For a current project, I need a tiny, but very high-ratio motor and gearbox, so modifying a servo to rotate continuously seems to be the perfect answer. The entire unit takes up less than 20x20x8mm and provides a motor, speed controller (I'm using it with R/C) and an approx. 120:1 ratio gearbox, while costing less than a fiver. But it does need some slightly fiddly modification first. I'm listing the process here, as much to aid my own memory next time I do this...

Small but hopefully powerful...

After levering off the base, you can see a tiny (4mm diameter) motor (the white circle on the right) and the circuit board. If you move the circuit board to one side, you find the potentiometer hiding underneath. This is what tells the servo what position it is in, so it can adjust correctly. All we need to do to make it rotate continuously is remove this feedback capability. To do this...

Lever out the potentiometer so it doesn't have the connecting shaft attached (it has a shaft connecting the pot to the output gear which you can see in the cover to the left). File down or drill out the potentiometer shaft hole (the little white bit sticking out of the small black box in the photo) so the shaft cannot turn the potentiometer. This has removed the servo's ability to sense where it is - there is a better way of doing this for bigger servos by replacing the potentiometer with a two high resistance, low tolerance resistors, which will give you more stability, but I don't have room to do that in this tiny servo body.

Flip the servo over and lever off the top cover to reveal the gearbox - be very careful with this or you will have gears everywhere and will have to spend 10,000 years trying to work out what order they went in. I speak from experience...

The output gear (the one sticking out the top with a spline on it) will have a tab on the bottom to limit the range it can turn. Cut/file this off. Before you put the output gear back, measure the spline diameter - about 3mm is fairly common.

To add a gear to the spline instead of one of the 'horns' that came with it (no idea why they're called that) drill out a gear to slightly less than the spline diameter - drilling this gear to 3mm (by drilling to 2.5mm, then 3mm to reduce the risk of decentreing the hole) results in a snug fit. I used a crown gear as the servo has to sit flat under the chassis so needs a right angle before reaching the wheels.

Re-assemble the gearbox and cover, then test - you should have a continuous rotation servo! If it rotates when the control stick is in the centre position, adjust the potentiometer with a small screwdriver until it stays still. If you want, give it a blob of superglue to make sure.

The potentiometer can then tuck back into the body as it is not physically connected to the output gear any more, making the installation nice and compact. Job done!