These tiny geared motors are usually referred to as N20 motors. The gearbox measures 10mm x 12mm so it fits nicely inside a part 11a double bracket. The mounting bolts are 1.6mm and are so tiny it's extremely difficult to drill the holes and get the bolts in without them hitting the gears. There is another way to mount these motors however. The brass bush for the 3mm shaft is 4mm in diameter meaning it fits neatly into the Meccano hole. This locates the motor squarely and snugly into the double bracket so you can simply glue it in place with super glue if you didn't want to fiddle around with microscopic bolts. Of course you'd need to be careful not to get any glue in the works but if you kept the motor pointing downwards, then dropped the glue onto the bracket and lowered the motor onto it, you should be ok. I'm trying to standardise all my electrical connections to use these 2.54mm header pins. They're what's used in breadboards, Arduino, Raspberry Pi, Picaxe and all sorts of electronic projects. As a general rule you put the header sockets on whatever is supplying the power, and the header pins on whatever is receiving the power. Stan Baker sells these motors with a 4mm shaft but I've only been able to find the 3mm shaft motors on ebay. It's fairly easy to put a 4mm sleeve on the D shaft. Once again you could use glue, but I soldered it allowing the solder to flow into the gap created by the D shape of the shaft. Make sure the brass tube has a 3mm inside diameter and 4mm outside diameter.