You don't need to spend the extra $ for the racing bells. My Sav X came with the 15t Heavy Duty bell and an extra 18t HD bell. The 18t is the largest # of tooth bell you can install. The 15 t makes the Volcano just fly off the line! If that .18 had more guts and the diffs could handle it, I think it could pull the front wheels! ( well almost!) The aluminum exhaust also makes a big difference in low end power. I cut out the baffle in the old stock exhaust and tried it. Way lounder then the alum and no low end power. Top end showed an increase. I put the Alum back on and there is a remarkable difference. Also the HPI clutchbells are a direct fit. No modifications are needed.
Here's the part #'s for the heavy duty bells they may be cheaper but work just great:
a989-----14t
a990-----15t
a991-----16t
a992-----17t
a993-----18t
I never tried a 14t but there should be enough adjustment for it to work. I do know for sure the 15t thru 18t WILL work on the single speed Volcano STX.
Yes I checked and they are cheaper. To search Tower or Hobbytown just add HPI to the beginning of each # in the search window( HPIA991).