For starters, you should always swap the plastic gears out! They'll break before long and the resulting plastic teeth mess will clog up the rest of the gearbox (we discovered this at our first event) and more or less render it hopeless until completely stripped of all grease. I believe the cheapy drills from Argos are currently entirely metal gears, and we got lucky at Homebase a while ago but having bought another more recently they have returned to plastic first stage. If you don't happen to have enough metal gears to swap them, Gimson sell them, you want the 18T ones if you're staying 36:1.

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For the conversion of a standard drill you need:
A 15T carrier plate - http://www.gimsonrobotics.co.uk/spare-g ... _GR02.html

And a set of 15T gears per drill - http://www.gimsonrobotics.co.uk/spare-p ... _GR02.html

You may also want to make life easier on yourself and just get a new motor with the 15T pinion gear already attached so that you don't have to attempt pressing your own on if you don't have the kit to do so. This is what we did, and it saved a lot of headaches.

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Loosen the grubscrews in the front of the gearbox a little bit so that they don't force the whole gear setup upwards. Remove the motor and open up the gearbox of the old drill. Take the first stage of gears out, they'll be 18T so they're spares for the second stage should you need them, though if they're plastic they'll be eaten by the torque in the second stage in no time.

Take out the existing carrier plate. This plate only works for 36:1 setups, so keep it as a spare for your other unchanged gearboxes.

Put in the new 15T carrier plate from Gimson. It will take a bit of fiddling to seat it. I'd suggest putting some fresh grease on the underside, though not too much as on this small scale too much can cause resistance and run your motor harder than necessary.

Place the new 15T Gimson gears on the 3 pins of the new carrier plate, again this takes a bit of fiddling to seat them, and a bit of grease on this side is good as they're completely fresh parts.

If you have a motor with the new 15T pinion (they come with 9T pinions) on, either bought that way from Gimson or pressed on (if pressed bring the front edge of it flush to the end of the motor shaft, as per Gimson's images), grab it now. Move the plastic end plate that joins the gearbox and motor onto the new motor/put it back on the original, making sure to tighten the two screws, as they're often loose from the manufacturer and can loosen in combat.

At this stage you need the metal plate that came with the original drill that goes between the innards of the gearbox (sits over the new 15T gears) and the plastic plate that holds the motor onto the gearbox. They're like a big thin washer. Often the hole in the centre is only big enough to pass a 9T pinion. I don't know where you can purchase ones with a larger enough hole to pass the new 15T pinion. We have several of both types, as these cheap drills seem to come with both kinds. Ask around if you don't have the right thing, or perhaps someone could make up a load for our use. If you have a decent drill you could drill it out fairly easily.

Place the big washer whatsit over the gears, and then position your new motor into the gearbox. You have to line up the little nob that is on the plastic casing of the gearbox with the hole/notch in the plastic plate holding the motor, and of course seat the motor pinion between the gears in the end of the gearbox at the same time. Bit of grease here is good, not a lot, though, none if you don't think it needs it. Getting the motor back on and in can be fiddly but if you have done it right turning everything around a little bit as you put it in should seat it.

Bolt up the gearbox again and tighten the grubscrews once more. I'd suggest running it on a lower voltage battery than the motor, at least initially, to run the gearbox and motor in. If there are any major vibrations or audible clunks as the gearbox goes round, open her up and see if you can see any issues, like debris. A bit of vibration is to be expected from these, especially from the bearings, they're fairly nasty. Leave it running in both directions for 5-10 minutes, this smoothes out the new gearbox and can drop your current draw hugely, and if nothing sounds or feels wrong, you've converted your 36:1 drill to a 24:1 drill.

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If I have missed anything let me know. I will happily make a video of this, the mess from doing this myself is still on the table!