From March 1995 Muscle Cars magazine Bill Ceralli of Ceralli Competition Engines
explains the necessity and technique of crankshaft balancing.
You've completely rebuilt the 455-cubic-inch engine in your vintage Stage 1 Buick,
turning it into a lethal street weapon. You've got a high-lift cam, stainless steel
valves, superlight pistons and forged H-beam rods. You spared no expense because
you want this baby to scream when you give it the gas. One question: Did you
remember to have the crankshaft balanced? If not, the scream you hear may be your
engine crying out in pain. The crankshaft is a rotating mass, and any rotating mass
that isn't perfectly balanced will cause uneven loading on the bearing surfaces that
support it. The faster it turns, the worse the problem becomes. Unfortunately, you
probably won't realize you have a problem, sitting there in your reupholstered bucket
seat with a hundred watts of stereo assaulting your senses. But your bearings will.
An out-of-balance crankshaft will cause uneven wear, resulting in premature failure.
Before you know it, you'll be rebuilding your engine again and wondering what went
wrong. "But I never touched the crankshaft," you argue. 'Wasn't it balanced at the factory?"
Yes. But two things have changed since then. First, because the factory is mass-
producing engines, its tolerances are very loose. This is fine for the average street
engine, which spends most of its life running at less than 3,000 rpm. But since you
intend to race it, you'll be spending considerable time above that figure, where the
least amount of unbalance will magnify itself. Therefore, you need tighter tolerances.
The other problem is your new lighter pistons and rods. When we talk about crankshaft
balancing, we're not referring to just that squiggly hunk of steel that resides in the
bottom of your engine. We mean the whole crankshaft assembly, which includes
pistons, pins, rods and rings. Not only did the factory people balance the crank to very
loose tolerances, they also balanced it to match the set of components that came in
your engine. By going to lighter pistons and rods, you've just thrown all of their work
out the window. So how do you balance a crankshaft? First, a word of caution kids,
don't try this at home. Crankshaft balancing is a very precise science that requires
the use of computerized equipment. It should be performed only by experienced
professionals. To find out just how sophisticated the process is, MUSCLECARS
consulted our resident engine expert, Bill Ceralli, of Ceralli Competition Engines
(318 E. 18th St., Paterson, NJ 07524; 201-7424972). Ceralli, who specializes in
building high-performance street and racing engines, has balanced close to 700
crankshafts. His cramped but well-equipped shop includes a state-of-the-art Hines
computerized balancing machine. The key to his success, however, is knowing how
to use it. The first step in the balancing process is to carefully weigh each of the
piston and pin combinations. The object is to make them all weigh the same.
According to Ceralli, factory tolerances allow a difference of 4 to 8 grams between
piston and pin assemblies. That's fine for the average low-rewing passenger car engine,
but the kiss of death for a high-performance powerplant. Even aftermarket high-
performance pistons rarely weigh the same, so they have to be machined. After the
piston/pin assemblies have all been equalized, Ceralli weighs one set of rings and one
set of bearings. Variations in ring and bearing weights are so insignificant, he says,
that it is not necessary to weigh more than one from a given set. Ceralli enters all of
these in dividual weights on something called a bob weight card, which serves as a
permanent record of that particular balance job. A bob weight is simply a device that
bolts to the crankshaft and simulates the piston, pin, ring, rod and bearing assembly.
It's necessary because were you to attach the actual assembly to the crankshaft, it
would flail about during the spin process, creating havoc, damage and incomprehensible
balance readings. The numbers entered on the bob weight card depend on whether the
component is considered rotating mass or reciprocating mass. Pistons and pins, for
example, are obviously reciprocating mass, while journal bearings are clearly rotating
mass. In accordance with the accepted formula, 100 percent of the weight of rotating
parts and 50 percent of the weight of reciprocating parts are used to calculate the total
bob weight. The tricky part is weighing the connecting rods, since the upper part of each
rod is reciprocating weight while the lower part is considered rotating weight. To do it
properly, Ceralli has a special fixture that allows him to weigh each end of the rod
separately. It also gives him some indication of where to remove material from each
rod to make them all weigh within 1/2 to 1 gram of each other. Add an oil allowance
of 4 grams, another 3 grams for the pin locks, and the bob weight card is complete.
It may be called "crankshaft balancing," but so far we've spent alot of time weighing
and machining other parts without touching the crankshaft. "Exactly," Ceralli says.
"You could spend an hour balancing a set of pistons, an hour balancing a set of rods, or
you could get lucky. Occasionally, we'll take a set of pistons out of the box and they're
all within half a gram. It does happen. Then again, we'll take a set out of the box that
are within 18 grams, and we can't figure out where to take the metal out!
"It's been a practice in the past for some people to put a light piston with a heavy rod.
I don't do that because we've found when servicing engines on rebuilds, it doesn't always
go back together the same way. That's something the gypsy balance guys will do." Once
the weights of the piston/rod assemblies are known, the bob weights are assembled
and made to weigh the same by filling them with lead shot. The crankshaft is then set
in the balance machine and the bob weights are installed on the journals. 'What's
important in setting up the machine is what I call bob weight orientation," Ceralli
explains. 'We install the bob weights with dial indicator accuracy. A common problem
with balancing is that many shops will install the bob weights by eye. We top dead
center (TDC) the crank journal first, before we install the bob weight. We then use
a level to make sure the bob weight is exactly perpendicular to the crank stroke. The
third step is to position the bob weight front to back so it is in exactly the correct
position. If you put these bob weights on and you move one of them back 50 or 60
thousandths [of an inch], it throws the readings off. "Another shortcut some balance
shops take," he continues, "is they just push the damper on halfway. It's important
that the damper be all the way on because when you're balancing something that's
longer than its diameter, weight on one end will affect the weight on the other end.
So the unit has to be assembled the way it's going to operate." Speaking of weight
on the other end of the crankshaft, proper flywheel installation is also important. This
is particularly true on externally balanced engines like Pontiac V8s, where the flywheel
(also called a flex plate) functions as a counterweight. According to Ceralli, "Even on
engines with a locating dowel there is a little bit of clearance on the flywheel bolts.
The flywheel should always be tightened up against crankshaft rotation because even
a little variation here can produce anywhere from 7 to 15 grams of out-of-balance."
One other precaution Ceralli takes in this area is the use of a calibrated original
equipment manufacturer (i.e., Ford, Oldsmobile, etc. ) flexplate whenever he balances a
crankshaft. If the customer plans to use an aftermarket flex plate, Ceralli will first
balance the crank using his calibrated flex plate, then install the aftermarket plate
and alter the plate to match the crankshaft assembly. Some balance shops, he claims,
do just the opposite; they alter the crankshaft to match the new flex plate. That's
fine if you never switch back to an OEM flex plate. If you do, odds are your crank will
be out of balance. With the bob weights, damper and flex plate attached, it is finally
time to spin the crankshaft. This is probably the simplest part of the procedure.
Ceralli programs the balance machine with the correction radius, which is simply
the distance from the center of the crank to the edge of the counterweight where
the necessary adjustments will be made. He then zeroes the meters, brings the
machine up to 500 rpm, and takes the readings, which indicate how much material
must be removed or added to the counterweights to bring the crank into balance.
As Ceralli said earlier, all corrections are made to the crankshaft, not to the damper
or flex plate. Removing material from the counterweights is a relatively simple
procedure, but in certain cases, such as the use of heavier pistons and rods, material
may have to be added. When that happens, holes are drilled in the crankshaft
counterweights and filled with a substance called "heavy metal," a tungsten alloy
approximately twice the weight of steel. One advantage of having your crankshaft
balanced by a reputable shop besides having the job done right, of course is
accountability. A high-performance engine may go through several rebuilds during its
lifetime. If accurate data exist, you can avoid the expense of having to rebalance the
engine each time. For example, Ceralli has the bob weight cards on file for every crank
he's balanced all 700 of them! Each crankshaft is stamped with a job number, but unlike
some balance shops, Ceralli does not stamp them with bob weight numbers. Why? "If an
engine is rebuilt and the pistons and rods are changed," he explains, "nobody ever bothers
to grind the bob weight number off. So, three engine shops down the road, somebody uses
the bob weight number stamped on the crank, not knowing the pistons and rods have been
changed. Guess what? Now the crank is out of balance." What does it cost to have your
crankshaft professionally balanced? About $150, Ceralli says, although he admits that
the price may vary slightly from one shop to another. Nevertheless, that's cheap insurance
considering what you have invested in your engine. Not to mention the piece of mind you'll
get from knowing that when you step on the throttle and your engine screams, it's
screaming in anger, not in pain.