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Mercury Mark 35a - upper unit is in order - on to the lower unit

Boatsnhoes

Contributing Member
The good news is I've (we've) sorted out quite a few problems, and now the motor runs pretty well. Water is flowing through it, and all in all everything above the exhaust hole looks good.

Bad news is this means I have to start thinking about the lower unit, crankshaft, etc.

So I have a few questions before I get started.

1) I stuck a borescope up the lower unit to get a look at the drive gear. It looked pretty darn rusty. Not irrecoverably so, and I'm pretty sure the crankshaft would still engage, but it's worrisome. Is this normal? Do I need to replace it, and if I do have to replace it, how difficult/time consuming is that? I was really hoping to keep the powerhead mounted and un-split, so hopefully that's avoidable.

2) I got a lower unit as part of the deal for the motor, but I'm not really clear on what's missing. It appears as though I'm missing parts 27-33 and part 3 of the shift shaft. What am i missing from the prop side, though? What you see in the pictures below is everything I have. Mostly I'm trying to match a part number to that exposed piece with 4 open screw holes on it, so I can tell what numbers above it I need.

Part numbers I'm referencing are here: https://www.marineengine.com/parts/...rts/mark-35a/all/gear-housing-assembly-page-1

3) The lower unit sounds very...crunchy when i spin the crankshaft by hand. I'm not sure what it's supposed to look/feel like, but I can definitely feel and hear crunchiness when it first starts moving. After that the crunching subsides but I feel the each gear tooth engaging (or so it seems). Does anyone know what can cause these symptoms? I assume I have to pull the whole gearcase to get a look inside. Any advice on how to go about that?

Thanks everyone! Hopefully we're in the home stretch now.



IMG_4026.jpg
IMG_4027.jpg
IMG_4029.jpg
 
Those plates ( both of them ) with the 4 holes screw out.------The holes are for a special tool with 4 pins to turn those retainers out.----They may well be left hand threads too.-----And what part are you referring to that you call the CRANKSHAFT here ?----Picture of what you are calling the crankshaft ?
 
What I was calling the crankshaft is actually the driveshaft, my mistake. The thing you see in the pictures is what I'm referring to, the big long drive shaft.
 
The crunching sound is probably rusted bearings in the unit, take out the gear lube screw (35) and see what comes out I'll bet it's water. To take the LU apart you undo the large retaining ring, it should be marked OFF and an arrow pointing the direction; then put the propshaft in a vise between 2 blocks of wood and gently tap the housing away from the threaded end. the shaft should come out with the reverse gear on it. You'll have to undo the waterpump cover to remove the driveshaft.
Mark-35-LU.jpg
 
My opinion-----Just another example of --------" do a complete evaluation before fixing / spending money on one part of a motor "-----Hopefully you have a source of good used parts in your area.
 
Rusty gears you can get away with. Rusty BALL bearings you can replace, but if the areas on the drive and/or prop shaft where the needle bearings run is rusted and pitted, the shafts are scrap. That's what can happen if water gets in there and is not removed. Hope not, but the only way to tell is to pull her down.

Jeff

PS: Still a bunch of those LUs around. Got 4 myself.
 
Thanks everyone, I’ll post some pictures once I get it all apart.

Jeff - if it comes to it, are you selling those extras? I’m not too far from you
 
Drained the unit today. Pretty sure it's supposed to have grease, but instead a bunch of oil came out. Regardless, the oil looked clean, no milkiness, no metal shavings, so I'm cautiously optimistic.

Still waiting on the impeller removal tool to come in so I take it apart and post pictures.

The unit is definitely stuck in forward gear though. Any experience with that?
 
I can move the shift shaft and I can feel the three clicks I'm supposed to but none of that changes the way the propeller spins
 
If you have done the shifting in the manner described below then there's something wrong with the clutch mech or the cam; you're going to have to pull it apart I guess. As to the LU lube - 105 Lubriplate replaces original gear grease in all 1, 2, & 4 cyl. Mercurys through 1961.
*Turn the LU shift shaft by hand counter-clockwise until it
stops, then with Vise Grips tight on the bottom of the shaft turn it a
few more degrees CCW until it clicks into a stop, this
should be neutral - check by turning propshaft --
prop should spin freely.
Put control in N and go ahead and attach LU to driveshaft housing some turning of the flywheel will help the DS splines align.
Do not shift into reverse unless you turn the prop at the same time.*
lushiftsetup.jpg
The clutch is spring loaded so that when it is in F it's relaxed and shifting it into N or R compresses it, increasing resistance.
 
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