4-CYCLE OVERHEAD VALVE ENGINES - Small Engine Discount

Mechanical Compression Release (MCR) camshafts
have a pin located in the camshaft, that extends above
the intake or exhaust cam lobe, to lift the valve to
relieve the engine compression for easier cranking.
When the engine starts, centrifugal force moves the
weight outward and the pin will drop back down. The
engine will now run at full compression (diag. 38).
Some OHH engines use a composite camshaft. This
camshaft is used on OHH/OHSK 50-70 engines that
have been manufactured after DOM (date of
manufacture) 7048. This change that took place in
February of 1997 and is basic on the OHH engines,
except those units that use extended camshafts which
will continue to use a cast iron camshaft. A camshaft
kit replaces the old camshaft and includes
compression release components and two light tension
valve springs. It is CRITICAL that the new valve
springs be installed or lobe damage will occur on the
camshaft.
NOTE: You must install the thrust washer between
the cover and cam to prevent wear.
Some OVRM engines use a ramp compression
release (RCR) system that works with the intake valve.
The ramp is located on the backside of the intake cam
lobe. The ramp delays the intake valve closure,
resulting in lower emissions and reduced compression
for easier pull starting (diag. 39).
Valve Seats
COMPRESSION
RELEASE
INTAKE
CAMSHAFT
INSERT
SPRING
END "A"
COMPRESSION
RELEASE SPRING
Valve seats are not replaceable. If they are burned,
pitted, or distorted they can be regrounded using a
grinding stone or a valve seat cutting tool. Valve seats
are ground to an angle of 46 degrees to a width of
EXHAUST
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3/64" (1.91 mm).
The recommended procedure to properly cut a valve seat is to use the Neway Valve Cutting System, which
consists of three different cutters. OVRM and OHH engines have a small combustion chamber and require the
use of a special Neway cutter #103 for the 46 and 31 degree combination cutter. The 60 degree cutter is Neway
cutter # 111. The tapered pilots required are; Neway # 100-1/4-1 for the .249" (6.325 mm) exhaust guide, and
Neway #100-1/4 for the .250" (6.35 mm) intake guide. Consult the cutter's complete procedure guide for
additional information.
First, use the 60 degree cutter to clean and narrow the seat from the bottom to the center (diag. 40).
A
THRUST
WASHER
Second, use the 31 degree cutter to clean and narrow the seat from the top toward the center (diag. 41).
Third, use the 46 degree cutter to cut the seat to a width of 3/64" (1.191 mm) (diag. 42). Check the contact area
of a new or reconditioned valve face on a finished valve seat. Using fingers, snap the valve sharply against the
corresponding valve seat and view the line imprinted on the valve face. The contact area or line should be
continuous and appear on the upper 1/3 of the valve face. Re-cut the seat to move the contact area higher or
lower if necessary and recheck the contact area.
B
38
BOTTOM
NARROWING
CUTTER
15 0
TOP NARROWING
CUTTER
46 0
TOP
31
NARROW
0
31 0 SEAT CUTTER
46 0
60 0
SEAT
SEAT
3/64" SEAT
72
BOTTOM
NARROWING
40
41
BOTTOM
NARROW
Not For Resale
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