1979 40hp lower unit stuck


Petty Officer 1st Class
Feb 21, 2011
I tore apart what was left of the 40hp's lower unit from the boat, since the lower housing was pretty much gone getting the shaft and gear out were pretty easy.
I have a pinion gear, shaft, a spacer, a stack of shims, and a tapered seat bearing at the top. The seal and retainer is the lower water pump housing.
The shaft has no pin, but like I said has the o ring.
If preload is required on the bearing or gears below, then what did the '79 use?
What does the that pin make contact with in the crank?
Am I missing something that's suppose to be in there?
The way it was, the pin had 1/2" between it and the top of the hole into the crank.

Stuck lower units or shafts seem to be a common issue with Mercs here. I had a 70hp that wouldn't come apart when it needed a water pump at only 2 years old back in the 80's, I fought with a 9.8hp for a month before it let go of its shaft to get a new water pump and then this one.

A local junk man brought me another Merc twin thats seized up this morning, this one says 35hp, its a 20" shaft, no recoil, with what looks to be the same lower. By the decals its an 84/85 model. Like the 40hp, it dumped out a bunch of debris when I turned it over on its flywheel.
Its got a minty clean stainless prop on it, so at the very least I got a free prop. He also gave me a 1979 Johnson 35hp that's seized, and a seized 1996 Mariner four stroke 9.9hp that looks like its been sunk. In turn I gave him the junk parts from the 40hp I cut apart.

A quick try at dropping the lower unit was a no go, this one is stuck too.
Like the two 40's this looks to have the same electronics and power head.
This one though is locked up completely. Its pretty clean looking but that means nothing. Something tells me its going go the same way as the last one. The lower is clean looking with clean oil. I pulled the starter and it works so its a spare starter too. I won't waste as much time on this one, if it don't come apart in a day or two, I'm going straight for the saw and cutting it up like the last one. When a power head is able to drop internal parts out when you flip it, there's not going to be much salvageable inside anyhow.

As to the pin, what years used the conical top bearing and what years used the roller bearing in the lower?
I can't find any comments anywhere online about this being an issue. I see some listings showing the lower unit being the same from 71-77, others say 71-79, and a few say 71-97, and a few are listing 77- 88.
Which is correct? (If any)


Mar 19, 2007
I had a few later model twins in that engine family that failed due to broken reeds. The petals of the reeds got atop the lower piston on each of them and pounded it to bits, one broke the crank, one mashed up the piston so bad it stuck midway covering the ports, and one blew a rod and opened up the block with a hole large enough to reach in and unbolt the remains of the rod. The one that broke the crank broke diagonally across the corner edge of the lower rod journal, likely from the force of slamming the piston against an immovable object. All three had stuck splines, one I was able to force apart, one I deemed not worth the hassle and just tossed in the scrap pile since it had broken fins, and another I had to cut it apart in pieces to save the lower.
Out of about 30 or so of those motors I've had apart, most had the small pin but I never gave much thought as to why. Basically what it seemed to be was that the short shaft motors had the oring shaft and no pin, and the long shaft motors got a shaft with a pin. The bearings and all else looked the same. The only one I still own is a 1971 model that I got off a buddy who has since passed, he bought it new to run on a homemade duck boat but found it too heavy to use so it just sat in a closet by his front door for 40 years. Being a 40hp tiller steer short shaft motor it occasionally gets hung on my 14ft Starcraft when I want to go way too fast for comfort.
With the condition of the river here lately I've refrained from any speed. Between logs, new sandbars, and a lack of dredging upstream getting down stream to fish is like navigating a minefield.
They were good motors but the later models seemed to slack off quality wise. I don't think I've ever seen one badged as a '400' or '402' blow apart, its always the 80's models, with most being the last that were badged as 35hp.


Jun 3, 2022
I've got an '88 35hp here right now that I picked up for parts to fix another one with a busted up skeg and anticav plate. I've had it soaking for a year and its not budged. The power head won't turn and when I flipped the motor over on the bench it sounded like a box of rocks. One spark plug is smashed to bits so I'm guesing its getting cut apart too in order to save only the lower, and the spare carb i suppose. Its too bad since its a super clean looking motor from the midwest..


Supreme Mariner
May 24, 2004
Beware that in the Late 80s, the 2 cylinder 35hp had a Higher WOT Rpm Range than the earlier 2 cylinder 40 did. There might have been some differences in Carburation(size and Jetting), Porting, Reeds, to name a few things.


Jun 3, 2022
I pretty much run and prop these to run around 5800 RPM max, so the carb won't be a big issue if I do use that carb.

I ended up burchering the mid and power head to get the driveshaft free.
I cut the mid in two places, removing a chunk from the middle with a circular saw, that went quick. I then dropped the upper part of the mid away form the power head, and cut the exhaust tube flange and dropped the pan out of the way. I then could get at the bottom of the crank. I removed the bearing retainter bolts, tapped the bearing retainer down the driveshaft and then Iw as able to split the power head open. the top cylinder is hammered to bits, the piston is mushroomed over the rings and was stuck in the bore by the dents around the edges of the piston. There's 6 reed plate leaves hammered into the top of the bore and the piston. There's a seventh steel leaf shoved through the rear half of the case that nearly cut all the way through it. Both rods look untouched but the lower piston is battered on the skirt with a chunk missing from the back around the wrist pin. The cylinder walls still show crosshatch and the rings are free and undamaged. What ever happened it happened fast and it stopped suddenly. With the crank out, and only the lower and the crank still attached I made a saw cut across the lower counter weight cutting slightly inward till the crank end separated. I didn't have to cut very deep. Then I cut a slot in both sides of the crank down to between two splines on opposite sides.
Even doing that the crank would not come off the driveshaft, I had to cut pie slices out of the crank between each and ever spline and even then I had to use a brass drift and hammer to get the crank sections off the splines. It was not rusted, but appeared to have some sort of brown sealer, maybe Indian head or Permatex Aviation on it. It could also have been some sort of petrified grease I suppose but it was hard as stone.
After cutting the crank away, I could see the shaft has a pin on the end, but the crank splines are not deep enough for that pin to contact the bottom (upper) part of the crank hole. The splines bottom and end before the pin hits anything. A rough measurement is about 5mm between the end of the pin and the bottom of the whole that looks to be more of a hollow area than a drilled hole beyond the splines.
The end of the 35hp crank is different than the end on the 40hp crank, the OD around the splines is smaller on the 35hp and the splines are counter sunk further on the older 40hp. Otherwise they look the same but they definitely aren't identical castings. The 35hp had a seal that incorporated a sleeve that was pressed over the end of the crank, the 40hp had only a seal that rides on the crank shoulder and an O ring around the spines that fit into the recess in the bottom of the crank.
With the presence of the O ring on some and seeing sealer applied to the splines, is the inside of the bottom of the crank somehow a potential crank case leak?

The bottom crank bearing is a ball bearing on this motor with a plastic insert or seal on the bottom side. The bearing is nearly completely locked up, it'll only turn about a 1/4 turn either way.
The inside of the motor is a sticky mess, its nastier than tearing apart an old diesel engine. The exhaust tube though is fairly clean, almost new looking.