@bob2 posted:"Quarter" is your problem. All drivers must have crankpins at the same angle - usually 90 degrees or thereabouts.
Pull the rods off, set one side at top or bottom dead center, and make sure the other side is in a very straight line - in line with the axle centerlines. A loose driver can be fixed at this point with Loctite and your special screwdriver.
To hold the drivers at top/bottom dead center, you can drill an aluminum plate with tight-fitting holes - spacing must be exactly the same as axle spacing. If the rods are tight (unlikely) they can be used.
Then the rods must be re-fitted after the Loctite sets. You fit them at front and back dead center. You will see interference if there is any - the rod must be adjusted so there is no resistance. Use a magnifying glass, and pins without caps. You may have to make test pins.
Thanks Bob; sounds like this is what I'll have to do. A couple questions:
1. can you explain what you mean by "axle centerlines"?
2. "...front and back dead center"?
3. What do you mean by "pins", and what are these used for?
Please forgive the elementary questions, but I've never had to quarter a steam locomotive before...believe it or not! 
Appreciate the assistance...
Mark in Oregon
