Never really had problems with the Brass Bushings, even at ~10'000rpm, i always had problems with the Bottom End Needle bearing on those cranks, the 2/3 Speed Sachs (pre 504/505) 25kmh Models seem to have the same problem, but there is a guy Racing here with a 30kmh Sach 503 and a Brass Bushing Crank doing i'd guess about 11-12'000rpm. (edited)