Two or four exhaust valves are provided for each cylinder (Fig. 1), depending upon the engine model. The
valve heads are heat treated and ground to the proper seat angle and diameter. The valve stems are ground to
size and hardened at the end which contacts the rocker arm (two-valve head) or the exhaust valve bridge (four-
The exhaust valve stems are contained within exhaust valve guides which are pressed into the cylinder head.
Exhaust valve seat inserts, pressed into the cylinder head, permit accurate seating of the exhaust valves under
varying conditions of temperature and materially prolong the life of the cylinder head. The exhaust valves are
ground to a 30seating angle while the exhaust valve seat inserts are ground to a 31seating angle.
The exhaust valve springs are held in place by the valve spring caps and tapered two-piece valve locks.
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.
Figure 1. Location of Exhaust Valves
Exhaust Valve Maintenance
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 leak-proof seating, valve stems must work freely and smoothly
within the valve guides, and the correct valve clearance (Exhaust Valve Clearance Adjustment) must be
Proper maintenance and operation of the engine is important to long valve life. Engine operating temperatures
should be maintained between 160-185 (71-85 ). 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.
Unsuitable fuels may also cause formation of deposits on the valves, especially when operating at low
When carbon deposits, due to partially burned fuel, build up around the valve stems and extend to that portion
of the stem which operates in the valve guide, sticking valves will result. Thus, the valves cannot seat
properly, and pitted and burned valves and valve seats and loss of compression will result.
Lubricating oil and oil filters should be changed periodically to avoid accumulation of sludge.
Valve sticking may also result from valve stems which have been scored due to foreign matter in the lubricating
oil, leakage of antifreeze (glycol) into the lubricating oil which forms a soft sticky carbon and gums the valve
stems, and bent or worn valve guides.