short out the spark plugs one at a time. The noise will  be  greatly  reduced  when  the  piston  in  the cylinder   that   is   responsible   is   not   delivering power. Piston-Pin  Knock Piston-pin  knock  is  identified  more  as  a metallic  double-knock  rather  than  a  regular clicking  sound  like  that  heard  in  valve  and  tappet noise.  In  addition,  it  is  most  noticeable  during  idle with  the  spark  advanced.  A  check  can  be  made by idling the engine with the spark advanced and then shorting out the spark plugs. Piston-pin noise coming from a cylinder will be reduced somewhat when the spark plug for that cylinder is shorted out.  Causes  of  this  noise  are  a  worn  or  loose piston-pin,  a  worn  bushing,  and  a  lack  of  oil. Piston-Ring  Noise Piston-ring noise is also similar to valve and tappet  noise  since  it  is  identified  by  a  clicking, snapping,  or  rattling  sound.  This  noise  is  most noticeable  on  acceleration.  Low-ring  tension, broken or worn rings, or worn cylinder walls will produce  this  sound.  To  avoid  confusing  this  sound with other engine noise, make the following test: remove the spark plugs and add an ounce or two of  heavy  engine  oil  to  each  cylinder.  Crank  the engine  for  several  revolutions  to  work  the  oil down past the rings. Replace the spark plugs and start the engine, If the noise has decreased, it is probable that the rings are at fault. Piston Slap Piston slap may be detected by a hollow, bell- like  knock  and  is  due  to  the  rocking  back  and forth  of  the  piston  in  the  cylinder.  If  the  slap occurs only when the engine is cold, it is probably not  serious.  However,  if  it  occurs  under  all operating  conditions,  a  further  examination  is called  for.  The  slap  can  be  caused  by  worn cylinder   walls,   worn   pistons,   collapsed   piston skirts,  or  misaligned  connecting  rods. Crankshaft Knock Crankshaft  knock  is  a  heavy,  dull,  metallic knock  that  is  noticeable  when  the  engine  is  under load  or  accelerating.  When  the  noise  is  regular, it  can  be  contributed  to  worn  main  bearings. When irregular and sharp, the noise is probably due  to  worn  thrust  bearings. ENGINE  TESTING In most shops, the Navy provides accurate and dependable  testing  equipment.  But  having  the testing equipment in the shop is NOT enough. The supervisor  and  the  crew  must  know  how  to  use this  equipment  since  proper  use  provides  the quickest and surest means of finding out what is wrong  and  where  the  fault  lies. Four of the most widely used testing instru- ments are the cylinder compression tester, vacuum gauge,  cylinder  leakage  tester,  and  tachometer. Compression Test As  you  have  learned,  engine  power  results from igniting a combustible mixture that has been compressed  in  the  combustion  chamber  of  an engine cylinder. The tighter a given volume of fuel mixture  is  squeezed  in  the  cylinder  before  it  is ignited, the greater the power developed. Unless approximately  the  same  power  is  developed  in each cylinder, the engine will run unevenly. The cylinder  compression  tester  (fig.  3-9)  is  used  to measure  cylinder  pressure  in  psi,  as  the  piston moves  to  TDC  on  the  compression  stroke. By  measuring  compression  pressures  of  all cylinders   with   a   compression   gauge,   then comparing  them  with  each  other  and  with  the manufacturer’s  specifications  for  a  new  engine, you  get  an  accurate  indication  of  engine  condition. The  compression  pressures  in  the  different cylinders  of  an  engine  may  vary  as  much  as  20 pounds. The variation is caused largely by the lack of  uniformity  in  the  volume  of  the  combustion chamber. It is nearly impossible to make all the combustion chambers in a cylinder head exactly the same size. For example, in a given engine with Figure 3-9.—Cylinder compression tester. 3-11