Yaroslavl Engine Plant YMZ-536 Operation Manual page 72

Each cylinder has four valves — two intake valves and two
exhaust valves.
The main valve train components are: a camshaft with a drive
gear, rear bearing and a thrust flange; tappets; pushrods; rocker arms
with adjusters; yokes; rocker arm shaft; rocker arm shaft supports;
valves; valve springs, fasteners and valve guide bushings.
The camshaft is made of structural steel and hardened to
improve wear-resistance of cams and bearing journals.
The camshaft is located in the upper part of the cylinder block
and is driven from the rear end of the crankshaft through a pair of
straight-tooth gears. The gears are installed according to the marks
embossed on the faces of the toothed rims. Split steel-bronze bushings
serve as camshaft bearings. All bushings, except the rear one, have the
same width and are press-fitted into the cylinder block. The rear
bushing has the same diameter but a larger width; it is press-fitted into
the cast iron rear bearing housing with a flange. The camshaft axial
shift is limited by a steel thrust flange installed between the gear hub
and the rear bearing journal. The thrust flange is fixed to the rear face
of the cylinder block with two bolts through the holes in the rear
bearing flange (Fig.20).
The valve drive system is shown in Fig. 21.
Tappets are roller-type, reciprocating. They consist of a body,
a roller, a pin and a lock press-fitted into the cylindrical part of the
body. The roller and the pin are made of bearing-grade steel and
hardened to a high degree of hardness. To prevent the tappets against
rotation, the tappet bores in the cylinder block have special grooves.
Pushrods are steel tubes with spherical ball ends: the upper
end with a spherical ball inside, and the lower end with a spherical
ball outside. The spherical surfaces are hardened for a better wear-
resistance. The pushrods of intake and exhaust valves are the same
design.
Valve rocker arms are forged steel, with a steel-bronze thin-
walled split bushing press-fitted into the hub.
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