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 TM 55-1905-219-14-6
3-165.2. EXHAUST VALVE - MAINTENANCE INSTRUCTIONS.
a. General.
(1) 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.
(2) Pre-finished, replaceable valve guides, are pressed into the cylinder head. Reaming of these
guides is unnecessary.
(3) 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 30 angle.
(4) The exhaust valve springs are held in place by the valve spring caps and tapered two-piece
valve locks.
(5) 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.
b. 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 loss of compression, mis-firing cylinders,
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.
c. 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 clearances 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 F and 185 F (710C to 850C). Low operating
temperatures, 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.
3-2793
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