I took a look at specs on both machines and I was mistaken. the br550 is actually 64.9cc and the more expensive br600 is 64.8cc not that that little difference would matter much. both are rated at 4.0 hp, i think i'm running a 24x10 on mine but it has only been run on a test stand without a tach. soon to be mounted on a 30% EDGE

sorry, but i think the shortblock is only avail for 4mix trimmers(34cc), probably not all that feasible anyway.