To disassemble the 773, I would you start by removing the front and rear trucks. When you do this there is a whole collection of springs, washers, bolts, pins, and keepers. These are the parts that you need to kept track of. There is a metal washer on the rear truck that should have been changed to a plastic washer. If this was not done, it should be done. If it was not done the spring there may have been annealed by short circuit current during a derailment and it is probably flat. If this is the case, it also needs to be replaced. Next is to open the door on the front of the boiler and unsnap the headlight bulb and socked from the clip. The boiler front goes with the shell and the headlight wire stays with the frame. You will need a cradle to safely support the engine when it is upside down. Put the locomotive in the cradle and remove the three screws that retain the frame to the shell. Actually the loco should have already been in the cradle when you took the front and rear trucks off. The frame should lift freely off the shell. As you lift the frame the two sets of steps at the front of the loco will fall of. Be expecting this. The only thing that holds them in place is being captured between the shell and frame. Also the smoke stack gasket could fall out at this point, but it is usually glued in place with old smoke pill residue.
Tie the loose head light bulb up someplace and you can run the loco with the shell and trucks off. Do not get carried away, it is much more likely to derail with all the weight removed. In reassembly, it is much easier with a couple of extra hands to hold those steps in place while the frame is being placed on the shell. Make sure none of the wires get pinched between the frame and the shell. Prior to tightening the screws, check to be sure the short flange on the top of the smoke unit is lined up with the smoke stack and the felt smoke stack gasket is in place. I have gone to gluing the gasket to the smoke unit with a couple of drops of Goo prior to reassembly. Only other thing to say is if you take the valve gear off the drivers, be sure to get them reinstalled properly. If the crank goes on backwards it could be broken first time you try to run the engine. After reassembly turn the wheels one revolution by hand to be sure everything works freely. This would be a good place for a few pictures. Also in normal operation it is a good idea to check those two crank screws to be sure they are tight.
Dennis Waldron at Just Trains, has written an excellent article on how to service the 773. It is online. Probably best way fine it is with Google.
