With high spring pressures and psycho rollers a few. That's why I epoxy magnets in the heads and block as a safety measure. I guess it depends on how hard you push your equipment.
My spring pressure neededs shouldn't be to high, using 280ish & .525ish with 1.6, beehive springs, titianum retainers, ford under cut hollow stem 1.85-1.55 valves.
