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. I could have bought these as spare parts for much the same outlay, and really the only reason to buy a complete model was to check the feasibility of the project. It would be 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 sheet, cut to be a snug fit inside the bottom of the body. I cut a rectangular hole and added a panel underneath to hold the motor, this lets the two drive shafts sit fairly level:
I trimmed the drive shafts to length before taking this photo. The drive shafts are in two telescopic parts and are made from plastic, so shortening them is straightforward. The method is to attach the parts to their respective universal joints, put the bogies and motor into the chassis, and look where to trim the drive shafts. Then cut to length with a razor saw.
This chassis ran 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 2 mm closer together, that is 1 mm inwards at each end. This would make sure they cannot foul the insides of the buffer beams on sharp curves.
The body shell 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 added two long strips of styrene to hold the whole moulding level on its new chassis. The strips are recessed 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 mouldings 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 on the bogie to provide somewhere to attach the new sideframes.
It is important to glue the new sideframes on the right way round. Everything is black and looks fairly nondescript, but 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.
I used two different methods to make up the extra millimetre. The bogie on the left has the cross-piece between the sideframes re-set 0.5 mm inboard at each end. This keeps the guard irons in line with the rails but the corners of the assembly are a bit fragile. 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:
With the bogies complete 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, then built up the chassis to set them at the correct height and glued them on with cyanoacrylate:
One buffer beam will have a dummy 'scale' coupler and the other will have a Kadee. I attached 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:
These sides are angled inwards 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 fixings sufficiently strong to hold it if I turn the model upside down. I put a couple of tiny dabs of cyanoacrylate 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, and the pantograph is 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 the radius 2 curves on my layout.
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 every curve. I wish I had put the bogies a little closer together, and I will do this if I ever make another one.
With thanks also to 'CloggyDog', 'Enterprisingwestern' and 'BernardTPM' for their help and encouragement through this project.