cylinder, and an expander screw is inserted in the pilot from  the top of the guide.  [Figure 10-17]  The pilot must be tight  in the guide, because any movement can cause a poor grind.  The fluid hose is inserted through one of the spark plug inserts. The three grades of stones available for use are classified as  rough, finishing, and polishing stones. The rough stone is  designed to true and clean the seat. The finishing stone must  follow the rough to remove grinding marks and produce a  smooth finish. The polishing stone does just as the name  implies and is used only where a highly polished seat is desired. In the actual grinding job, considerable skill is required  in handling the grinding gun. The gun must be centered  accurately on the stone holder. If the gun is tilted off- center, chattering of the stone results, and a rough grind is  produced. It is very important that the stone be rotated at a  speed that permits grinding instead of rubbing. This speed is  approximately 8,000 to 10,000 revolutions per minute (rpm).  Excessive pressure on the stone can slow it down. It is not a  good technique to let the stone grind at slow speed by putting  pressure on the stone when starting or stopping the gun. The  maximum pressure used on the stone at any time should be  no more than that exerted by the weight of the gun. The stones are installed on special stone holders. The face  of the stone is trued by a diamond dresser. The stone should  be refaced whenever it is grooved or loaded, and when the  stone is first installed on the stone holder. The diamond  dresser also may be used to cut down the diameter of the  stone. Dressing of the stone should be kept to a minimum  as a matter of conservation; therefore, it is desirable to have  sufficient stone holders for all the stones to be used on the job.  Another practice, conducive to good grinding, is to ease off  on the stone every second or so to let the coolant wash away  the chips on the seat. This rhythmic grinding action also  helps keep the stone up to its correct speed. Since it is quite  a job to replace a seat, remove as little material as possible  during the grinding. Inspect the job frequently to prevent  unnecessary grinding. The rough stone is used until the seat is true to the valve  guide and until all pits, scores, or burned areas are removed.  [Figure 10-18]  After refacing, the seat should be smooth  and true. The finishing stone is used only until the seat has a  smooth, polished appearance. Extreme caution should be used  when grinding with the finishing stone to prevent chattering. The size and trueness of the seat can be checked by several  methods. Runout of the seat is checked with a special dial  indicator and should not exceed 0.002 inch. The size of the  seat may be determined by using Prussian blue. Prussian  blue is used to check for contact transfer from one surface  to the other. To check the fit of the seat, spread a thin coat  of Prussian blue evenly on the seat. Press the valve onto the  seat. The blue transferred to the valve indicates the contact  surface. The contact surface should be one-third to two-thirds  the width of the valve face and in the middle of the face. In  some cases, a-go and no-go gauge is used in place of the valve  Pilot Expander screw Stone Grinding Fluid hose Stone holder Spacing washers Seat out of alignment with guide Needs further rough grinding Excessive pitting True and clean ready for finish grind 10-14 Figure 10-17.  Valve seat grinding setup.  Figure 10-18.  Valve seat grinding. 