Careful planning will enable you to fit the best possible motor/gearbox combination, so you get optimum performance with minimum impact on the appearance of the loco. You can also take into account your own individual requirements, preferences and factors that are specific to your particular model.

With so many options and variations on offer, the prospect of working out your own gearbox solution may seem daunting, but it’s actually a lot easier than many people imagine. With a basic knowledge of the components, you simply follow our guidlines as you work through the procedure in a logical and organised manner - there’s no technical theory or complex calculations involved.

​Making a Start

For accurate planning, a decent line drawing of your loco is a big advantage, but it must be to scale. You can check this by taking a sample, known dimension from the prototype (the bigger the better) e.g. length over the bufferbeams, and compare this with the same dimension on the drawing. A loco measuring 30ft 6ins from beam to beam should be 30.5 x4(mm) so this dimension on the drawing should be 122mm. If you don’t have a drawing you can still use the Gearbox Profiles sheet (see below) by laying it over the actual model, although this is not as accurate. If your kit is unbuilt, arrange the chassis frames on paper and draw in lines to represent key features of the structure, such as the boiler, firebox etc.

The Gearbox Profile Overlay

Click here to view the Gearbox Profiles which you can print onto a transparent A4 sheet. Make sure you select to print at ‘full size’ and check the scale at the bottom of the print-out is 500mm long. Do the same for the Motor Profiles sheet.

If you’ve read through this planning guide and studied the gearbox and motor specifications, you may already have some idea of what you need. Lay the Gearbox Profile transparency over the side view of the loco and try the images for size - this should indicate roughly where the front face of a particular gearbox will be - and then you can overlay a motor profile. If things aren't working out, try a different motor, or gearbox, or both. Remember, whatever combination you're considering, it's important to match gear ratio with motor RPM, taking into account the wheel size. Our Speed Calculator will do the calculations for you.

If your side elevation's looking promising, project the motor centreline from the gearbox across to the front elevation of the loco. Now lay the end view of the gearbox over this view of the engine - don’t overlook this stage; the inside of a round-topped firebox may catch the top corners of the gearbox, whereas the Belpaire version won’t, even though they both look the same size in side elevation!

If the motor has to fit into a confined space, it may be easier to select the motor first, then find a gearbox of suitable ratio to span between this and the driven axle. Which axle you choose to drive can depend on a lot of things, for example, do you intend to compensate the loco and, if so, will it be fully sprung or have a rigid axle which allows more room for the gearbox? Frame width is also a consideration, as some of our gearboxes may not fit between the frames of some OO chassis.

Considerations and Modifications

Take the kit’s material thickness into account, allowing 0.4mm inside the loco outline for etched kits, and about 1.2mm for whitemetal. It’s a good idea to draw this material onto the drawing, along with other castings and spacers. Study your model’s instructions and assembly sequence. Look out for things like chassis spacers, crosswires, fixing screws, cab details, or any other obstructions that might get in the way of the drivetrain, or features you particularly want to preserve, or are prepared to sacrifice in favour of better performance, should it come to that. . .

Some kit manufacturers give little thought to how the models will be motorised, so you may consider making modifications to the kit itself, which is best done before it’s assembled. A simple alteration can be effective, like moving a frame spacer to make room for a bigger motor or higher ratio box. On small prototypes, we sometimes suggest making saddle tanks removable to allow the motor to be fitted into the boiler space, like in our own kits. This kind of forward-thinking approach, combined with the number of motor/gearbox options we offer, allows you to make the drivetrain an integral part of the overall design.

Remember also that, in a lot of instances, the motor/gearbox unit must be able to pass up into the body/boiler space as you offer up the chassis, unless there is special provision for the motor to be uncoupled from the gearbox (e.g. removable boiler or saddletank or backhead). Allow some clearance around the motor/gearbox and consider the possibility that the drawing (or kit) may not be 100% accurate. In other words, don’t cut it too fine unless you have to and, if you do, have the burr-grinder handy!

Some Key Points

MicroMisers use a 2mm axle with push-fit gear only so, for bigger diameters, you’ll need to bush the wheel bores and frames to suit this. All other boxes are supplied with a grubscrew-fixed final drive gear to suit axle diamater of either 1/8in, 2mm or 3mm.


1/8in is by far the most common size for UK driving wheel axles and note that, to the naked eye, the difference between 3mm and 1/8in (3.125mm) is almost imperceivable, so double-check before ordering. . .

For articulated boxes, you can position the axle anywhere on the inner radius shown on the diagrams. The larger, outer arc shows how a DriveStretcher can increase the throw of the box, and states which one to use (D1 or D2). All boxes, (apart from the LoLoader) include etches so they can be driven them remotely, with the motor mounted away from the gearbox. You’ll need to source the bearings, shafts and other bits and pieces for this yourself.

The higher the gear ratio the faster the motor turns for every rotation of the wheels, giving smoother performance, particularly at shunting speeds, but with a lower top speed.


Large motors run more smoothly at low revs and have a higher power ouput, so a lower ratio box (e.g. 34:1) can be used to give a decent top speed with good low-end performance.

For smaller engines, space limitations may dictate a smaller motor which has less power and revs higher, but you can still achieve excellent slow running by using one of our compact, high-ratio gearboxes and these combinations are surprisingly powerful.

Corelsss motors have considerably more power and higher torque than the equivalent-sized iron core, so a relatively small motor and a lower ratio will still produce high tractive effort and smooth performance.

Wheel diameter is also a major factor, so it's wise to use our downloadable Speed Calculator to double-check the motor/gearbox combo will give you the road-speed you require.

The Final Choice

In many cases, you could end up with a number of options, involving different combinations of motor size, gearbox type/ratio and driven axle location, and there’ll be even more if you’re prepared to modify the loco in favour of a better drive system. At this stage, it’s up to you to make a decision based on your own preferences, weighing up the trade-offs and benefits for each combination.