Original pacific, generations 1 and 2

 There is quite a difference in the first 2 generations. This is a common theme in Aristo where the original manufacture was in Korea, and then it changed to China (Sanda Kan, who was later purchased by Kader, who manufactures and owns Bachmann and all the old Aristo molds)

The Korean built Pacific had a lot of extras, and the first copy made in China was pretty close in design.

Motor Block:

The motor block pre-dates the current "prime mover" design. The wheels seem to be made of something else too, not as good as the current metal, so wheel cleaning was needed more often. Only the outer 2 drivers were powered, the center "gearbox" is a dummy/hollow. It is designed to swivel along the long axis of the loco, to keep more of the wheels on the track.

Drivers

The driven wheels are keyed to the axle ends that keeps them from slipping on the axles (wish this was true with the new locos)

There are 2 styles:

This one has the "double flat" type of axle that fits into a corresponding recess in the wheel:

The other has a pin in the axle, which engages slots:

Notice that the axles halves are pinned to a plastic tube in order to keep them electrically isolated

Wiring:

The picture below shows some of the electrical connections with the boiler removed. 

Below, the smoke box is removed. This loco has been modified by R.J. DeBerg to use the new style "prime mover" smoke unit.

Below, you can see the modification RJ made to the smokebox to accommodate the new smoke unit. Notice the tubes with springs in them to bring smoke down to the cylinders with the original smoke unit.