TM 1-1500-204-23-1 uniform.      Temperature      changes      affect      air-cooled engines to a greater extent than other engines, and there must be sufficient valve clearance to prevent valve from holding  open  when  extreme  cold  is  encountered,  since holding  open  will  make  cold  weather  starting  extremely difficult.  Inadequate clearance or excessive clearance is conducive  to  valve  failure;  therefore,  proper  settings  are essential to engine operation. (a) Measuring and adjusting valve clearance  When  rough  engine  operation,  backfiring,  loss of  compression,  hard  starting,  or  any  malfunction  which can  be  attributed  to  improper  operation  or  clearance  of valves  occurs,  valve  clearance  shall  be  measured  and adjusted, using the following procedures 1 Measure and adjust valve clearance in accordance with specific instructions contained In applicable engine maintenance manual. 2 Locate     top     dead     center     piston position. 3 Use  feeler  gauge  (shaped  so  that end  of  gauge  can  be  slipped  in  a  straight  line  between valve  and  rocker  arm  roller  or  adjusting  screw)  specified in tools section of maintenance manual. NOTE Do  not  use  excessive  force  to  insert gauge between valve stem and adjustment screw or rocker arm roller. 4 Adjust valve clearances found to be out   of   adjustment   according   to   specific   procedures   in engine maintenance manual. (b) Reclamation and disposition of sodium-filled   valves.      The   reclamation   procedure   for sodium-filled  valves  consists  primarily  of  rebuilding  the facing of exhaust valves and repairing the stems WARNING Do   not   attempt   to   mutilate   sodium- filled   valves.      The   metallic   sodium used in these valves, when contracting  moisture,  gives  off  highly flammable   hydrogen   gas,   and   when brought   into   contact   with   the   skin often results in severe personal injury. 1 Sodium-filled valves definitely determined to be nonrepairable as a result of cracked or broken    stems    shall    be    processed    for    disposition    In accordance    with    procedures    established    by    Part    3, Chapter XV, DOD directive 4160 21M.  Disposal of these nonrepairable   valves   shall   be   affected   by   one   of   the following methods. Burial, at least 30 inches below ground level, in a   select   location   where   future   excavation   is unlikely. By dumping at sea. 2 Values   determined   to   be   suitable for    reclamation    shall    be    prepared    for    shipment    in accordance with provisions of AR 746-1. (c) Establishing crankshaft position using top center indicator and protractor.  The proper crankshaft position  must  be  established  prior  to  checking  or  setting valves using the following procedures. 1 Determine ratio of top center indicator,   as   shown   in   figure   7-15,   pointer   travel   to crankshaft travel. 2 Install Indicator in either spark plug hole of number one cylinder. 3 Adjust  arm  of  indicator  so  that  full scale deflections will be obtained as piston moves through its full stroke. 4 Install protractor and pointer specified in engine maintenance manual on one of engine accessory drive pads. 5 Rotate crankshaft in normal direction    of    rotation    until    piston    is    coming    up    on compression stroke. Figure 7-15.  Top Center Indicator 7-15