The valve head has a ground face that forms a seal against  the ground valve seat in the cylinder head when the valve is  closed. The face of the valve is usually ground to an angle  of either 30° or 45°. In some engines, the intake-valve face  is ground to an angle of 30°, and the exhaust-valve face is  ground to a 45° angle. Valve faces are often made more  durable by the application of a material called stellite. About  1 ⁄ 16 inch of this alloy is welded to the valve face and ground  to the correct angle. Stellite is resistant to high-temperature  corrosion and also withstands the shock and wear associated  with valve operation. Some engine manufacturers use a  nichrome facing on the valves. This serves the same purpose  as the stellite material. The valve stem acts as a pilot for the valve head and rides in  the valve guide installed in the cylinder head for this purpose.  [Figure 1-21]  The valve stem is surface hardened to resist  wear. The neck is the part that forms the junction between the  head and the stem. The tip of the valve is hardened to withstand  the hammering of the valve rocker arm as it opens the valve. A  machined groove on the stem near the tip receives the split-ring  stem keys. These stem keys form a lock ring to hold the valve  spring retaining washer in place.  [Figure 1-22] valve may be reduced as much as 300° to 400 °F. Under  no circumstances should a sodium-filled valve be cut open  or subjected to treatment which may cause it to rupture.  Exposure of the sodium in these valves to the outside air  results in fire or explosion with possible personal injury. The most commonly used intake valves have solid stems,  and the head is either flat or tulip shaped. Intake valves for  low-power engines are usually flat headed. In some engines,  the intake valve may be the tulip type and have a smaller stem  than the exhaust valve or it may be similar to the exhaust  valve but have a solid stem and head. Although these valves  are similar, they are not interchangeable since the faces of the  valves are constructed of different material. The intake valve  usually has a flat milled on the tip to identify it. Some intake and exhaust valve stems are hollow and  partially filled with metallic sodium. This material is used  because it is an excellent heat conductor. The sodium melts  at approximately 208 °F and the reciprocating motion of  the valve circulates the liquid sodium, allowing it to carry  away heat from the valve head to the valve stem where it  is dissipated through the valve guide to the cylinder head  and the cooling fins. Thus, the operating temperature of the  For a reciprocating engine to operate properly, each valve  must open at the proper time, stay open for the required  length of time, and close at the proper time. Intake valves  are opened just before the piston reaches top dead center,  and exhaust valves remain open after top dead center. At  a particular instant, therefore, both valves are open at the  same time (end of the exhaust stroke and beginning of the  intake stroke). This valve overlap permits better volumetric  efficiency and lowers the cylinder operating temperature.  This timing of the valves is controlled by the valve-operating  mechanism and is referred to as the valve timing. The valve lift (distance that the valve is lifted off its seat) and  the valve duration (length of time the valve is held open) are  both determined by the shape of the cam lobes. Typical cam  lobes are illustrated in  Figure 1-23 . The portion of the lobe  that gently starts the valve operating mechanism moving is  called a ramp, or step. The ramp is machined on each side of  1-18 Figure 1-21. View of valve guide installed on a cylinder head. Figure 1-22. Stem keys forming a lock ring to hold valve spring  retaining washers in place.  Valve Operating Mechanism steel. Exhaust valves are usually made of nichrome,  silchrome, or cobalt-chromium steel because these materials  are much more heat resistant.