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 ½ 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.