They're plastic. The rear wheels get a little heat in them, softening the hexes, and when you get them hot enough, they melt, stripping them, and the wheels. HPI makes an aluminum set for the MT2 that fits. you get four, so if you ever do break one, you have spares. This is what I did even before I stripped any hexes, since I've seen it happen so many times before - and i didn't ruin any rims that way. I'll brb with the part number.