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TM 55-1905-221-14-3
The following is an index to the maintenance procedures:
Gear Train and Engine Timing
Balance Weights and Cover
Camshaft and Balance Shaft
Camshaft and Balance Shaft Gear
a. General.
A completely enclosed train of five helical gears is located at the rear end of the engine. A gear bolted to the
crankshaft flange drives the camshaft and balance shaft gears, as well as the blower drive gear, through an idler gear
mounted between the crankshaft and balance shaft gears on the RB and LD engines, and between the crankshaft and
camshaft gears on the RD and LB engines.
The camshaft gear and balance shaft gear mesh with each other and run at the same speed as the crankshaft. Since
these two gears must be in time with each other, 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 a corresponding mark at the root of the mating teeth of the other gear.
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 right-hand gear, whether on the balance
shaft, as shown on RD, and LD engines, or the camshaft, as shown on RB and LB engines, has left-hand helical teeth.
The idler gear rotates on a double-row, tapered roller bearing mounted on a stationary hollow hub. This hub is
accurately located on the cylinder block end plate, at the right-hand side of the LB and LD engines and at the left-hand
side of the RB and RD engines, as viewed from the gear train end.
A blower drive gear is located on the blower side to transmit power to the blower, governor, fuel pump and water
Since, as stated above, the cam and balance shafts 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
or balance shaft gear.


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