Abstract: In a petroleum coke operation, a pilot hole is drilled in the coke by high pressure water emitting from downwardly directed nozzles in a drilling assembly. A full hole is eroded in the coke by diverting the high pressure water to a series of full hole nozzles in response to shutting off, and then resupplying, high pressure water to the drilling assembly. This conveniently converts pilot hole operations to full hole operations, which is the normal mode of cutting coke from a drum. Thus, the modes of operation are switched each time water is resupplied to the drilling assembly.