Manley SBC 1.6 Severe Duty Exhaust Valves (Set of 8)Proven Fatigue Tests: The Severe Duty? Valves have undergone rigorous fatigue tests. In normal operation, valves experience 20,000 to 25,000 lbs of applied stress. When valves bounce on the seat, this stress soars to 40,000 to 60,000 lbs. Manley exhaust valves, tested at 40,000 psi and 1400? F, impressively endured up to 100,000,000 (one hundred million) cycles. In contrast, competitors' offerings failed after just 100,000 cycles, showcasing a