The older style Athearn locomotives (blue box kits) used a metal clip between the power
trucks and the motor. On top of this, the unit also has a headlight mounted to a stud
that is attached to the frame. It has a small brass wiper that makes contact to the metal clip between the motor and the trucks. If the headlight mount is bent down to much, it will produce sparks if it shorts out. If you did nothing in the way of removing the
truck from the frame while replacing the truck sideframe, then that's not what's causing the sparks that you are seeing. Also check to see if the motor brushes are clean and
not gunked with lubrication.
trucks and the motor. On top of this, the unit also has a headlight mounted to a stud
that is attached to the frame. It has a small brass wiper that makes contact to the metal clip between the motor and the trucks. If the headlight mount is bent down to much, it will produce sparks if it shorts out. If you did nothing in the way of removing the
truck from the frame while replacing the truck sideframe, then that's not what's causing the sparks that you are seeing. Also check to see if the motor brushes are clean and
not gunked with lubrication.