TM 1-1500-204-23-1 6 Set calibrated scale until zero mark on scale and scribe on pointer are aligned. 7 Move pointer back to top of slot or until it contacts pivot arm. 8 Turn crankshaft in direction opposite  normal  rotation  until  pivot  arm  returns  to  top  of slot. 9 Recheck   zero   mark   of   calibrated scale against reference mark on slide pointer. 10 Move  slide  pointer  to  top  of  slot  or until it contacts pivot arm. 11 Rotate   crankshaft   In   direction   of rotation.  Movement of slide pointer by the pivot arm will Indicate  crankshaft  position  In  relation  to  true  top  dead center on the calibrated scale. (e) Alternate   methods   of   locating   leaky intake   exhaust   valves.     Alternate   methods   of   locating leaky intake and exhaust valves are listed In the following paragraphs. 1 Engine  run  test.    Prior  to  stopping engine, run it up to 1000 rpm and move mixture control to IDLE  CUT-OFF.    The  engine  will  turn  over  six  or  more revolutions    after    combustion    ceases.        A    very    slight burning   of   an   exhaust   valve   will   be   evidenced   by   a pronounced  slush  from  the  affected  cylinder  This  sound occurs each second revolution of the engine. 2 Wheeze  test    Perform  wheeze  test for  locating  leaky  intake  and  exhaust  valves  by  pulling piston to top dead center, locate faulty valve by listening for    noise    in    exhaust    collector    or    Intake    duct    which indicates air leaking past respective valve. 3 Compressed air test Admit compressed   air   into   cylinder   through   spark   plug   hole Restrain piston at top of dead center Locate leaking valve by listening for noise in exhaust collector and intake duct. (f) Built-m timing marks.    Some  engines have   reference   timing   marks   built   into   the   engine,   as shown    in    figure    7-17.        Engines    without    a    propeller reduction gear will have the timing mark, a scribe mark or a    pin,    on    the    propeller    shaft    and    a    corresponding reference mark on the surface of the nose section next to the  shaft.    Refer  to  the  applicable  maintenance  manual for location of such marks. NOTE When  using  timing  marks  to  position the   crankshaft,   sight   straight   across the  stationary  pointer  or  mark  on  the nose  section  to  the  other  mark.    Any angle  incurred  in  sighting  will  cause error when positioning the crankshaft. h. Accessories.        Accessory    units    such    as    fuel pumps, vacuum pumps, oil pumps, generators, alternators, magnetos, starters, filters, superchargers, and turbochargers  are  attached  to  the  engine.    Refer  to  the applicable maintenance manual for specific maintenance procedures i. Ignition System.  The ignition system must deliver a  high-tension  spark  to  each  cylinder  of  the  engine  in firing order at a predetermined number of degrees ahead of  true  top  dead  center.    Voltage  output  of  the  system must be such that the spark will jump the spark plug gap under    all    operating    conditions    with    no    tendency    for electrical  leaks.    Timing  and  replacement  of  parts  make up most of the Ignition system maintenance. (1)  Magneto  system.    The  magneto,  a  special  type  of engine-driven  ac  generator,  uses  a  permanent  magnetic source.    The  magneto  develops  the  high  voltage  which forces a spark to jump across the spark Figure 7-17.  Built-In Timing Marks 7-17