i cant remember but if you said something about about burning out drill motor or knackering the gearbox but if it is to do with the motors i was thinking if you went for some 14.4v drill that had a 650 rpm output and over volted them to 18v you would only loose about 12 rpm and would put less strain on the motor

http://www.ebay.co.uk/itm/BRAND-NEW-GEN ... 4ced488dea

this is the same guy i get my 12v ones from and they seem better built than the last argos ones i got