I'm rebuilding a 671 turbine engine. All has gone well but when I replaced the side rods they lock up. Anyone have an idea what might be happening or what I may have done wrong. HELP
I assume you're talking about a Lionel model of the PRR S2 class direct drive turbine loco.
Do you have your drivers properly quartered? Try putting a single siderod on, then rotating the drivers slowly. All of the crankpins on the rod-free side should rotate in sync - all at the same angle. If they don't, you will have to readjust the driver on the mis-quartered axle.
If the drivers were not quartered exactly 90 degrees apart, turning one driver set end for end will cause the lockup you mentioned.
For additional info, I would suggest placing this on the Classic Toy Trains forum. That gets a lot more traffic and attention from knowledgeable people.
Chuck