Modifications to the Design of a Miniature Steam Locomotive
Notes on the Construction of a Miniature Steam Locomotive using commercially available parts
By Keith Appleton
My own small locomotive, a 7 1/4 inch gauge "Tich" from the Kennion Brothers drawings is modified to use small ball races on all the valve gear and little end of the connecting rods. The ball races that I used for the valve gear are 1/4 inch OD & 1/6 inch ID. (available from Spen Bearings, Cleckheaton, West Yorkshire UK). In the axle boxes, I used 2 ball races per box (also available from Spen Bearings) - a bit over the top, but I don't anticipate any wear in my lifetime!!
No wear is evident so far & it must have run well over 100 miles around my garden track - over a long period!
If you are about to build one, bore out the cylinders to one & a half inches - far better, much more power & the boiler supplies more that enough steam. I used "Sweetpea" cylinder & valve chest castings with"Simplex" slide valve castings which are all available from www.blackgates.co.uk
I recommend using 1/4 inch stainless steel for the valve rods, as the specified 3/16" stainless steel shown on the drawing is not strong enough & bends too easily as the rod is unsupported. I also used 5/16" stainless steel for the main piston rods. An observation: On the drawing, the reversing lever is the wrong way round, if you build it as shown, then pushing the lever forward makes the engine go backwards! Easily rectified by moving the position of the reach rod..
Most miniature steam locomotives are built & run almost in the same way as full size & so they wear out to scale ..... very quickly!..... even the amount of ash in the smokebox after a long run is also to scale - usually half full!