The biggest contributor to engine clatter is when the crankshaft isn't running true within tolerance...Runout of the mainshafts after a crank rebuild should be no more than .002"...
Even slightly larger runouts lead to the crank pinion running eccentrically. That in turn causes alternately too tight and too loose a clearance of the pinion and cam gear teeth and that amplifies the problem detailed in Rons post leading to a lot more clatter...
I've experimented with crank run out and all the other features and it must all be correct to result in an engine that runs mechanically quietly..Ian
And lets not forget cam wear & cam follower wear
BSA put "quietening ramps" on their cams to gently close the cams rather than have them slam shut as you do on race engines
Cams wear over time
