Hardened keepers are recommended for all high performance engines with larger camshafts to eliminate broken keepers. The factory stock keepers are made from powdered iron, compressed to shape.
Berg keepers are made from solid bar stock, machined on the ID and OD, then split to provide a gap, so the ends do not touch, just like Chevy and other domestic keepers. No more burred up valves from the keepers moving up and down all the time. This proper method of manufacturing eliminates stem wear at the keeper grooves on your valves.