HPI Racing Roto Start System Savage .21. For .21 and .25 HPI engines. he HPI Roto Start System for the Savage 21 replaces the pullstarter with a durable metal gear backplate and a handheld shaft driven starter. Starting is a simple as pressing a button. Requires a 6-cell, 7.2v Stick battery pack and a charger. Complete installation instructions are also included.