TM 1-1500-204-23-1 plug gap In each cylinder.  Magneto operation is timed to the engine so that a spark occurs only when the piston is on the proper stroke at a specified number of crankshaft degrees    before    the    top-dead-center    piston    position. Maintenance  of  magnetos  consists  primarily  of  replacing defective  magnetos  or  components  as  explained  in  the following procedures: (a) Prior  to  installation,  perform  internal timing check. 1 Check breaker point adjustments, breaker point dwell on master cam lobe, and distributor alignment check. 2 Use   reference   marks   on   breaker cam, distributor rotor, and magneto housing incorporated on most assemblies to perform internal timing check. (b) Time    high-tension    Ignition    system magneto-to-engine,  at  a  definite  crankshaft  position  as follows. 1 Position    crankshaft    to    timing    or spark advance position using a piston position indicator 2 Align   internal   timing   marks,   and slide magneto into place. 3 Shift   magneto   to   retard   position, and turn magneto to advance until breaker points begin to open as indicated by timing light. 4 Install mounting nuts, magneto cover, and leads. (c) Check  timing  of  high-tension  ignition system  magneto  after  a  new  magneto  is  Installed  or  if incorrect timing is suspected, as follows. 1 Determine correct crankshaft position and check internal magneto timing marks. 2 Turn  crankshaft  one-fourth  turn  in opposite direction of normal rotation. 3 If magneto has an impulse coupling,   rotate   crankshaft   in   normal   direction   beyond timing   cylinder   top-dead-center   position;   when   impulse coupling  clicks,  rotate  crankshaft  in  opposite  direction  to approximately 5 degrees advance of timing position. 4 Connect timing light and turn ignition switches to ON position. 5 Apply   even   and   equal   torque   to each of cam retaining screws In direction opposite normal rotation to remove gear train backlash. 6 While   holding   backlash   out,   turn crankshaft   until   points   begin   to   open   as   indicated   by timing light. 7 If   necessary,   adjust   magneto   so that timing light indicates points are beginning to open at proper firing position. (d) Install   and   time   low-tension   Ignition system     magneto-to-engine,     at     a     definite     crankshaft position, using the following procedures. 1 Depress    magneto    timing    plunger and   turn   magneto   shaft   until   plunger   engages   fully   In notch of magneto shaft. 2 Position   magneto   In   place   while holding plunger. 3 If   magneto   does   not   line   up   with centerline  of  engine,  remove  magneto,  release  plunger, and  turn  magneto  drive  shaft  90  degrees.    Repeat  until magneto lines up properly. 4 Hold   plunger   depressed   and   turn magneto    counterclockwise    on    its    flange    as    far    as necessary to remove gear backlash. 5 Install   magneto   bolts   securely   in place. (e) Check  timing    of  low-tension  ignition system   magneto   after   a   new   magneto   is   installed   or incorrect timing is suspected. 1 Determine correct crankshaft position. 2 Rotate   engine   one-fourth   turn   in opposite  direction  of  normal  rotation  and  slide  pointer  of piston position Indicator to top of scale. 3 Turn  engine  in  direction  of  rotation until slide pointer is starting to move down. 7-18