The Knight type of sleeve valve enjoyed a long period of popularity. The principle underlying the sleeve valve is the same as the one for the poppet type of valve. Instead of a cam and a poppet valve and valve lifter, as discussed in the foregoing, the Knight valves are sleeves. The sleeves surround the pistons, and are operated by means of small connecting rods and an eccentric shaft. Although considered obsolete in America, this design has some foreign adherents.
The poppet valve is well known to practically everyone. It is made in a variety of shapes and sizes. Some are of one piece and some are of two pieces. The two-piece valves usually have a noncorrosive steel head welded to a wear-resisting steel stem. Single-piece valves of tungsten or other special alloys are used. Some valves are so hard that they cannot be filed. Heating in the engine does not draw this temper. Other valves are soft. Valves are provided with some means of holding their springs in position. In rare cases, the valve stems are threaded, and a nut, type N, is used.
Valve seat inserts
Owing to the fact that the softer metal of the cast-iron head or cylinder blocks is prone to pitting in conditions of high-speed hard service, manufacturers introduced the practice of fitting valve-seat rings of special nonburn, non-wear characteristics in the block or head for valve seats. In factory practice there are several methods of installing these inserts. One of the most interesting is the use of "dry ice" to chill the insert ring to a temperature many degrees below zero which contracts the metal enough to allow of easy installation, and then the return of the metal to normal temperature causes it to expand to a very tight fit in the surrounding metal.
The guide is fitted into the cylinder block just above the camshaft. It is locked tight so that it may not move. The lifter is fitted into it, and is a free-sliding fit in the lifter guide. Oil from the splash of the engine keeps it lubricated. The complete assembly of lifter and adjusting screw in the lifter guide appears at D.