The D3 is unusual in many ways. It's unusual for Wilesco in being a simple no-frills engine, with no artificial brickwork, no ladders, plain brass instead of nickel. The main exception to this is the rather superfluous chimney cap which is not only pointless but looks silly. Also, being plastic, I'm sure it wouldn't last long if kept in place while the burner was operating. Fortunately, it is just a clip on thing that is easily clipped off. The overall dimensions are just 190mm high (including the cap) with a 125mm x 158mm base.
There are also a couple of unusual design features, not just for Wilesco. The piston rod and the piston itself are all one continuous brass rod, all the same diameter (well almost). This looks more like a hydraulic piston than a steam one. The oscillating cylinder pivot point is at the end instead of the more usual mid point. The crank-shaft has a real crank in it; unusual in simple toy steam engines.
The price has been kept down in more ways than simplicity. Some parts have been made from recycled materials. This certainly applies to the tinplate parts and maybe others as well. If you look inside the firebox, it can be easily seen that the metal originally came from an Agfa film container! The base also seems to have a similar origin.
The boiler is a vertical type with a single central flue. It's dimensions are 45mm diameter x approx 45mm height. It looks taller than that because the firebox extends up beyond the square section to form what, in a real boiler, might be called a "combustion chamber". The firebox contains the mounting for the standard Wilesco fuel tablet burner which is adjustable to provide a degree of control of the flame. The boiler itself has a pair of (rather small) sight glasses to indicate minimum and maximum water level.
The single action oscillating cylinder is 6mm diam x 16mm stroke. The running characteristics are a little disappointing for Wilesco; possibly due to the unusual cylinder / piston design. Like many small single cylinder engines, it has a tendency to walk across the table when running at full speed. The instruction leaflet includes a recommendation to screw the base down onto something solid - a wise recommendation.
For all it's faults, it is a generally innovative design and, given it's low price tag, it is good value for money.