Lionel used some hex head drive screws. Usually the way to tell the difference is if the head is recessed a little it is a drive screw. If the head is flush it is a threaded screw. On the 682 I believe that the hex head fastener that holds the lubricator linkage to the shell is a drive screw. If you put a wrench on it and try to turn it it will break off. Leave the linkage attached to the shell and take it loose at the wheels.
From the description you gave of your smoke problem, I would guess one of two things. As others have said, you need to check that the air pump is working and getting air into the smoke unit to blow the smoke out. Could be a mechanical problem that is not letting the smoke piston go up and down or it could be the air passages are plugged up. Second problem is if the smoke unit heater has been changed, the reproduction elements have the resistance too high. Resistance should be in the 11 to 15 ohm range. It it is 20 ohms or above the resistance is too high.
I woukd recommend against changing the smoke unit to the liquid style heater. The pill type unit uses liquid just fine. The liquid type will not work with pills.
