The Wagner replacement cylinder F51088 uses an F6858 bleeder screw, 3/8"-24. Assuming they make the cylinder identical to the original GM 5455122 cylinder, that would be the size.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars