The Pilot's Lounge #27: Radial Engines and Hydraulic Lock
Round engines are different than flat piston engines because, well, they're round. In addition, one of their characteristics is for oil to flow into a cylinder after shutdown. Unless the oil's presence is discovered and the cylinder drained before the engine is rotated, catastrophic damage can occur. AVweb's Rick Durden explains what hydraulic lock is, why it happens and what you can do about it.