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 TM 55-1905-220-14-5
3-90.2. EXHAUST VALVE - MAINTENANCE INSTRUCTIONS.
a. Four exhaust valves are provided for each cylinder. The valve heads are heat treated and ground to the
proper seat angle and diameter, and the valve stems are ground to size and hardened at the end which
contacts the rocker arm or exhaust valve bridge.
b. Pre-finished replaceable valve guides are pressed into the cylinder head. Reaming of these guides is
unnecessary.
c. Exhaust valve seat inserts pressed into the cylinder head permit accurate seating of the exhaust valves
under varying conditions of temperature and materially prolongs the life of the cylinder head. The inserts are
ground to very close limits and the freedom from warpage, under ordinary conditions, reduces valve
reconditioning to a minimum. The exhaust valves and valve seat inserts are ground to a 30seating angle.
d. The exhaust valve springs are held in place by the valve spring caps and tapered two-piece valve locks.
e. Excess oil from the rocker arms lubricates the exhaust valve stems. The valves are cooled by the flow
of air from the blower past the valves each time the air inlet ports are uncovered.
f. Exhaust Valve Clearance Adjustment.
Correct valve clearance adjustment is important for proper operation of the engine. Too little clearance
between the exhaust valve stem and the rocker arm causes a loss of compression, misfiring cylinder, and
eventual burning of the valves and valve seat inserts. Too much clearance results in noisy operation of the
engine, especially in the idling speed range.
g. Exhaust Valve Maintenance.
(1) Efficient combustion in the engine requires that the exhaust valves be maintained in good operating
condition. Valve seats must be true and unpitted to assure leakproof seating, valve stems must work freely and
smoothly within the valve guides, and the correct valve clearance must be provided.
(2) Proper maintenance and operation of the engine is important to long valve life. Engine operating
temperature should be maintained between 160 and 185 (71 to 85 ). Low operating temperatures,
F
F
C
C
usually due to extended periods of idling or light engine loads, result in incomplete combustion, formation of
excessive carbon deposits and fuel lacquers on valves and related parts, and a greater tendency for lubricating
oil to sludge.
3-1517
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