I have a 621 Jersey Central NW 2 switcher that the gearing jams. I found that the commutator rides up when the direction is changed and jams. I have always done my own repairs and never have run into this. From what I have read it seems this condition is caused by the black piece of plastic at the bottom of the motor being loose. I have also read the repair is shimming this piece so it isn’t loose. First is this the cause of the problem and second how do you find the correct position for the shim/shims?
thanks
Gary Burgin