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.
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.
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.