[Franks 72]

Anyone ever had a rocker arm come off the valve? This motor was rebuild from the ground up and ran fine all last season. It has (twice ) slipped the rocker arm off the valve. Rocker nut had not backed off and preload was correctly set (hydraulic lifters). What should I look for ? Replace the lifter? Camshaft issues? Bent valve stem? Any ideas would be greatly appreciated, Im out of them. Note ,the heads are not original GM by iron "swirl" type. Thankyou