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I have six green books. They all say 10 degrees intially & the maximim total varies from 30 to 36 degrees depending on which engine.
The engines my green books cover are B20A, B, E & F & B30A, E & F.
I'd love to know what publication says 21-24 initial, I doubt it's anything from Volvo.
I attribute the valve damage to insufficient clearance (0.018") which reduced seat pressure on hot valves due to thermal expansion of the stems. 0.020" valve lash seems to have eliminated that problem.
The difference beetween .018" & .020" isn't that great. Were the valve stellite faced or plain? Fuel & what you are doing with the engine are more likely factors for valve seat wear. Any B20 valves will last indeffineately on unleaded fuel driven casually around town, however, hit the highway or drive up some mountains & their life is drastically reduced without leaded petrol.
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