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V8 Engine Balancing

Sept/Oct 2006

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

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.  This simply means that they are selected to be the same weight as others in the engine set.  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.  However, most of these old engines have been rebuilt a time or three and the balance may 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 loosen bolts and nuts, and 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.

How is it done?

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

Static balancing is done first.  All machine operations, and anything else that changes the weight, need to be done before balancing. The first step is weighing each piston and rod.  The weights are recorded and the heavier rod ends are weight matched to the lightest parts.  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, just as if each rod is two pieces. The weight of each end of each rod is equalized by removing metal from the heavier parts to match the lightest.  Care must be taken to avoid localized overheating.   Localized overheating would distort the part.

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 noted. All the weights are put in the following formula to determine the bobweight. The weights used here are from the last Flathead Ford V8 engine I balanced. All weights are in grams.

Piston                     250
Pin                            72
Retainers                    4
Ring Set                   95
Reciprocating Rod  130                
___________________________
                                             551

Rotating Rod       295
Bearing Pair         55                
___________________________
                           350 X 2       700

Crank pin plug                           5
Est. oil in crank pin                    4
___________________________
Weight of bobweight              1260

Note that the formulas used  are 1X the reciprocating weight of one assembly and 2X the rotating weight of each assembly.

For the V8-60, V-6 bobweights may be used as bobweights for most V8's are too wide and too heavy for the V8-60.  The bobweight simulates the weight acting on the crank pins for the dynamic balance. The bobweights are made to be able to clamp to the rod journals. They allow weight changes as small as gram.   After the bobweights are made to be the same correct weight, they are attached to the crank pins. Each bobweight is installed so that its center of gravity is at the center of its rod journal.

The crank, with bobweights, is placed in the balancing machine.  This starts the dynamic portion of the balancing job.  The balancer is set up to determine 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 where it is light, or taken off where it is heavy, based on the weight and position of the clay.

When the crankshaft with bobweights is fully balanced with metal, other components, such as: crankshaft gear, crankshaft pulley(s), flywheel with ring gear, and clutch pressure plate are added. As each part is added, the balancing rpm may need to 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 maintain balance. 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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