TM 5-3895-359-14&P
GEAR TRAIN AND ENGINE TIMING
GEAR TRAIN
Since the camshaft must be in time with the crankshaft,
A completely enclosed train of five helical gears is
identification marks are located on two teeth of the idler
located at the rear end of the engine as shown in Fig 1.
gear with corresponding match marks stamped on the
A gear bolted to the crankshaft flange drives the
crankshaft gear and the camshaft gear as shown in Fig.
camshaft and balance shaft gears, as well as the blower
drive gear, through an idler gear mounted between the
crankshaft and balance shaft gears.
Before removing or replacing any of the gears, it is
advisable to line up and make a sketch indicating the
The camshaft gear and balance shaft gear mesh with
position of the timing marks. To do this, rotate the
each other and run at the same as crankshaft gear.
crankshaft until the timing marks are aligned on the
Since these two gears must be in time with each other,
camshaft gears. Then check whether the "R" timing
and the two as a unit in time with the crankshaft gear,
mark on the crankshaft gear is aligned with the "R" on
the letter "O" is placed on one tooth of one of the gears
the idler gear and record this information for reassembly
with a corresponding mark at the root of the mating
purposes. Standard gear train timing is used with
teeth of the other gear.
advanced timing being incorporated in the design of the
camshaft.
The camshaft and balance shaft gears are keyed to their
respective shafts and held securely against the shoulder
on the shaft by a nut. Viewing the engine from the
Gear
Number of Teeth
flywheel or gear train end, the right-hand gear,
(camshaft) has left-hand helical teeth (Fig. 1).
Crankshaft
78
Idler
68
The idler gear rotates on a double-row, tapered roller
Cam or Balance Shaft
78
bearing mounted on a stationary hollow hub. This hub is
Blower Drive
40
accurately located on the cylinder block end plate at the
left-hand side of the engine as viewed from the gear
TABLE 1
train end.
Balance weights, one fastened to the inner face of each
A blower drive gear is located on the blower side to
gear (camshaft and balance shaft) are important in
transmit power to the blower, governor, fuel pump and
maintaining perfect engine balance. These are in
water pump.
addition to the weights cast integral with the gears.
Fig. 1. Gear Train and Timing Marks-Right-Hand Rotation Engine (Standard Timing Shown)
10-2-103
