Failed exhaust valve when guide worn allowing valve to cock at rest. This prevented it from sealing and letting hot gases by heat stressing the valve, guide, seat, and bosses. The heat was enough to coke the exhaust valve springs. Caught during annual on compression test. 18/80. After staking valve, we got 20/80. Air could be heard in exhaust. 2013.52 Total Time this cylinder, installed in 1 first run 1976 Cheetah. This is the first engine work other than the pepperbox conversion on the carb since the plane left the Grumman factory in 1977.