Figure 3-14.—Excessively band-ground valve. assemble and disassemble a valve assembly. Before you reassemble  a  valve  assembly,  be  sure  to  thoroughly clean  and  inspect  the  valve  spring.  (Use  kerosene  or diesel  fuel  for  cleaning.  NEVER  use  an  alkaline solution;  it  will  remove  the  protective  coating.)  The condition  of  the  surface  of  a  valve  spring  is  the  best indication of impending failure. (Use magnafluxing to help find cracks that would otherwise be invisible.) The free length of a valve spring should be within the  limits  specified  in  the  manufacturer’s  technical manual. If such information is not available, compare the length of a new spring with that of the used spring. If the length of the used spring is more than 3 percent shorter  than  that  of  the  new  spring,  replace  the  used spring  immediately.  Remember,  however,  that  loss  of spring tension will NOT always show up as a loss in overall  length.  Springs  may  be  the  proper  length,  but they   may   have   lost   enough   tension   to   warrant replacement. Figure 3-15.—Facing a valve on a lathe. Do   not   reinstall   springs   with   nicks,   cracks,   or surface  corrosion.  Replace  them.  To  minimize  corrosive conditions, use clean lube oil, eliminate water leaks, and keep vents open and clean WORN VALVE KEEPERS AND RETAINING WASHERS Worn  valve  keepers  and  retaining  washers  may result  if  valve  stem  caps  (used  in  some  engines)  are improperly  fitted  Caps  are  provided  to  protect  and increase  the  service  life  of  the  valve  stems.  Trouble occurs when the cap does not bear directly on the end of the stem, but bears instead on the valve stem lock or the spring  retaining  washer.  This  transmits  the  actuating force from the cap to the lock or the retaining washer, and then to the stem, causing excessive wear on the stem groove and the valve stem lock As a result, the retaining washer wiIl loosen and the valve stem may break An improper fit of a valve stem cap may be due to the  use  of  improper  parts  or  the  omission  of  spacer shims.  Steel  spacer  shims,  required  in  some  caps  to provide  proper  clearance,  are  placed  between  the  end  of the valve stem and the cap; leaving out the shims will cause the shoulder of the cap to come in contact with the lock.  When  you  disassemble  a  valve  assembly, determine whether or not shims are used. If shims are used, record their location and exact thickness. Valve caps must be of the proper size, or troubles similar to those resulting from shim omission will occur. Never attempt to use caps or any other valve assembly parts that are worn. BROKEN  VALVE  HEADS Broken  valve  heads  usually  cause  damage  to  the piston, liner, cylinder head, and other associated parts. This   damage   is   generally   repairable   only   by replacement of these parts. 3-11