TOPIC: Removing driveshaft

How does one remove the driveshaft on a Mark 78 lower unit. Thanks, Skip.

take water pump apart. take propshaft out take nut off the pinion gear. then driveshaft will pull out as an assy.( put DS in vise and tap on the gear housing with a block of wood and hammer heat may be needed.)

I have done a few of these and here is what you will typically face:
The waterpump base (the recess where the impellor goes) is a real bear to get out. It is a round piece of aluminum with the stainless cup insert. The aluminum outer part tends to corrosion weld itself to the aluminum lower unit casing. I soak for a few days and then LOT'S of heat on the outer lower casing. These pry out using two screw drivers in the slots of the lower unit to lever against, it just sits in the lower unit and has a keyway to keep it from rotating so you are trying to pry it up. It still may not come out, I have resorted to destroying ones to get out. These waterpump bases are hens teeth to find. I found one NOS and made a blueprint so I can have new ones machined. I made a few from solid Stainless with a few details added to facilitate future removal. I run one in my Mark 75, no issues, the others I horse traded for other parts so I don't have any but, more could be made.
Do battle and try to save the one that is in there.
Below the waterpump base (once removed) there is a threaded retainer which also holds the driveshaft seal, you will need the spanner wrench tool to get this out. Again heat helps, in general these do come out OK. I forget if these are reversed threads or not, manual will tell you. Look for shim spacers too.
To remove the prop shaft you again will need a spanner tool to unscrew the end ring, again corrosion can be an issue so soak and heat the lower unit. I find it best to rap on the spanner wrench arm to snap the retainer free rather than brute arm strength.I forget if this is reverse thread or not.
Once that is out the next thing you will see is a cap that has the prop shaft seal in it. This will wiggle out but has an O-Ring on the outside diameter to seal it to the lower unit bore so it's tight. You will now see a bearing with it, Prop shaft will slide out. You cna pull on the propshaft to take out the seal ring and the bearing all at the same time, that is the easiest way. If stuck, clamp the prop shaft in a vise and tap on the lower unit to seperate. There are shim spacers in all this so be mindful of that.
Now you can get at the drive shaft lower nut so pinion can be removed. The drive shaft will come out as described by Dave but beware there nmay be spacers under the top ball bearing so look for them upon removal of the shaft.
You will find the drive shaft is probably corroded where the seal area/impellor is, they are steel and very few were hard chromed for corrosion protection. These lower units are not that robust and are of poor design / materials as much as I hate to ding Mercury design. Another area of probable poor condition is the impeller key flat on the shaft, these get trashed because a roller pin is used rather than a flat key and this area gets beat up as the motor is started in forward and reverse the roller beats it's way into the shaft flat. I use the more modern flat key and ONLY start the motor in forward to reduce further destruction.
I use speedi-sleeves to fix the driveshaft seal area and they work great if installed correctly and at the right location to engage the seal.
The bearing MUST be in the drive shaft before the speedi sleeve is bonded on, bearing will not slide over it. I got lucky when Bob Grubb was still in business and he knew someone that would weld the shaft with Stainless in the seal area and then grind to size, THAT was the best. The prop shafts are hard chromed and generally the seal area is OK but you can speedi-sleeve that are area too if needed, I do that as a matter of course when rebuilding.
A word of caution, these lowers are easy to crack, use a lower unit stand to hold it in place while working on it. Use HEAT and penetrating lube. Get the spanner wrenches, these come up on ebay all the time and buy a manual form East Coast that covers these.
Once done right they hold up fine, I have run mine for many years with the speedi-sleeves and have had zero water intrusion. If you have a good lower use lower gear oil rather than the Lubriplate grease, if it leaks some then use the Lubriplate grease and check it frequently. These lowers are just really not that good which stinks becasue the motor is so COOL!
The pump valves, those rubber tube things can be trouble. Where I only start my motors in forward I do not use the valves but you need to block one of the holes off for proper pump operation. I forget which one you block off but I use a solid disk and use a washer in the one I want water to pump through. I also have these made out of Stainless.
Good luck, if you have a real clean lower to start it's a much easier job.
Randy