Hi Simon I had the same thoughts but couldnt find any cheap metal gears this is why I used heng long gears with the help of Rob Fv 432 the gears in heng long gearboxes are the only cheap gears I could find even then I had to fill the sides with metal filler to stop the ringing then file it down then I had to machine the shafts to fit the bearings I bought couldnt find any cheap ones the size of the shafts I used 1/8 i/diax 3/8 o/dia these are for the gears the final drive bushes are 8mm i/dia x16mm o/dia this is the one through the hull the other side is 3/16x3/8 which means turnning the shaft to fit.Then you have to plan where the motors fit find material to build buy drills and reamers to cut the holes for bearings mark out where you want the holes drill ream fit bearings shim gears assemble run in the gears /motors speed 400 7,2 volt with the extra gearing of 3,5 to 1 and speed 400 motors you should have a gearbox that will pull anything have white cap motors on at the momment connected it to a battery put a cloth around drive sprocket and tried to stop it it nearly sprained my wrist have a look at photos .have changed the left side from original post to made it thinner to allow for bigger motor if i need one

I would start by close inspection of your gearboxes for side play you cant do much about the actual meshing but you can make sure you have full face contact I.E. not have half gear but full gear alinement then strip them down fill the gears with metal filler seal the bushes into postion with metal filler to stop them turnning around and assemble fitting shims as required to get best possible gear alinement this is about all you can do with these gearboxes this along with a close fitting cover should reduce the noise to a level that should be O.K. that is you will still here them but only just the sound of the tracks shouuld mask the gearboxes