The pipes stopped breaking, but the new springs broke instead. Observation of engines screeching on the dyno revealed that, at certain revs, those retaining springs (two per pipe) vibrated like bowstrings. The fix for that one was easy: Just slip a short piece of rubber hose over the spring to act as a damper. The bowstringing was gone, but the springs now broke in a new way, at their ends, where they were formed into hooks. One end of the spring hooked into a tab welded to the exhaust pipe and the other into a hole drilled through a nearby cooling fin.