LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train

1.6. Valve Lapping

Valve Lapping
Because contemporary valve head and seat alloys, such as Stellite®, are so hard, it is very difficult to machine them. Instead, the valve parts are precisely cast and finished with some grinding, which means minimal machining is required. It also means that field servicing of valves is not practical apart from using some abrasive lapping compound to seat new valves in a cylinder head or to check valve seat-to-face contact. Lapping a valve involves applying some abrasive compound to the valve face and rapidly turning it back and forth in its seat using a stick with a suction cup attached to the valve head Figure 35. After the valve is lapped and cleaned, pencil marks or a purpose-made machinist dye is applied to the valve face. The valve is reinstalled, and the contact pattern is checked to verify that a uniform contact line is centered in the valve face Figure 36.

FIGURE 09-35: To help seat a valve, abrasive lapping compound is used in the face contact area while the valve is rapidly twisted back and forth using a lapping stick.

FIGURE 09-36: To check the width and uniformity of the valve face-to-seat contact, vertical pencil markings are made around the circumference of the valve face. The valve is placed back in the seat and turned several times before removing and rechecking the contact pattern.