging from a hoist in hopes the combined weight of the lower, mid and some hydraulic pressure will pull the two apart.
Something tells me that if it didn't soak free after 6 months and with all the banging, prying, and tapping I've done, it may never move.

Re: Mercury 70hp stuck lower unit

I bought a used 50hp 3 cylinder power head from a used parts dealer a while back on which they cut the driveshaft and lower unit off after not being able to separate the splines. They all but gave me the thing for free because even after getting the power head off the middle they couldn't get the 3" piece of driveshaft out of the crank. I ended up cutting it off flush and drilling it out and removing the remains via chisel and Dremel tool. The drive shaft itself was rusted up and scaly, the crank was spotless inside after I got it all cleaned out. It had its O ring installed, the grease where there but dried up. That power head came from out in central Missouri, far from any saltwater.
I've tinkered with all brands of outboards over the years and I've had quite a few Mercs with stuck drive shafts and stuck props for some reason. I had an older Mercury 500 that had a stuck prop, I went so far as to remove the nut and took it out on the water for day in hopes it would just work free and fall off, but it never budged. It took me two hours to cut that prop off with a die grinder and 3" cut off wheel.

Re: Mercury 70hp stuck lower unit

I didn't read anything about you using the suggested wedges. It does work!

Re: Mercury 70hp stuck lower unit

I made up four steel wedges from a piece of 1" bar stock cut on a sharp angle. They've been hammered in place now for three weeks with no luck. I've tried with the motor upside down, with the thought that the PB Blaster and Aero Kroil would run down the driveshaft and get where it needs to be, and its been hanging from a hook, with a rope around the lower unit and the hydraulic lift pulling on the top. I've not gained so much as a fraction of an inch. There's not enough space to get into to cut the driveshaft off without sacrificing the mid section. Its been soaking now since last year.
My guess is that there's an issue with the driveshaft metal vs the crank metal? For some reason, nearly every Mercury I've pulled apart here has a rusted up driveshaft but this is the first one that just won't budge.