The  principal  cause  of  burned  exhaust  valves  is small particles of carbon that lodge between the valve head  and  the  valve  seat.  These  particles  come  from incomplete  combustion  of  the  fuel  or  oil  left  by  the piston rings in the cylinder. The particles hold the valve open   just   enough   to   prevent   the   valve   head   from touching the valve seat. The valve is cooled by several means, including its contact with the valve seat. When carbon  particles  prevent  contact,  the  heat  normally transferred from the valve head to the seat remains in the valve head. The valve seat seldom burns because the water   jackets   surrounding   the   seat   usually   provide enough   cooling   to   keep   its   temperature   below   a dangerous  point. When  cleaning  carbon  from  cylinder  heads,  remove all  loose  particles  from  the  crevices;  be  extremely careful that you do not nick or scratch the valve or seat. Removing the valves from the engine will make it easier to clean the passages and remove the carbon deposits from the underside of the valve heads. Check the tappet clearance adjustments at frequent intervals  to  be  certain  they  are  correct  and  that  the locking  devices  are  secure.  The  adjustment  of  valve clearances is discussed later in this chapter. Most engines are equipped with valve seat inserts made  of  hard,  heat-resisting,  alloy  steel.  Occasionally,  a seat will crack and allow the hot gases to leak, burning both the insert and the valve. Sometimes a poor contact between   the   valve   seat   insert   and   the   counterbore prevents the heat from being conducted away, and the high temperatures deform the insert. When this occurs, both the seat and the valve will burn; the seat insert must be  rep1aced. LOOSE VALVE SEATS You  can  avoid  causing  loose  valve  seats  only  by installing  them  properly.  Clean  the  counterbore thoroughly to remove all carbon before shrinking in an insert. Chill the valve seat with dry ice and place the cylinder  head  in  boiling  water  for  approximately  30 minutes; then drive the insert into the counterbore with a valve insert installing tool, as illustrated in figure 3-10. Never   strike   a   valve   seat   directly.   Do   the   driving operation  quickly,  before  the  insert  reaches  the temperature of the cylinder head When replacing a damaged valve with a new one, inspect the valve guides for excessive wear. If the valve moves from side to side as it seats, replace the guides. Figure 3-10.–Driving a valve iusert into the cylinder head counterbore. PITTING If the valve seat is secured firmly in the counterbore and is free of cracks and burns, you may remove slight damage  such  as  pitting  by  hand  grinding  (fig.  3-11). Generally, you will use prussian blue to check the valve and valve seat, but if this is not available, use any thin dark oil-based paint. Allow the valve to seat by dropping it on the valve seat from a short distance. If the surfaces fail to make complete contact, regrinding is necessary. 75.72 Figure 3-11.–Hand grinding a valve and valve seat. 3-9