I've managed to find time recently to sit down and modify another Hornby 040 chassis by including a second stage of gearing to improve the slow running capabilities. Hopefully I can now find the time post a 'How to ...' guide.
Just to make sure there is no confusion here is a picture of the model in question, and a picture after the modifications have been made. I recommend using the chassis with cylinders, as they should help retain strength. The connecting rods are irrelevant.
Here are some drawings.
Since the article was written for the Railway Modeller, I have improved and simplified the design.
My remit was to improve the slow running characteristics of these models whilst using the basic Hornby controller. Although not relevant here, this design should still fit within the confines of the Class 06 Diesel and the Saddletank Loco (after some modifications to the running plate), and require the minimum of expenditure.
You will need:
Styrene sheet (plasticard) of thickness 1mm (40 thou) and 0.25mm (10 thou)
Styrene cement (I like Revell Contacta Liquid) and superglue or contact adhesive
Junior Hacksaw Blade
Bent nose pliers
Drills (1.5, 2.0, 2.5, 3.0)
2.0mm top hat bearings
Gearwheels and Worm (Squires GS0220, GS0200 (optional), GS0305)
Biro tube (empty) to fit over 2mm rod
Small screw similar to that fitted to chassis
Soldering iron and cored solder
Start by removing any weights fitted, then the motor mount and motor. Discard the motor mount. Using the bent nose pliers carefully lever the worm from the shaft. Clean all parts.
Using the smallest drill make holes in the side plates behind the cylinders and then enlarge up to 3mm diameter.
Cut down to the holes with the side cutters and trim with a knife to form two slots for the gear wheel.
Now drill a 1.5mm hole in the base of the chassis. Located between the axle and the cylinders inside the chassis moulding you should find a circular moulding mark. Drill the hole in the centre of this circle. This will be used to secure the bearing support.
The spur gear is obtained by modifying the double gear wheel. Push the gear part way onto the shaft of the motor. Temporarily apply power to the motor while lightly holding the saw blade against the gear so that it cuts a slot and separates the large gear wheel. Remove any burrs from the edge of the newly made spur gear with a knife.
[Edited 9/8/14 10.55 for the following addition]
This is where to drill the hole to secure the bearing support.