Valve Reconditioning One of the most common jobs during engine overhaul is  grinding the valves. The equipment used should preferably  be a wet valve grinder. With this type of machine, a mixture  of soluble oil and water is used to keep the valve cool and  carry away the grinding chips. Like many machine jobs, valve grinding is mostly a matter  of setting up the machine. The following points should be  checked or accomplished before starting a grind. True the  stone by means of a diamond nib. The machine is turned on,  and the diamond is drawn across the stone, cutting just deep  enough to true and clean the stone. Determine the face angle  of the valve being ground and set the movable head of the  machine to correspond to this valve angle. Usually, valves  are ground to the standard angles of 30° or 45°. However, in  some instances, an interference fit of 0.5° or 1.5° less than  the standard angle may be ground on the valve face. The interference fit is used to obtain a more  positive seal by means of a narrow contact surface.  [Figure 10-23]  Theoretically, there is a line contact between  the valve and seat. With this line contact, the load that the  valve exerts against the seat is concentrated in a very small  area, thereby increasing the unit load at any one spot. The  interference fit is especially beneficial during the first few  hours of operation after an overhaul. The positive seal reduces  the possibility of a burned valve or seat that a leaking valve  might produce. After the first few hours of running, these  angles tend to pound down and become identical. Notice that the interference angle is ground into the valve, not  the seat. It is easier to change the angle of the valve grinder  work head than to change the angle of a valve seat grinder  stone. Do not use an interference fit unless the manufacturer  approves it. Install the valve into the chuck and adjust the chuck so that  the valve face is approximately 2 inches from the chuck.  [Figure 10-24]  If the valve is chucked any further out,  there is danger of excessive wobble and also a possibility of  grinding into the stem. There are various types of valve grinding machines. In one  type, the stone is moved across the valve face; in another, the  valve is moved across the stone. Whichever type is used, the  following procedures are typical of those performed when  refacing a valve. Check the travel of the valve face across the stone. The valve  should completely pass the stone on both sides, yet not travel  far enough to grind the stem. There are stops on the machine  that can be set to control this travel. With the valve set correctly in place, turn on the machine and  the grinding fluid so that it splashes on the valve face. Back  the grinding wheel off all the way. Place the valve directly  in front of the stone.  [Figure 10-25]  Slowly bring the wheel  forward until a light cut is made on the valve. The intensity  of the grind is measured by sound more than anything else.  Slowly draw the valve back and forth across the stone without  increasing the cut. Move the work head table back and forth  using the full face of the stone, but always keep the valve  face on the stone. When the sound of the grind diminishes,  indicating that some valve material has been removed, move  the workhead table to the extreme left to stop rotation of the  valve. Inspect the valve to determine if further grinding is  necessary. If another cut must be made, bring the valve in  front of the stone, then advance the stone out to the valve.  Do not increase the cut without having the valve directly in  front of the stone. An important precaution in valve grinding, as in any kind  of grinding, is to make light cuts only. Heavy cuts cause  chattering, that may make the valve surface so rough that  much time is lost in obtaining the desired finish. After grinding, check the valve margin to be sure that the  valve edge has not been ground too thin. A thin edge is called  a feather edge and can lead to preignition; the valve edge  would burn away in a short period of time, and the cylinder  would have to be overhauled again.  Figure 10-26  shows a  valve with a normal margin and one with a feather edge. The valve tip may be resurfaced on the valve grinder. The  tip is ground to remove cupping or wear, and also to adjust  valve clearances on some engines. The valve is held by a clamp on the side of the stone.  [Figure 10-27]  With the machine and grinding fluid turned on,  the valve is pushed lightly against the stone and swung back  Line contact 45° 44° Figure 10-23.  Interference fit of valve and valve seat.  10-16