TM 5-3810-300-24&P-3
1.7
ENGINE BALANCE AND BALANCE WEIGHTS
Both rotating and reciprocating forces are completely
The front balance weights are keyed to the front end of
balanced in the engines. The eccentric rotating masses
the camshaft and the balance shaft (Fig. 1). Current
of the crankshaft and connecting rods are balanced by
balance weights are of one-piece construction.
counterweights on the crankshaft cheeks.
Formerly, spring-loaded balance weights (Fig. 2) were
The reciprocating masses (the piston and upper end of
used. The balance weight bushing oscillates on the
the rod) produce an unbalanced couple by virtue of an
hardened surface of the hub during engine operation.
arrangement on the crankshaft in which reciprocating
Torque variations are transmitted from the hub to the
masses, though equal, are not opposite.
This
weight through the spacer and spring leaves.
unbalanced couple, which tends to rock the engine from
end to end, is balanced by an arrangement of rotating
Remove Front Balance Weights
counterweights, mounted at the front and rear ends of
the camshaft and balance shaft, which produce a couple
1. Remove the balance weight cover.
equal and opposite in magnitude. Consequently the
engine will operate smoothly and in balance throughout
2. Place a block of Wood between the balance weights
its entire speed range.
Each set of weights (weights on one shaft comprise a
3. Loosen the balance weight retaining nuts on the
set) rotates in an opposite direction with respect to the
camshaft and balance shaft with a 1-1/2" socket wrench
other. When the two weights at either end of the engine
and remove the nuts and internal tooth lock washers.
are in a vertical plane, their centrifugal forces are in the
same direction and oppose the unbalanced couple; when
4. Force the balance weight off the end of each shaft
they are in a horizontal plane, the centrifugal forces of
with two heavy screw drivers or pry bars between the
these balance weights are opposite and are therefore
heads of the bearing retaining bolts and the balance
cancelled. The front balance weights are eccentric in a
direction opposite to the rear balance weights.
Therefore, rotation will result in the desired couple
effective only in a vertical plane.
The balance weights consist of two eccentric weights at
each end of the engine. On three cylinder engines
(through serial number 3A-48439), the weights at the
rear are integral with the gears. On three cylinder
engines effective with 3A-48440 and four cylinder and six
cylinder engines, additional weights are attached to the
Fig. 2 - Spring-Loaded type Front Balance
Weight
Fig. 1 - Typical Front Balance Weight
Mounting.
Page 110