Currently I plan to use 18mm plastic tube for the boiler, which has a 1mm wall thickness - this is oversized by about 2" (or 0.6mm in scale) - so any motors etc. should be less than 16mm wide.
Unfortunately I don't have the budget to buy RTR chassis to test. My issue with rewheeling one is that the wheels are by far the most expensive part of the model, so if you buy an RTR chassis on top of that it's going to get very expensive. For example, my R1 chassis (fully motorised) without wheels costs about £20, the wheels are another £25 extra and there are far fewer of them on an R class than an F class!
That said, if someone does have a chassis they want to try, I would be happy to modify the loco body to fit. Maybe there's something to be said for a mechanism designed by a professional over one designed by me! But I am a mechanical engineer(ing student) so I shouldn't be too awful...
You are quite right. It would be more expensive. Peter K produce (well, advertise) a GER No.1 Class, but just the etches. I worked out what it would cost to buy the etches, source all the castings and the motor, gears and wheels etc. It came out at about the same as buying the etches, the castings, the leading and tender wheels and a RTR chassis that I could adapt. Here, though, I would have been able to retain the RTR driving wheels, as these were the correct diameter.
Once you add replacement coupled wheels, you are likely to make things more expensive than assembling all the components necessary for a completely kit-built alternative.
With wheels, axles, crank pins, I find £30 can easily be added to the cost of a 6-wheeled loco. I estimate that to re-wheel the Radial chassis with Romfords would cost in excess of £25, certainly if including etched balance weights etc.
Then, of course, you still have the bogie and tender wheels - the Radial saves you nothing here - though you'd need to buy these anyhow.
The point is that 3D printed chassis in WSF/BSF, motor, mount, gear box, and all wheels/axles/crank pins/rods/bearings etc, is likely to be cheaper than RTR chassis plus all wheels/axles/crank pins/bearings etc. The only thing you'd save on would be the rods, and your bits box would love you for the conversion, of course.
You could certainly motorise your kit yourself for less. Some might, however, prefer to use a proprietary chassis as an easier alternative, and, perhaps, a more reliable one (albeit one no longer under warranty!).
EDIT: For clarity, I mean more reliable than some people may feel they could build, and I do not intend to cast aspersions on your design for motorising. Indeed, it may be that you can plan a relatively easy and user-friendly means to build this.
I would welcome this, as I'm likely to want more than one, and cost is a factor.
Edited by Edwardian, 15 June 2018 - 09:44 .