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Balancing the V8 Engine

Jan/Feb 2000

Why do the racers and hotrodders do it?  Should you do it in a restoration?  How is it done?

Balancing of moving parts is commonplace.  Generator and starter armatures are balanced. Engines are balanced from the factory. Ford put new machinery in place to balance the new V8's in 1932.  The drilled holes in the crankshaft counterweights differ from engine to engine and are the result of factory balancing.

The factory has many parts to choose from and usually balances parts like pistons and rods by selection.  If when the parts are made they are weighed and ID'd, then sets may be selected that are in close balance without metal being removed.

If your Ford engine has never had any parts replaced, it will be in fair balance, very good for its time.  Most of these old engines have been rebuilt a time or three and the balance will not be close.

If you are trying to make all the horsepower possible, balancing prevents loss of the power otherwise used to create the vibration.  This vibration can also break parts such as mounting brackets, especially in racing conditions.

On a restoration, vibrational forces will not be as great because rpm is not as great. A balanced engine may add to your driving pleasure by running more smoothly than an unbalanced engine. It should help the engine last longer by decreasing bearing loads at the same power settings.

There are two parts of engine balancing, static and dynamic.

Static balancing is done first.  All machine operations, anything that changes the weight, need to be done before balancing. The first step is weighing each piston and rod.  The piston weights are equalized by swapping pins and/or by machining metal from inside of the piston.

The rods are weighed "end-for-end", because one end of each rod rotates and one end reciprocates. These two weights are handled differently in the formula for determining bobweight. The weight of each end of each rod is equalized by removing metal from the heavy parts.

I think that Ford, and other manufacturers, used rod total weight only.  Because flathead Ford rods have no "balance pads" or waste weight, balanceable sets need to be selected before machine work is done.

The weights of rings, pistons, pin retainers, and rod bearings are also determined.  All the weights are put in the following formula to determine the bobweight. The weights used here are from the last engine I balanced. All weights are in grams.

Piston                   250
Pin                          72
Retainers                  4
Ring Set                 95
Reciprocating Rod 130                 

Rotating Rod       295
Bearing Pair         55                  
                           350 X 2       700

Crank pin plug                           5
Est. oil in crank pin                  4

Weight of bobweight              1260

The bobweight simulates the weight acting on the crank pins for the dynamic balance. After the bobweights are made to be the same correct weight, they are attached to the crank pins.

The crank, with bobweights, is placed in the balancing machine.  This starts the dynamic portion of the balancing job.  The balancer determines the rpm that the crankshaft would like to vibrate at - this determines the speed that the crankshaft is turned in the machine.

As this assembly spins, a strobe light flashes to show the operator where the unbalance is, and meters show how much.  By using clay, the crankshaft is temporarily balanced. 

When balance has been achieved by the correct amount of clay in the right place(s), metal is added to the crankshaft, or taken off, based on the weight and position of the clay.

When the crankshaft with bobweights is balanced, other components, such as: crankshaft gear, crankshaft pulley(s), flywheel, and clutch pressure plate are added. As each part is added, the rpm may be adjusted, and the new assembly is rebalanced with all weight added or subtracted only to the additional parts.

The orientation of the parts when the assembly is balanced is the orientation in which they must be assembled when in or on the block. To do this, index marks are painted and/or stamped on the crankshaft, flywheel and clutch, and anything else that could go together in more than one position.

While this procedure may be time consuming for the engine rebuilder, the benefits will endure through the life of the engine.

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