What part of the gear case do you want to dissemble? I am going to assume that you are pulling it down completely, so this is what I do.

Remove the water pump and the attaching bolts on the water pump assembly and remove the pump housing from the lower unit. There is an O ring on the outer surface of the housing, it will come out. Next, remove the prop, and the bearing carrier nut. This nut is right hand thread, and you can make a tool to fit into the slots in the nut. Don’t be tempted to knock it out with a punch, it usually will only wreck the nut, and cause you issues later. Once the bearing carrier nut is off, place the prop shaft in a vice and with a block of wood to protect the housing, drive the housing off of the prop shaft with a mallet. The prop shaft, bearing carrier and reverse gear assembly will come out as a unit. (Note shims that are behind the reverse gear retainer.) There will be a pin in the end of the drive shaft that rides on the shift cam. This pin will fall out of the prop shaft or will pull out fairly easily. Once the prop shaft assembly is removed, you can access the bolt on the drive shaft pinion gear. Place the drive shaft in a vice to hold it. Looking into the open end of the housing where the prop shaft assembly was. There will be a lock tang that you bend out of the way, and then you can get a wrench squarely on the bolt. This is a thin head bolt, so be sure that you have your wrench square on the bolt. Once you remove the bolt, you can then with a block of wood to protect the housing, you will be able to drive the housing off of the drive shaft. This will remove the upper roller bearing, and any shims under it. The pinion gear will fall out as the drive shaft moves out of the way. Next, place a block of wood on the bench, and strike the housing open end squarely on the block of wood. This will cause the forward gear to fall out of the housing. A few smart blows on the block of wood are all that is usually needed to remove the forward gear. Again note the shims under the forward gear bearing. Finally, if you want to remove the shift shaft and cam, there is a retainer on the shift shaft at the top of the housing forward of the water pump. Remove the retainer set screw and the assembly will pull straight out. The shift cam will then fall out of the lower unit. Be sure to remember which way the cam is installed and that it moves out with proper rotation of the shift shaft.

Hope this helps! See this link for an exploded view of the unit. http://www.marineengine.com/parts/mercu … omplete-17
http://www.marineengine.com/parts/mercu … omplete-16

Steve