I usually spend 4 Sat. nights a month operating at different model RRs in the area. I generally take the "yard job" and can spend 4 hours happily making up and breaking down outgoing and incoming trains.
One of my favorite engines is an old (late 50s) die cast English Yardbird (PRR A-5 0-4-0) that I picked up many years ago. Eventually the front (geared) drive axle slot in the frame became so worn that the engine wouldn't run any more. A while back I picked up another one of these that showed little evidence of wear with the intention of using it's frame to replace my worn out one.
Last week I started the rebuild. I slipped the "good" frame out of it's engine and the "bad" frame out of it's engine and then put the "good" frame in the engine I was trying to save. Before re-installing the motor I checked the wheels with their attached siderods and valve gear to make they turned freely. The wheels went about 1/2 turn and locked up solid.
Further examination showed that the frame castings were slightly different with the axle slots on the "good" frame being about 1/32 closer to the cylinder location than the "bad" one. This caused the valve gear to hit the backs of the cylinders. I figured I could fix that by elongating the slot in the valve gear hanger and moving it aft so I separated the valve gear assembly from the wheels which exposed the siderods.
Well, the holes in the siderods where the eccentrics pass through to the wheels were elongated too (which may have been part of the original problem). So I went to replace the siderods from the worn engine with the ones from the "good" engine. Turns out those castings are different too but I proceeded on. This model was only produced for about ten years so I was surprised to find so many differences. When I went to remove the screws that hold the siderods to the front wheelset one came out very hard. Turns out it wasn't a shoulder screw but a regular screw that had been screwed in to the right height and then staked (hit with a punch) from the back to hold it in place. So I took the other screw (a shoulder screw) and went searching in my junk boxes for a mate. After two + hours of digging I found one. Problem solved.
Wrong! The new siderods are thicker so both shoulder screws are too short. More searching turns up one screw with a slightly longer shoulder so I install it as a check before continuing the search. The crossheads slide back and forth right next to the front wheelset and the longer screw sticks out just enough to interfere with them on their journey. So now I have to thin down the "new" siderods. If I thin down the bosses on the front of the siderods I'll wipe out the cast detail so I have to thin down the whole siderod from the rear.
During this test I find out that the wheels on the worn engine have small bosses on the back, the wheels on the "parts" engine don't. This is because the frame on the "parts" engine is slightly wider than the one on the "worn" engine. About 15 minutes work with a file cures this problem but wipes out the fresh paint I applied to the replacement frame early on in this project.
I'm now about 8 hours into a one hour project and both engines (one of which was running at the onset of this madness) are in pieces all over the kitchen table along with several boxes of assorted parts, junk and leftover screws.