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Fig. 117 Cross Section of Intake and Exhaust Valves Installed in Head
TM-5-4210-230-14P-1 Aerial Ladder Fire Fighting Truck Manual
When to Service
ENGINE DIVISION SERVICE MANUAL TM 5-4210-230-14&P-1 ENGINE Hydraulic Valve Lifters (Tappets) The hydraulic valve lifter or tappet consists of a body (cylinder), plunger, plunger spring and check valve assembly and is removed from the engine as an assembly.  The lifters are removed after removing the valve cover, rocker arms and push rods. NOTE: If   a   tappet   cannot   be   removed   due   to carbon  buildup,  use  a  carburetor  solvent in    the    tappet    bore.        However,    if    any excessive amount of solvent is used, it will be necessary to change the oil. Optional sources of hydraulic valve lifters have been established.    When  disassembling  the  lifters,  do  not  mix  the components of any lifter.  Figure 120, illustrates the difference in   construction   between   types   "A"   and   "B."   Generally,   the service  instructions  are  the  same  for  all  types  of  hydraulic lifters.  The push rods are common for all types of lifters. Fig. 120 Sectional View of Hydraulic Tappets Showing Types "A" and "B" 1.Type A 6. Plunger  oil  hole 2.Type B 7. Plunger 3. Snap ring 8. Check valve 4. Identification rules 9. Spring 5. Tappet oil hole 10.   Oil Chamber Operation of Valve Lifter Oil  from  the  engine  lubricating  system  is  fed  to  a supply  chamber  in  the  hydraulic  lifter  plunger  from  which  it can  flow  directly  through  the  plunger,  past  the  check  valve and  into  the  compression  space  between  the  bottom  of  the body   and   the   plunger.      When   the   engine   is   started,   oil pressure   is   built   up   in   the   system   which   fills   the   supply chamber. With the lifter on the base circle of the cam and the engine   valve   seated,   the   light   plunger   spring   moves   the plunger outwardly to remove all lash from the valve gear.  As the   plunger   moves   outward   increasing   the   volume   in   the compression chamber, the check valve moves off its seat and the chamber is supplied with oil.  As the camshaft rotates and lifts the body upward, a pressure is immediately built up in the compression chamber, forcing the valve onto its seat. Further rotation    of    the    camshaft    lifting    the    valve    gear    against increasing valve spring pressure increases the oil pressure in the  compression  chamber,  thus  acting  as  a  solid  member  of the valve operating mechanism.  As long as the engine valve is  off  its  seat,  the  load  of  the  mechanism  is  carried  by  the column of oil.  During initial running of the engine, there might be  some  air  remaining  in  the  compression  chamber  which, under pressure, tends to leak through the clearance between the plunger and cylinder, thus decreasing compression space volume. However, when the lifter is again on the base circle of the cam, the plunger moves outward, drawing in more oil so that after a number of cycles, the air is completely eliminated and  a  solid  column  of  oil  is  maintained  in  the  compression space.  During  this  period  when  some  air  is  present  in  the compression  space,  the  valve  gear  may  be  somewhat  noisy, since it has the same effect as excessive lash with aerated oil. However,  once  the  unit  is  completely  cleared  of  air,  it  then becomes a true hydraulic system. The compensation required to take care of variations in length from the cam to the valve seat is accomplished by the original fit   of   the   plunger   in   the   cylinder   bore.      These   parts   are selectively  fitted  to  give  a  predetermined  slight  leakage  of  oil (called    leakdown)    under    load.        The    purpose    of    this predetermined    leakage    is    to    permit    the    valve    to    seat irrespective  of  the  change  in  length  between  the  cam  and valve seat.  If the valve gear is expanding, thus reducing the distance  between  the  cam  and  valve  seat,  the  load  imposed by the valve spring on the lifter causes the above mentioned slight leakage between the plunger and cylinder, reducing the clearance   volume   and,   therefore,   the   overall   lifter   length. When CGES-210  Page 44 PRINTED IN UNITED STATES OF AMERICA

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