Reviving a Minitrains Porter

Some time ago I started a Narrow Planet kit for a little Bagnall tank using a Minitrains Porter 0-6-0 chassis. The Porter body had to be removed, which involved cutting part of it away from around the motor as I recall, but after having done so the chassis didn't run smoothly. I then noticed the motor looked wonky, and on closer inspection I found one of the retaining screws had a stripped thread and didn't tighten. After some frustration I put the kit away to think about what I could do about it.

Having made some progress with the Kerr Stuart Haig chassis I figured I'd pull this kit out to see if I could revive the chassis too. The first discovery was that the two screws that hold the top plate of the chassis down are the same M1.4 thread as those that hold the motor, although very slightly shorter and countersink head. However, one could be carefully screwed into the motor in place of the stripped screw, the motor thread is damaged too but not so bad that the new screw couldn't bite. This is made far more difficult by wheels being in the way preventing a screwdriver being lined up straight. The old screw went into the top plate of the chassis where it is not under strain. 

The motor still looked wonky, and the chassis still ran badly, jerking and clunking. With the screws backed off slightly the worm slipped. I tried adding packing (10 thou plastic) under the front of the motor tilt it back slightly, but then it wouldn't run at all. I then rotated the motor 180 degrees, which seemed to result in less of a lean - I've no idea why. The chassis was still jerky and clunky. 

With the motor removed it span freely under power - but the chassis didn't roll smoothly, with a definite clunk once per revolution. Gently, the rear crank-pins were prised out to free the con-rods, but the clunk remained. Finally I had to accept I would have to do what one should never do with these chassis... open it up!

Tiny screws, tiny gears, even tinier washers. Scary. Anyway, on close inspection I discovered one of the gears (on the middle axle) has a damaged tooth. These teeth are tiny so it is very hard to see, but just visible in the picture below - look for the unequal gap between teeth. 

I have no idea how this can have happened, but the real question is how to fix it. I can't fix the gear, but I could swap the axles so the damaged gear is at the front, and remove the idler gear, leaving the just rear and middle axles powered. Of course, getting the two axles in place with the idler gear and their crank pins aligned was rather tricky, but I suppose the same challenge with 3 axles and 2 idlers is even harder. Fortunately it all went back together in the end, and the chassis ran smoothly as a 2-4-0. Encouraged, I then re-attached the coupling rods and connecting rods, first omitting the middle axle crank pins to drive the front axle from the rear, the middle axle appeared to rotate with it's crank pin holes in line so I added the middle pins too. Somewhere along the way one of the tiny washers from behind the coupling rods pinged off the tweezers into space, but it seems to manage just fine. 

Yes, amazingly after all that work, the chassis actually works, and appears to run as smoothly and sweetly as you'd expect a Minitrains to, despite it now lacking an idler gear and relying on the coupling rods to drive the front axle. 

You see from the video the body is placed on. Before giving up on the chassis I'd spend some time smoothing the visible parts of the 3D print and given it a coat of primer. I added another coat, but found the lightweight body blew away from the spray can! It needs weight anyway to balance the chassis and aid pick-up and traction, so I added some lead. The "Swiss rolls" are slid into the boiler, and the flat pieces into the small side tanks, wiped with a little contact adhesive to hold them in place. As you see from the video this is enough to aid good running. 

Chassis problems fixed, I can now continue the build of this dinky little loco.