Shift Shaft Wont Move on LU for 1973 Mercury 115HP

BHassett

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Aug 24, 2016
Messages
17
Hi All,

Motor is stuck in reverse. Even if we changed the gear on the linkage arm it remained in reverse. We isolated the shifting issue to the LU. Dropped the LU and the shift shaft does not twist or go up and down. I know it should do one of those. Any idea what's going on? I've wrapped a rag around it and tried to move it manually with some vice grips with no luck.

I also noticed some substance in there that didn't look like lube to me. I included a bunch of pictures so you guys can see what I'm talking about.

Theres also a black cap that was threaded through the shift shaft sitting on top of what I surmise is the bearings. Also included pictures.
 

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gap1949

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Jul 17, 2003
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13
What does the lower oil look like? If lower has had water intrusion, find another lower. If not you need to pull the carrier and prop shaft. Pay close attention to the shift dog, pin and retaining spring.
 

BHassett

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Aug 24, 2016
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17
When I drained it some oil came out but not much. The oil that was there looked decent.

To be clear, if theres any water in the gearbox I need a new LU? Did any of that gunk in the pictures hint towards water intrusion?

Anything in particular I should be looking for when I pull the carrier and prop shaft?
 

Texasmark

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Dec 20, 2005
Messages
14,544
1. Pictures are blurry and of little help.

2. Shift shaft protruding from the lower unit is a short steel rod, with splines on both ends. It has no bearing. It is contained by 2 (drilled out to it's diameter) bosses, one above and the other below the shift cam (part of the LU housing) that TWISTS to select the gear, with each gear being roughly 20 degrees apart....F-N-R. What you are looking at is the seal(s), usually 2 front (open area) to front.....you're looking at the back of the top one, to seal lube in and water out.

3. The LU should be full of what may look like blue/black 30 wt. motor oil viscosity but it's not motor oil, it's a special lower unit oil that you can get at WW. When you dropped the drain plug you should have gotten out roughly ? of a quart of blue-black oil with few if any metallic shavings. If your drain plug had a magnet there could be a thin layer of very fine metallic "dust" for normal operation. Any chunks are bad news as is any water, as are any water bubbles, as are colors of the oil other that that specified, as is inadequate fill.....where'd the oil go and why wasn't the LU full, and what didn't get cooled/lubed as a result......latent damage?

4. With what you have said and the year model of that engine a functional replacement really wouldn't be a bad idea as it also should contain a new/rebuilt water pump that you will surely need. One of the reasons I say this is that to get at the contents of the lower unit, after you pull the prop and the thrust that is on the prop shaft and the prop presses against, you have to remove the spanner nut holding the contents in the LU. If you look at the rear inside of the casting, right in front of where the prop rides, you will see that the housing itself is threaded and running on those threads is a 4" spanner nut that requires a special wrench to remove (or your creativity) and severe corrosion. The diameter and corrosion is such that it is almost impossible to remove that nut without adequate tools to cut the nut out without damaging the housing or bearing carrier that you are trying to get out.

I'll leave it there.
 

BHassett

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Aug 24, 2016
Messages
17
There was way less than 3/4 a quart of lube/oil in the LU. Would that explain why its not turning at all? If I get some in there do you think it'll loosen up or is it completely shot?
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
There was way less than 3/4 a quart of lube/oil in the LU. Would that explain why its not turning at all? If I get some in there do you think it'll loosen up or is it completely shot?

1973 is a long time back. Lots of years for unprotected metal, ferrous metal at that even though some of it has been tempered for strength, some stainless steel but SS is only carbon steel with nickel in it, amount determines the type. If your LU was full all that time, changed periodically, didn't leak, and was used periodically to keep moving parts lubed up, I'd say you wouldn't be on here asking these questions.

That's my take. The proof is in the pudding....aka if you really want to know tear it down.
 

BHassett

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Aug 24, 2016
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[FONT=arial, helvetica, sans-serif]Thanks for your response, Mark. There very much appreciated.[/FONT]

One piece of info I left out..every time I'd try to start the boat it the prop would in reverse. [FONT=arial, helvetica, sans-serif]That makes me hopeful that the gears aren't total shot. No help with why I can't turn the shift shaft though. [/FONT]
 

Texasmark

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Dec 20, 2005
Messages
14,544
I'm deliberately going over this again, rethinking the problem, looking for a trigger mechanism.

The shaft is a short, splined on both ends steel rod induction hardened. The little black thing was probably a protector for the exposed seal that rides on top of the LU casting where the shaft is inserted. The LU casting has 2 bosses with a clearance hole drilled to accept the shaft. No bearings are involved. You have the shaft seals, the outer shown in your pictures, one under that turned the opposite direction (seal oil in and seal water out), and the shaft. The shifting mechanism is a hardened steel "cam" with 3 detents (dimples) for FNR, which is inserted between these bosses and moved radially by the lower spline of the shift shaft. Each dimple is a different distance from the pivot point...the shaft. (Not moving, the shaft could be rusted to the cast alum. bosses for one possible answer).

The prop shaft is hollow on the front end and a pointed "cam" follower protrudes from the front with several parts behind it, including a spring which keeps pressure against the dimples in the "cam".....these parts are brass for the follower and an adapter with a hole in it, and the rest including the spring are stainless steel like the prop shaft...nothing much to rust here and this is low enough that low oil probably wouldn't be a big deal from a corrosion standpoint if the engine storage was vertical.

Part of the front end of the prop shaft outer diameter is splined and riding in this splined area is a sliding gear called a "clutch dog". This dog has sawtooth teeth on the front end and cogs on the rear. The parts in the previous paragraph are locked to this dog via a SS pin that goes through that hole I mentioned above, so that as the cam follower follows the dimples in the shifter cam, the dog moves back and forth along the prop shaft engaging the rear of the F gear, engaging neither, or engaging the front of the R gear. The gear is "gear grade hardened" as are the F and R gears and that process lends itself to little oxidation (corrosion) damage. The radial and thrust bearings are SS also....nothing much here to corrode.
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The fact that nothing move, not even slightly brings me back to the cast alum bosses into which the shift shaft is mounted.

The shift shaft is hardened steel and you aren't going to hurt it.
You are looking at the rear of one seal when you look down at the shaft. Directly below it should be another. Directly below that a cast alum boss which is part of the main LU casting.

Get a small drill, like a 1/16" and drill several holes in the seal to a depth of ?". Get a scribe or something sharp and using the holes gouge out the seal. Do likewise for the one under it if it has one...most have 2 seals.

With the seals out you are looking at the upper boss. Do you see any corrosion? Give it a squirt of a quality penetrating oil like PB Blaster and let it soak a bit. Grasp the spline with your vise grips and methodically attempt to rock the shaft back and forth. If you get movement, more PB and more twisting. Let me know when/if you get movement of any kind. Don't attempt to shift just yet.
 

BHassett

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Aug 24, 2016
Messages
17
Thanks, Mark. Ive been on the road for work last 3 weeks so Im just getting to working on it again today. The good news is the special shift shaft bushing removal tool I ordered came in.

My thought is I can just remove the shaft and get the same results you're looking for? I'll let you know how it goes and what I see if I ever get it off. Damn bushing doesn't make any progress loosening after a few turns.

Im just trying to get the damn thing apart so I can see if anything stuck in there. I've read in couple places the prop shaft needs to come out to remove the shift shaft....so I'm off to get what I need to take off the prop shaft now..which I'm hoping is just a couple sockets but I'm sure I'll find I need another F'n special tool for that too.
 

BHassett

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Aug 24, 2016
Messages
17
So I got the pushing off and i'm looking down. Don't see anything stuck in there. There's a whole lot of pressure on the shift shaft though. Still can't turn it.
 

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