TM 5-3895-346-14
GEAR TRAIN AND TIMING
ENGINE OVERHAUL
GEAR TRAIN AND ENGINE TIMING
A train of helical gears, completely enclosed between the engine end plate and the flywheel housing, is located
at the rear of the Series 53 engine.
The gear train on an in-line engine (Fig. 1) consists of a crankshaft gear, an idler gear, a camshaft gear, and a
balance shaft gear. The governor drive gear and the blower drive gear for the four-cylinder engine are driven
by the camshaft gear or balance shaft gear, depending upon the engine model.
On in-line engines, the crankshaft gear is pressed on and keyed to the end of the crankshaft.
The idler gear rotates on a stationary hub.
The camshaft and balance shaft gears on in-line engines are pressed on and keyed to their respective shafts
and each gear is secured by a retaining nut and lock plate.
The camshaft and balance shaft gears on an in-line engine mesh with each other and run at the same speed as
the crank-shaft gear. Since the camshaft gears must be in time with each other, and the two as a unit in time
with the crankshaft gear, timing marks have been stamped on the face of the gears to facilitate correct gear
train timing.
The symbol system of marking the gears makes gear train timing a comparatively easy operation. When
assembling the engine, it is important to remember the engine rotation. Then, working from the crankshaft gear
to the idler gear and to the camshaft and/or balance shaft gear in that order, line up the appropriate circle
symbols on the gears or the appropriate triangles as each gear assembly is installed on the engine. Refer to
Fig. 1.
CAUTION
It is advisable to make a sketch indicating the position of the timing marks
BEFORE removing or replacing any of the gears in the gear train.
The circle and the triangle are the basic timing
symbols stamped on the gears. The letters stamped
on the crankshaft gears identify the proper timing
marks for the particular engine: I represents in-line
engine, V represents V-type engine, R represents
right-hand rotation engine, L represents left-hand
rotation engine, and A represents advanced timing.
Figure 1. In-Line Engine Gear Train
Timing Marks (Standard Timing Shown)
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