Is there an optimum distance for how far the exhaust pipes go into the silencers on a 1956 swinging arm A7 mine go in approx 4 inches which seems quite a bit or does it not make any difference to performance.
With a normal setup I don't think it will make any difference. For information on optimal exhaust lengths, read Tuning for Speed. Only applies to unsilenced exhausts though. For the curious, a Velocette goes better with a fishtail silencer than without.