I've had this unit for a good four years and have been working on it on and off for most of that time.
IT IS DRIVING ME NUTS! 
The diagonal drive has not functioned properly for more then 30 seconds.
A history of repairs
1. re-quarter and remount the wheels - wheels loosened again
2. replace frame/wheels - due to a broken ladder apron this needed to be done anyway. Wouldn't run with boiler on.
3. At this point I attempted to remove the armature and damaged the shaft bearings.
4. A seller at the Renningers flea market in Kutztown, PA suggests rebuilding the bearing ball assemblies (I have another 671 motor frame and brushplate).
5. I buy the works (armature, balls, washers) from Olsen's. Upon the arrival of the parts, I find that not only did I not order enough balls (I ordered 20), but the armature shaft was slightly too short to allow the E-clip to slip on to the notch (
).
Questions:
1. Has anyone else had any experience with postwar Lionel diagonal drives?
2. Are there any hobby shops in the greater Philadelphia area that can work on Lionel postwar engines?
(I'm just north of Coatesville on the old Reading line)
I've put so much money into this thing I don't want to give up on it.
Thanks,
Don't give up now, you can do it. Get everything smoothed out with out the motor. (side rods) and then add the motor.
