Hot answers tagged set-modification
If there is no need for the bricks to belong to specific sets (as in, it's not important to be able to build some official models from them) then I'd say the best way is to look for ebay bulk brick sales. For around 100$ you can get several pounds or around a thousand semi-random pieces. Try searching for "lego pieces".
You can switch 4 pairs of gears on each end of the crawler (2 pairs in the portal hubs and another 2 pairs inside the gearbox infront of the motor) do this on both ends or you run the risk of broken gears or premature motor wear. I ran 12 tooth gears all the way from the motor to my hubs for a dramatic increase in speed however the torque suffered severely, ...
I suggest using google to search for them. When I did that I found two threads on EuroBricks discussing this - here and here. The first thread links to a youtube video that has downloadable instructions. Note that the instructions are 19MB and the website is fairly slow.
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