Mercury Lower Unit 101

[Roberta]

I'm in the middle of repairing my Merc 500 lower unit and I thought I would share this with some of you who wondered what was all in those things and how they come apart and go back together. I have never had one apart before and was a bit apprehensive about ripping into mine. Water in the oil and oozing out a vent that should not be oozing prompted my curiosity.

For safety sake, pull the spark plug wires off the plugs to make sure the motor cannot start. It is a good idea to have the proper manual for your motor, too. Drain the oil out of the lower unit before you start.

First off, removing the unit from the drive shaft housing is no big deal. There are four nuts with washers and one socket head bolt. Before removing anything, it is important to place the shifter in forward so the lower shift shaft will be aligned with the upper shift shaft when you put it all back together. The socket head bolt retains the trim tab and is removed from a hole in the drive shaft housing above it. This will reveal the fourth nut and washer that holds the lower unit on. The first three are fairly obvious, two up front and one near the middle.

Once you have the trim tab and the four nuts and washers removed, you can pull the lower unit off. It may move down about an inch, then feel stuck. A small tug and it will come out. The exhaust tube will come down slightly and the water tube to the pump may pull down These can be reinstalled when you put the lower unit back on.

Once you have it off you will see the drive shaft (the long thing), the shift shaft (the short thing), and the studs that held the two front nuts and washers on. In the middle you will see the water pump housing with the drive shaft going through it. There are three nuts that hold the pump into the lower unit. To remove the pump, first remove the O-ring from the top of the drive shaft and the rubber slinger. The pump can be unbolted and the top lifted off to reveal the impellor and pump base. The impellor can be slid off the drive shaft. Be careful not to lose the key for it. The flushing screw in the side of the lower unit needs to be removed so you can lift out the pump base. There are gaskets, dowel pins and the faceplate. Keep track of the order. On the bottom of the pump base is a gasket, and O-ring and two seals placed one on top of the other. One seals the water in the pump while the other seals the oil in the gear case. The lips of the seals are oriented to face to media they are sealing.

Once the pump base is removed you will be able to see the drive shaft bearing. The pinion gear in the gear case will prevent the removal of the drive shaft. The propeller shaft needs to be removed to access that.

Take a look at the shift shaft. On top will be the reverse locking cam. As the shift shaft is rotated, the cam lifts a rod in the drive shaft housing to latch the motor tilt mechanism when in reverse or neutral. Since you pulled the lower unit out in forward gear, it will be oriented correctly and should be marked and noted should you decide the remove it. Removing it is simple as it slides off the shaft. Below it is the base for it, a rubber washer, and the bushing assembly with seals. The bushing assembly is screwed in and has an O-ring below it. A special tool is made for removing the bushing, but a tap or two with a pin punch will loosen it so it can be thumbed out. DO NOT PULL THE SHAFT OUT UNLESS YOU PLAN ON REMOVING THE PROP SHAFT. The shift cam on the lower end will disengage and if it moves, you will not get the shaft back in. You will then have to pull the prop shaft.

Now you can remove the prop nut and retainers and pull off the prop and hub. Inside is the threaded gear housing cover and a locking tab washer. A special tool is used to remove the cover, but I used a pin punch to loosen and thumbed it out. Once the cover is removed the bearing carrier, prop shaft, and reverse gear can be pulled out. There are shims for the bearing and a key at the bottom of the carrier. Unless there is a problem with the gears or the bearings, just a good cleaning is all that is necessary.

Now you will be able to see the end of the drive shaft with the pinion gear attached. Beyond that is the forward gear and it’s bearing. There are also needle bearings in these assemblies. They are caged so they won’t fall apart, but need special drivers to remove and install them. I will not be getting into that. My real purpose for getting into my lower unit was to replace seals and the drive shaft.

There is a nut on the end of the drive shaft that holds the pinion gear on. Hold the driveshaft in a “V” jaw in a vice while turning the nut to remove it. You may need to allow the gear housing to move along with the wrench that holds the nut. Once the nut is off, you can tap the gear housing with a mallet to pop off the pinion gear. The drive shaft and bearing can now be removed.

What you will see where the driveshaft was is the bearing race and an oil sleeve that helps lubricate that bearing. The race has shims below it to adjust pinion depth to set the proper gear lash between the pinion and the forward and reverse gears.

Behind the pinion is the forward gear and its bearings. There is a needle bearing in the center to support the prop shaft and a cone to support the gear itself. Shims are installed behind the cone race and should need no attention unless replacing the gear or cone. The whole prop shaft is supported on needle bearings and the gears are supported by the shaft and the taper cone bearings and clearances are set by various shims.

And finally, you can see the shift cam in its little slot with the shaft coming down from above. Pulling the shift shaft will release the cam. Note its orientation and where the little detents are. These engage the follower cam on the end of the prop shaft to move the shift sliding clutch ratchet.

Now that we have seen most of the parts, how does it all work? The unit was designed to be in forward gear when all is at rest. The pinion and drive shaft constantly turn the forward and reverse gears whenever the engine is running, as well as turning the water pump. The sliding clutch ratchet is spring loaded to be engaged in the forward gear. The follower cam in the forward end of the prop shaft pushes the ratchet to the neutral position and, further, to the reverse position. In neutral the ratchet is disengaged from both drive gears. The follower cam is actuated by the cam and shift shaft as it is turned by linkage in the engine compartment. Since the ratchet is splined to the prop shaft and the drive gears are not, the prop shaft turns with the gear that the ratchet is engaged with. The drive gears spin opposite of each other due to the rotation of the pinion that is always engaged in them. So the prop shaft spins in the direction of the drive gear that is engaged with the ratchet, unless the ratchet is in neutral, between gears. Very nifty setup.

Getting back to the shift shaft and the reverse locking cam. Coming down through the drive shaft housing is a rod that engages the reverse lock on the tilt mechanism. The cam lifts that rod as the shaft is turned to change gears and the lock will occur in neutral and reverse to prevent the motor from lifting up during backing and slowing down quickly in neutral. It is important to make sure that when you re-engage the lower unit to the drive shaft housing, that this shaft and cam are oriented properly to allow the shift to properly occur and the lock to engage when it is supposed to. When the shaft is turned to the forward gear position the cam should allow the rod from the drive shaft housing to be down to release the lock. It should also be able to lift that rod as the shift shaft nears neutral going towards reverse.

Looking down on the shift shaft in the lower unit, turning it counterclockwise will disengage the forward gear and into the neutral position. The prop should spin freely . Further turning will engage reverse. Now the prop will turn in reverse. Continued turning will force the cam around and you will re-engage the forward gear, but the cam may not be in the correct position. You must continue to turn CC until you feel a slight detent and the mechanism is in forward drive. In either forward or reverse, the prop will drive the drive shaft in one direction and you will hear a ratcheting noise in the other. This is the ratchet dogs slipping on the drive gears and is normal.

Once you are in the forward detent, you can place the reverse lock cam on the shift shaft in the correct orientation so it can properly operate the pushrod. My book shows a line from a cam lobe to a stud on the gear housing. Having the shifter in forward, the cam properly oriented, and the motor control in forward should ensure that the unit will go together and operate correctly.

Since the drive shaft on my motor was so badly pitted, it tore up the seals and allowed oil to escape and water in. Fortunately I discovered the problem before damage extended to the gears and bearings. I was able to get all the seals and minor parts from Crowley Marine in Denver. The drive shaft was located on Ebay and was NOS (new old stock). The parts are on their way and I will be assembling everything later next week.

In the mean time, I have been busy cleaning up the parts and making tools to reassemble. I am not found of using a pin punch on the parts to get them apart. I made a driver for the gear carrier cover nut and will be making one for the shift shaft bushing. These are fashioned from a heavy wall plumbing connector. The tabs were cut with a Dremmel tool and should be sufficiently strong to re-assemble with maybe a slight tap with a punch to set them.

I will post more pics later during the re-assembly.

Roberta

This photo shows the top of the lower unit with the pump, drive shaft and shift shaft bushing removed. At this point it is easy to remove the shift shaft and lose the shift cam in the gear case. With the bushing in place, a clip will prevent to shaft from being removed.



This photo shows the bottom of the drive shaft housing with the shift linkage and reverse lock pushrod. You can see the splined socket that accepts the shift shaft from the lower unit.



This photo shows the shift shaft with the cam lobe that operates the follower cam in the prop shaft to move the ratchet that shifts the gears.



Looking into the gear case you can see where the cam is that operates the follower cam that moves the ratchet.



Pieces parts



Here you see the forward and reverse gears, the bearing carrier, the pinion and the prop shaft with the ratchet. The ratchet engages the selected gear to drive the prop shaft. The follower cam is the small pin sticking out of the right end of the prop shaft.



Here is the shift shaft bushing and reverse lock cam.



This shows how badly the drive shaft was pitted. I tried to clean it up, but it is too badly pitted to be reliable.



Here is the tool I made from a plastic plumbing connector to be used to screw the case cover nut back on. A similar one will be made for the shift shaft bushing.

[upspirate]

Great post Roberta,thanks!

[fergal butler]

Very handy post thanks for taking the time to put it up.

[wright6627]

Super post, Roberta.

Your post really illustrates how good Mercury lower units were back in the mid 70s, and had been for years. It's interesting to note that this design goes all the way back to 1952 and has been incorporated in Mercury outboards and outdrives to this day. OMC didn't come up with a robust design like this until the 70s. There's virtually no wear on your your unit except where a little too much water pitted the drive shaft and where seals put slight groves into the prop and drive shafts. It's actually very typical for a 35 year old Merc lower unit that has had a reasonable amount of maintenance.

[wright6627]

Roberta,

Is that a 2 bladed prop I see in one of the pictures? If so (and if your tach is working now) what RPM are you getting at wide open throttle? Over the years, I've had trouble getting 40 and 44 cubic inch Mercs up to speed with 2 bladers when mounted on delux boats like your Zip. I've gone to 3 blade props with pitch around 10-11 in order to get engine speed, boat speed, and acceleration up (sometimes way up). For the Zip, I'd be looking for a 3 blade prop that would run a little past spec when the boat is very lightly loaded. Then I'd cup the prop to bring the RPM into spec.

I've really enjoyed the adventures of Roberta and her superfine boat posted in these forums. Good luck with the new project--a little like eating popcorn, ain't it?

Randy

[Roberta]

Thanks, Everyone. It's been kinda fun exploring the old Merc. I will be posting more pics and thoughts as I begin re-assembly later this week. Yeah, Randy, the design has been a very good one. Everything seems to be in Great shape for being 42 years old. I see areas where a good waterproof grease will help adding more years, like that area of the drive shaft by the water pump and by the shift and lockout shaft and cam.

I have been running at 5400 rpm and doing about 34 mph with that two blade prop. I'm not sure what pitch it is, but it is AL and I think it is 10 3/4" diameter and either 13 or 12 pitch. Seems to be the right prop for my zip and it is the one that came with the motor, but I don't know if it is original to the motor. I can't find any numbers on it to positively identify it.

Roberta

[upspirate]

I'll bet ya could do a little better if you had it re-worked during the winter.....look's a bit chewed up on the edge

[Roberta]

I'll bet ya could do a little better if you had it re-worked during the winter.....look's a bit chewed up on the edge

Any suggestions as to who might be able to do it right?

Roberta

[wright6627]

Roberta,

I know you can do it yourself. Straighten the edges with a file. Whatever you do with one blade, do to the other. Eyeball the curvature of each blade and compare. Any small dents in the curvature can be straightened out with the anvil on your vice and a hammer. Finally, taper the blades to a blunt edge with your grinder. I like a taper about 1/2" wide on the back face (closest to the lower unit) and an edge about 1/16". Some guys go for a knife edge, but that's a little fragile and you'll end up repeating the process. Don't grind or file the front face of the prop.

I think you're getting marvelous performance from your 500 considering you're using a 2 blader of 12" pitch. I think there's a good possibility that your engine is exceptional. I wouldn't have been surprised to hear you were pulling 4,500 to 4,800 RPM with that prop on the Zip. I still think there's room for improvement by going to a 10" or 11" 3 blade prop. This would probably allow your engine to run a little faster than what's best for it. Then you cup it to get the revs back to 5,500 to 5,600 RPM. I would not expect much of an increase in top speed but there might be a couple mph left in the thing. There would be a dramatic increase in acceleration and load carrying ability. Kind of like adding about 10 horsepower without any weight penalty. I love cupped props--you get the hole shot of the prop at its manufactured pitch but you get top speed of a higher pitched prop. Better performance all the way around.

People still consider propping a dark science, but it's really just fun experimentation. I've spent many hours bent over the trailer ball on the back of a pickup truck trying to cup another mph or 2 on our race boats. Aluminum props for your motor are available cheap on e-bay. You'll be able to figure out what best makes your Zip tick. Then, if your pockets are deep enough, you can pop for a stainless version.

Randy

[Roberta]

I've dressed enough airplane props, so I guess I could clean this little bugger up. I'll give it a go. Maybe I'll look for a 10 or 11" three blade I could throw on when hauling extra people like at the gatherings. I still plan on the Lenco trim tabs.

Roberta

[ttownshaw]

Good work Roberta...Yep, I replaced the seals on my Evinrude...process was very similar. Removing the bearing carrier was a bit of a chore though. Anyway, another good tip. Take a scrap piece of 2x8 or 2x10 (4' long is plenty) and cut a notch out of it so the lower unit can rest inside the notch (on the cav plate). Then clamp the board to your workbench. You can also add a small bungee to keep the lower unit from sliding around.

[Roberta]

Great tip, Bill!! Thank you!!

Roberta

[Roberta]

Well, I got the drive shaft about a week ago, but I'm still waiting for the rest of the parts. Apparently the water pump base is on backorder.

It took a few swats with my BFH to remove the bearing from the old shaft, so I decided to try and ease the bearing onto the new shaft. I put the shaft in my freezer to shrink it down bit and warmed up the bearing a tad with a heat gun to expand it a tad. I made a driver from a piece of pipe I ground down to the diameter of the bearing inner race. Using that I was able to easily pop the bearing onto the new drive shaft.

Then I ground the end of the pipe to make a tool for screwing in the shift shaft bushing. I made two tabs in the end of the pipe similar to the four I cut into the bearing carrier cover nut tool. This should help to properly snug that item into place when the time, and parts, come.

Roberta I hate waiting

[Roberta]

Finally got the parts yesterday and started putting the lower unit back together. I smeared a little waterproof grease on all parts bearing surfaces prior to assembly.

The first thing I did was to put the shift shaft in and the shift cam. They are secured by the bushing assembly and shaft seals. I used that modified pipe and pipe wrench to tighten the bushing down, rather than bump it in with a pin punch. Now that the shaft is in and secured, I won't have to worry that it might pull out and allow the cam to be let loose behind the forward gear assembly. I then dropped the forward gear assembly in and got ready to install the drive shaft.

The drive shaft was set in my vice and held in AL "V" groove jaws. Then I slipped the gear housing over the shaft. The pinion gear was then slipped on to the drive shaft and secured by the nut and a little locktight. I temporarily placed the bearing carrier nut in the housing to prevent chafing the threads for the nut while I used a breaker bar and socket to tighten the nut. Having the shaft secured in the vice allowed me to swing the housing to facilitate tightening the nut.

The next thing was to replace the bearing carrier shims in the housing. You need to be sure these are stacked neatly so the reverse gear passes through them and they don't get bumped out of position. A small dab of grease will hold the shift follower cam in the prop shaft while you place it into the forward gear assembly. Then the bearing carrier with the reverse gear can be slid in. A new O-Ring was used and the prop shaft seals were replaced as well. It is important to carefully set this in place to insure it goes all the way in. Once in, you should have only a few thousandths of end play in the shaft. If you can pull the shaft in and out more than that, the carrier is not properly seated and you may need to check to seat if the shims or anything has moved, causing the carrier to not bottom out in the housing.

Once the carrier is in place the retaining key, locking washer, and nut can be installed. For this I used the plastic plumbing tool I made with an oil filter wrench to turn it in. Prop shaft endplay and gear lash can be checked. I do not have the sophisticated tools to accurately check this, but my own experience gives me a reasonable idea of what feels proper. I checked the play prior to disassembly, so I had an idea of what to expect. Since I did not change the bearings or shims, I expected nothing would change from what the factory did when the lower unit was built, with the exception of the pinion depth. That could be different because I changed the drive shaft, but it appears that the clearances were all OK as the unit operated smoothly and felt to have proper lash. Had the pinion been tight or loose, I would need to reshim the bearings holding the forward and reverse gears to get the proper clearances. This could get very complicated and is beyond my expertise. But doing a routine reseal of the lower unit should not get that involved.

OK, the gear train looks good, I installed the prop, and I can now install the water pump and the rest of the parts.
The water pump base seals the gear oil in the gear case with an O-Ring and an oil seal and seals water in the pump. On my pump base there are two seals on the drive shaft. One seals water in the pump while the other seals the oil in the gear case. The seal lips need to face the proper direction so they can seal properly, so the pump side seal should have the lips facing up toward the water while the gear side needs to face down towards the oil.

When sliding the base down the drive shaft there is a danger of damaging the seals as they pass over the O-Ring groove on the upper end of the shaft. To prevent damaging the seals, I installed that O-Ring and greased the area to allow the seals to safely slip over the O-Ring and down the shaft.

The pump base also holds the drive shaft cone bearing in place. There should be no play on the shaft when the water pump base is pressed in place. Additionally, the base should bottom on the housing to properly seal. Reshimming of the bearing race would be necessary if the proper clearances are not met. The manual explains this in better detail.

I used a dab of gasket sealant on the base gasket and set the base in. It is a tight fit, so I temporarily set the pump cover on and carefully drew the base in using the nuts and washers that hold it in. I also used a dab of grease to help ease the o-ring in. Once that was seated I could check the play in the shaft and make sure the base was fully in. All OK.

The next gasket, the face plate, and another gasket were set on the base. I then placed the impeller on the shaft and engaged the key into it. The pump cover can now be installed.

The pump cover has a seal and washer for the tube that delivers the water to the engine. This was replaced. There is also an insert that the impeller rides in that should be checked for wear and damage and replaced, if needed.

You can seat the impeller into the cover by pressing down on the cover while rotating the drive shaft clockwise. This actually works very well and you will feel the cover slip down as the vanes of the impeller splay into position in the insert. Once down, the cover nuts and washers can be installed. Now the Slinger and the upper drive shaft O-Ring can be reinstalled.

The only thing left now is the reverse lock cam and spacer. The spacer has one or two tangs on it that fit into slots in the housing around the shift shaft and bushing. The cam slides on the splined shaft and must be in the proper orientation so that it can lock and unlock the motor latch.

Remember when we took this all apart? We placed the shift control in forward? Make sure it is in forward. Now the lower unit must be placed in forward. The cam in the gear case needs to be in the forward detent. To do this, use a pliers to turn the shift shaft counterclockwise. Careful not to damage splines. You will come to a point where the shaft easily turns about 180 degrees. Turning CCW, when you reach a point of slight resistance, turn just slightly further to feel that cam and follower engage in the first notch. Check to see that the drive shaft turns the prop, indicating you are in forward. Turn a little further and find neutral. The prop will now turn freely. Turn the shift shaft a little more to find reverse. You should be able to turn the drive shaft and see the prop go in reverse. Turn the shift shaft a little more and the unit will go back to forward, but make sure to find the detent again as above.

The reverse locking cam and spacer can now be placed on the shift shaft. You will see a slope and a flat spot on the cam. While in neutral and reverse, the reverse latching pushrod coming down from the driveshaft housing is lifted by the cam to latch the mechanism and will be supported on the flat portion on the cam. As the motor is shifted into forward, the cam is rotated to allow the pushrod to go down the slope and release the reverse latch. Because of this relationship it is necessary to place the cam on the splines of the shift shaft so that the slope of the cam is ready to start raising the push rod as soon as the control is starting to shift from forward towards neutral. Then you will have the proper position of the cam and shift shaft to properly shift the lower unit and latch the reverse lock. The latch should be engaged in neutral and reverse and should disengage during the shift from neutral to forward.

Now the lower unit can be installed onto the driveshaft housing. I put copious amounts of waterproof grease on the latching cam and pushrod and a little on the shift shaft and drive shaft to prevent rust and corrosion. Don’t forget to place the socket head screw in the lower unit for the trim tab. Now you should be ready to replace the lower unit gear oil.

Hope this helps anyone wanting to repair their Merc Outboard. I don't claim to be an expert, but this worked OK for me.

Roberta


Here is a pic of the shift shaft and cam in the lower unit and the drive shaft set in. You can see the detents in the cam where the follower cam engages the shift cam.



Here you see how I held the driveshaft to install the pinion gear and nut. I could swing the housing to enable me to turn the wrench.



Here you can see the pinion and the forward drive gear in the housing.



Here is the prop shaft and cam follower ready to be placed into the housing and into the forward gear bearing. The clutch dogs are what shift back and forth to engage the forward or reverse gear.



Now you see the bearing carrier inserted into the housing and secured with the key, washer and lock nut.



Here is the upper part of the lower unit with the pump base set, the impeller in place with key, and the shift shaft with bushing and seals.



Now the pump is fully assembled and the slinger installed. The shift shaft locking cam is also in place.



Here is that O-Ring on the top of the drive shaft. Putting it in place while you lower the pump base over it will protect the seals in the base.

[jamundsen]

Roberta You are simply amazing. Great post!