Since the 80s the Japanese have been putting at least one counterbalancer on all their thumpers, from 200 up to 800 ccs of displacement, and in some cases the larger sizes even get 2 counterbalancers. The Europeans do likewise these days, too.
Mass market machines must only meet a certain bare minimum of tolerances to pass quality control, therefore counterbalancer assemblies make sense since each machine's engine is not going to be individually balanced to the minutest degree possible, as though it were being assembled by a master craftsman taking great pains for the utmost in precision. Pre-1980s thumpers were of course most characterized by their boneshaking vibration, and the appearance of counterbalancing measures was inevitable. Imagine for instance the tooth shattering vibes that would accompany the DR650-800 single cylinder engines if they had not been counterbalance equipped.
But by all accounts Tom Lyons hot rod Fireball thumpers are individually balanced so well people are perplexed to outright shocked at the smoothness. And clearly no RE Bullet has a counterbalancer or rubber engine mounts or other vibration mitigation in place, so I'm left wondering just how big can a thumper get without needing a counterbalancer if a master engine builder such as Tom is the fellow putting it together?
700cc? 800cc?