118,292. Turner, J. A., and Harms, A. May 17, 1917. Cylinders, relief w a l v e s for.-An attachment for admitting air only when the throttle is closed consists of a ring e interposed between the plug shoulder and the engine and having an annular passage f with an opening i communicating with the inside of the plug. The a i r enters through a projecting portion, the passage through which may be restricted by a screw adjustment k and is closed by a ball j sufficiently heavy to be lifted only when the engine is running with the throttle closed. Instead of using this attachment a series of intersecting passages, similarly restricted and controlled, may be drilled in the plug body.