Valve Lifter Mechanism /Arrangement; Example Of Sohc-4 Valve System; Example Of Dohc With Valve Bucket Type - Honda Motorcycle Service Manual

CYLINDER HEAD
VALVE LIFTER MECHANISMIARRANGEMENT
The current camshaft arrangement i n 4-stroke engines can be divided into Single Over Head Camshaft (SOHC) and Double
Over Head Camshaft (DOHC) configurations.
The SOHC follows the basic design of 4-stroke engines, operating IN and EX valves through rocker arms with one camshaft.
Compared t o the DOHC, the SOHC type is less expensive t o manufacture and is easier t o maintain due t o the reduced number
of pans. However, "valve jump" (where the valve cannot accurately follow the cam when the engine rotates at high speed)
can occur, causing the valve t o contact the piston, and causing severe engine damage. To decrease valve mass and reduce
the possibility of engine damage during high engine speeds, 4-stroke engines requiring high power generally use the DOHC
design in which the valves are operated directly w i t h t w o separate camshafts for IN and EX valves.
The DOHC configuration can be of t w o designs: a type pressing the valve bucket directly, or a type lifting the valve through
the use of a rocker arm. For the former, a shim is provided in the bucket for valve clearance adjustment. The valve clearance is
adjusted by replacing the shim. The shim is usually provided between the bucket and the cam lobe.
Some types have a small shim inserted between the under side o f the bucket and the valve stem, allowing less valve ac-
tuating mechanism weight.
Some DOHC types are also equipped w i t h rocker arms, allowing easier adjustment of valve clearance.
The DOHC has a further advantage when combined with the 4-valves per cylinder engine type. A larger valve area can be pro-
vided i n the 4-valve type i n comparison t o that of the 2-valve type,. enabling a greater intake volume of airlfuel mixture and a
smoother exhaust. Valve weight is also less, consequently reducing the likelihood o f valve jump associated with high engine
speeds. Furthermore, with a 4-valve type the spark plug can be placed at the center of the combustion chamber, allowing an
easy flame propagation balance during combustion.
EXAMPLE OF SOHC-4 VALVE SYSTEM
EXAMPLE OF DOHC VALVE
CAMSHAFT
r ' "
ROCKER ARM
INTAKE VALVES
EXHAU~T'VALVES
BUCKET TYPE
VALVE SPRING
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Date of Issue: Sep., 1988
O
HONDA MOTOR CO., LTD.