This post is a record of how I motorised my AEI class 81 by Atlas Editions.
My project started with a topic on the RMWeb, where I asked about possible chassis. From a reply here I bought a Hornby class 67 as the donor model - the bogie wheelbase of a 4mm scale class 67 is spot-on for a 1:87 scale class 81.
I used only the bogies, motor and cardan shafts from the Hornby donor. It would have cost about the same to buy these as spare parts, and really the only reason to buy a complete model was to check the feasibility of the project. It would be much easier to sell on a complete model if I decided to give up.
I built a new chassis to hold the motor and the two bogies. I began with a piece of styrene, cut to be a snug fit inside the bottom of the body. I cut a rectangular hole and added a panel to hold the motor, this lets the two drive shafts sit fairly level:
I should mention, I trimmed the drive shafts to length before this photo. This is easier than it looks - the drive shafts are in two telescopic parts and are made from plastic, so the task reduces to attaching the parts to their respective universal joints, putting the bogies and motor into the chassis, and seeing where to trim the drive shafts. Then cut to length with a razor saw.
This chassis performed well on test - well enough to let me commit to the build. But if I was doing this project again, I would move the bogies about 2mm closer together, that is 1mm inwards at each end. This would make sure they cannot foul the insides of the buffer beams on model curves.
The body is almost unchanged for this project. I cut off the two cylindrical bosses (not in the photos but obvious when you dismantle a model) and then added two strips of styrene inside to hold the whole moulding level on the chassis. The strips are inside the body by a distance equal to the thickness of the chassis:
The next stage was to swap the bogie sides from Hornby for the ones from Atlas Editions. The Hornby sides are part of the moulding holding the wheels in place. I took one of these off and wished I hadn't when I came to put it back; it is easier to do the cutting with the sides in position. I left the solid plastic cylinders to provide somewhere to attach the new sideframes.
It is important to glue the new side on the right way round. Everything is black and looks fairly nondescript, but of course the cab steps need to be on the outer ends of the bogies so they end up under the cab doors. It is good how solvent sets slowly.
There is an option with the positioning of the new sideframes. If you want to keep the Hornby sideplay (as I did), the frames need to go on about 0.5 mm outside their prototypical locations. This will make sure the wheels do not rub on the frames. Alternatively, you could add some shim washers to remove the sideplay and the fix the frames in their 'scale' locations.
Here are the two methods I used to make up the extra millimetre. The bogie on the left has the cross-piece re-set 0.5 mm inboard at each end - this keeps the guard irons in line with the rails but the corners are a bit weak. The bogie on the right has the cross-piece cut and a scrap of 1mm styrene glued in, reinforced with styrene on the inside. This is stronger but the guard irons are now a little too far apart:
Then I returned to the chassis and added the buffer beams from the Atlas model. I cut these to give me a tab to glue them onto the chassis, built up the chassis to set them at the correct height and glued them on with superglue:
One buffer beam will have a dummy 'scale' coupler and the other will have a Kadee. I fixed the Kadee with a M2 screw right through the buffer beam casting and the chassis, so hopefully it is strong enough to pull a train:
The sides are angled inwards to add a little strength and to make sure the body will still go on if I add any reinforcement outside them at their top edges.
The motor gets equal and opposite torque reactions from the two bogies, so it only needs fixing in place to hold it if I turn the model upside down. I put a couple of tiny dabs of super glue to hold the motor, so I can prise it out if a repair is ever needed.
The underframe details went on next. I pulled these off the Atlas model and fixed them on with Carr's Butanone. For some reason, Plastic Weld would not hold them. When you fix on the underframe parts, you have to decide which end of the model will be number 1 end and number 2 end. For a modified class 81 like this with air tanks on the roof, the air tanks are at the number 1 end (pantograph at number 2 end).
I was a bit ambitious here. I had to pull some of these details off to let the model run round a model curve.
The buffer heads received some paint too.
I have left the body loose on the chassis for the time being, it is quite a snug fit. I want to stop now and enjoy the model on the layout.
The engine runs really well, but the clearances between the bogies and the insides of the buffer beam castings are very tight on any curve. I wish I had put the bogies a little closer together, and I will do this if I ever make another one. I have ordered another Atlas model from China so I can build an un-powered track-ready model. Then I can assemble a rake of mark 2 coaches and try for a double-headed Anglo-Scottish express.
With thanks also to 'CloggyDog', 'Enterprisingwestern' and 'BernardTPM' for their help and encouragement through this project.