i dont know on that one. but assume it would need modding. the SH .21 comes in the 1/8 scale redcats and they have a totally differnt chassis. the sh.18 is a direct bolt in as the tornado(that has the .18) and the shockwave have basically the same chassis. jsut different motor/center trans and radio.

unless you are looking to do only speed runs, i dont see why you would want the .21. it would totally over power the shockwave. most likely just end up with lots of broken parts. i have seen and aftershock 3.5 run(.21 motor) and it was shocking the acceleration. stick with the .18 if you want to have a buggy that wont tear itself apart.

the SH motors are also alittle better quality IMO. so even tho the .18 isnt much bigger, it will be much faster.