mechanical parts. As usual the hole in the boiler wasn't big enough to allow the motor and gearbox in. Kit designers have to take a guess how much of a gap is required and this varies enormously. Some people will use a tiny motor that barely pokes above the frames at the cost of power. Others will stick in a two-stage 'box that pushes the motor well into the boiler. To make it worse, kits are often intended to be produced in several scales widening the possibilities.
The combination I am using should give plenty of power (this is a working model after all) and a sensible speed range. But it's too big for the supplied hole.
Making space is a horrible task involving a mini drill fitted with a cutting disk to cut across the boiler. Then making the cuts parallel to the sides with anything sharp I could lay my hands on including the slitting disk, scissors and side cutters. The result needs tidying up a bit but at least the body now fits.