TM 5-3810-300-24&P-3
1.7.1
GEAR TRAIN AND ENGINE TIMING
GEAR TRAIN
A completely enclosed train of five helical gears is
The idler gear rotates on a double-row, tapered roller
located at the rear end of the engine, as shown in Fig. 1.
bearing mounted on a stationary hollow hub. This hub is
A gear bolted to the crankshaft drives the camshaft and
accurately located on the cylinder block end plate, at the
balance shaft gears, as well as the blower drive gear,
left-hand side of the engine, as viewed from the gear
through an idler gear mounted between the crankshaft
train end. A blower drive gear is located on the blower
and balance shaft gears on the RB and RC engines, and
side to transmit power to the blower, governor, fuel pump
between the crankshaft and camshaft gears on the RA
and water pump. Since, as stated above, the cam and
and RD engines.
balance shafts must be in time with the crankshaft,
identification marks are located on two teeth of the idler
The camshaft gear and balance shaft gear mesh with
gear with corresponding match marks stamped on the
each other and run at the same speed as the crankshaft.
crankshaft gear and the camshaft or balance shaft gear,
Since these two gears must be in time with each other,
as shown in Fig. 1.
and the two as a unit in time with the crankshaft gear, the
letter "O" is placed on one tooth of one of the gears with
For standard gear train timing, the letter "R" on the idler
a corresponding mark at the root of the mating teeth of
gear is aligned with the "R" on the crankshaft gear and
the other gear.
the camshaft or balance shaft gear. For advanced
timing, used on certain engines, the letter "A" on the
The camshaft and balance shaft gears are keyed to their
crankshaft gear is aligned with the letter "R" on the idler
respective shafts and held securely against the shoulder
gear.
on the shaft by a nut. Viewing the engine from the
flywheel or gear train end, the right-hand gear, whether
Balance weights, one fastened to the inner face of each
on the balance shaft, as shown on RA and RD engines,
camshaft and balance shaft gear, are important in
or the camshaft, as shown on RB and RC engines, has
maintaining perfect engine balance.
These are in
left-hand helical teeth (Fig. 1).
addition to the weights cast integral with the gears.
Fig. 1 - Gear Train and Timing Marks - Right Hand Rotation Engines (Standard Timing Shown)
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