HPI pinions are crap. Buy some of RCMikes hardened pinions.
Your truck is ballistic!!
EDIT: I see your are having some real pinions probs, sorry to hear that. RCM has been holding up great here (Flux XL, RC8Te and Hyper Mini STe). I'm moving to an MBX6Te soon (when Mike will ship the conversion kit that I ordered last week) so hopefully my pinions will hold on that new rig!
Mugen MBX6Te RCM + Mugen MBX6e RCM + Savage Flux XL FLM + MERV all with Spectrum DX3S || WTS&WTB