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 TM 55-1905-220-14-5
3-91.1. GEAR TRAIN (Cont).
c. 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 flywheel or gear train end, the righthand gear is the
camshaft and has lefthand helical teeth.
d. The idler gear rotates on a double-row, tapered roller bearing on the cylinder block end plate at the left-
hand side of the engines, as viewed from the gear train end.
e. A blower drive gear is located on the blower side to transmit power to the blower, governor, fuel pump
and water pump.
f. Since the camshaft must be in time with the crankshaft, identification marks are located on two teeth of
the idler gear with corresponding match marks stamped on the crankshaft gear and the camshaft gear.
g. However, the timing is advanced on certain engines by aligning the "A" on the crankshaft gears with the
"L" or "R" (depending upon engine rotation) on the idler gears.
h. Before removing or replacing any of the gears, note whether standard or advanced timing is used on the
engine. To do this rotate the crankshaft until the timing marks are aligned on the camshaft gears. Then check
whether the "A", "L" or "R" timing mark on the crankshaft gear is aligned with the "L" or "R" on the idler gear
and record this information for reassembly purposes.
i. Balance weights, one fastened to the inner face of each gear (camshaft and balance shaft) are important
in maintaining perfect engine balance. These are in addition to the weights cast integral with the gears.
j. Gear train noise is usually an indication of excessive gear lash, scoring, pitting or excessive bearing
wear. Therefore, when noise develops in a gear train, the flywheel housing should be removed and the gear
train and its bearings inspected. A rattling noise usually indicates excessive gear lash whereas a whining noise
is a result of too little gear lash.
k. Excessive wear and scoring may result from abrasive substances or foreign material in the oil,
introduced in the engine by such a means as removal of the valve rocker cover without first cleaning away the
dirt.
l. Since the camshaft and balance shaft gears each have the same number of teeth as the crankshaft gear,
they will turn at crankshaft speed. However, as the blower drive gear has only about half as many teeth as the
camshaft or balance shaft gear, it turns at approximately twice the speed of the crankshaft.
3-1547
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