User Manual

200 SETUP AND ADJUSTMENTS
Page 7-7
Some E7 engines are equipped with a pointer
marked VALVE on the timing gear cover. When
aligned with the TC mark on the vibration
damper, it provides the 30 degrees after Top
Dead Center (TDC) damper relationship for valve
lash adjustment. Make all adjustments in firing
order sequence.
E7 engine firing order is 1 - 5 - 3 - 6 - 2 - 4.
The Jacobs exhaust valve yoke originally used a
valve yoke screw with SAE threads. MACK yokes
had metric threads. SAE and metric threads are
different enough so that mixing threads should
not be possible. An incorrect screw can be
started into the threads but will lock up after a
quarter of a turn to two full turns.
Effective January 1994, on engine serial
No. 401269, the E7 Jacobs yoke has metric
threads utilizing the Spiralock™ thread form. This
special thread form has a wedge ramp at the root
of each thread to provide a locking effect. The
yoke adjusting screw is free-spinning in the yoke
threads until the locknut is tightened. When the
locknut is tightened, the crests of yoke adjusting
screw threads are pulled tightly against the
wedge ramp of screw threads. Because of this
feature, when the locknut is loosened, the
adjusting screw usually remains locked to the
yoke threads and can be difficult to turn with a
screwdriver. A light tap squarely on top of the
yoke adjusting screw may be required to loosen
the locking effect and allow screw to be turned.
Refer to Figure 7-8.
Figure 7-8 — Spiralock™ Thread Ramp
Yoke and valve lash adjustment must be made
under static conditions with coolant temperature
below 100°F (38°C).
Housing Identification
Each engine brake housing assembly has an
identification tag showing the model number and
is marked FRONT or REAR to show installation
location. Make sure to install front to front or rear
to rear. Engine brake housing serial numbers are
stamped on top of the castings. E7 engines use
the Jacobs Model 680B engine brake assembly.
201109a
SPIRALOCK YOKE THREADS
TM
FREE SPINNING YOKE THREADS
YOKE ADJUSTING
SCREW
YOKE ADJUSTING
SCREW
WEDGE
RAMP
WEDGE
RAMP
RADIAL
CLEARANCE
RADIAL
CLEARANCE
YOKE
YOKE
5-101.bk Page 7 Friday, March 17, 2006 11:05 AM