TM 10-3930-653-14&P 4-7-9.  SPARK PLUGS 4-7-9-1.   DESCRIPTION The spark plugs are of the  resistor  type,  having  14 mm (0.55 in) threads and 0.7 to 0.8 mm (0.028 to 0.031 in)   gap.      Inspection   and   cleaning   should   be   made   in accordance with the periodic maintenance schedule. Note All     spark     plugs installed on an engine,  must  be  of  the  same  brand and heat range. 4-7-9-2.   INSPECTION (1) Remove  spark  plug  wire  by  pulling  on  boot,  not  on wire itself. (2) Remove spark plugs. (3) Check electrodes and inner and outer porcelains of plugs,   noting   the   type   of   deposits   and   the   degree   of electrode erosion.  See Fig. 7-69. Normal:     Brown     to     grayish-tan     deposits     and     slight electrode   wear   indicate   correct   spark   plug   heat range. Carbon fouled: Dry fluffy carbon deposits on the insulator and   electrode   are   mostly   caused   by   slow   speed driving,  weak  ignition,  too  rich  a  fuel  mixture,  dirty air cleaner, etc. It  is  advisable  to  replace  with  plugs  having  hotter heat range. Oil    fouled:    Wet    black    deposits    show    excessive    oil entrance   into   combustion   chamber   through   worn rings  and  pistons  or  excessive  clearance  between valve   guides   and   stems.      If   the   same   condition remains after repair, use a hotter plug. Overheating:  White  or  light  gray  insulator  with  black  or gray    brown    spots    and    bluish    burnt    electrodes indicate engine overheating. Moreover, the appearance  results  from  incorrect  ignition    timing, loose  spark  plugs,  low  fuel  pump  pressure,  wrong selection   of   fuel,   a   hotter   range   plug,   etc.      It   is advisable  to  replace  with  plugs  having  colder  heat range. Fig. 4-149.  Spark Plug 4-83