kysi u know about the diff cup mod dont yer? run a search and u will suss it out. as for the .18, i have had a few probs with my rally, but i have been drivin it like a hooligan when ive had probs, smooth throttle control helps no end. the larger diameter wheels on the mt, i would imagine, would create a greater strain on the drivetrain. hi grip surfaces have resulted in the engine twisting the engine mount out of the chassis - get some nuts on the ends of the mount screws for the plate.

its only a case of reinforcing ur diff cups, and keeping an eye on the twisting of ur driveshafts. u will need to be a little more careful with the gear mesh - check it at 90, 180 and 270 degrees etc, and average it out if there is a variation.

dont panic peeps - a 21 will work in there, but the more power u put thru it - the more failures u are goin to get. it was only designed to take cens .16 remember.