Taking It For A Spin: Why Are Some Cranks Externally Balanced?

If you’ve ever driven with a missing wheel weight, you’ve experienced how just a little bit of imbalance can cause a whole lot of vibration. It’s not hard to imagine, then, how a properly balanced crankshaft can result in a smoother running engine, less bearing wear, and reduced parts breakage.

Although all stock engines leave the factory with an assembly-line-quality balance, the balance of the rotating assembly can often be improved. In balancing the rotating assembly, the weight of the crankshaft counterweights offsets the rotating and reciprocating forces of the piston/rod assemblies. But why are some engines internally balanced, while others are externally balanced? We’ll get to that, but first, let’s look at how an engine is balanced.

Balance is the Key to a Great Life

By weighing each reciprocating component, from the pistons down to the rod bearings, a custom engine builder can use a little math and simulate the rotating and reciprocating mass. Weight has more of an effect the further it is from the crank centerline. Because of that, the weight of the big end of the rod and the bearing — which rotate around the crank centerline — is assigned a value of 100-percent. But, the small end of the connecting rod, piston, ring pack, and wrist pin are assigned a value of half of their cumulative weight, as they are reciprocating weight.

After figuring the total weight in grams, bobweights are assembled to mimic the rotating and reciprocating weight and are bolted to the crank’s throws. By spinning the crankshaft to about 750 rpm in a crank balancing machine, the engine builder can see where weight may need to be added or removed from a counterweight. Weight is removed by drilling into the counterweight’s edge, or it’s added by drilling into the side of the counterweight and pressing in Mallory metal — a tungsten alloy that’s twice the weight of steel for the same physical size.

Bobweights are assembled to mimic the rotational and reciprocating mass of the pistons, rings, wrist pins, connecting rods, and bearings.

Are You In or Are You Out?

Some engine designs left the factory externally balanced, with weight added to the crank damper and flexplate or flywheel in a specific location to counteract an inherent imbalance in the design. Tom Lieb, the owner of SCAT Crankshafts in Redondo Beach, California, gives the small-block Chevy 400  as an example. “When they went from the 350 to the 400, they increased the stroke by a quarter-inch. But, they shortened the rod, and with the increased stroke, the piston would have come down on top of the counterweight at bottom dead center,” Lieb says.

When increasing the stroke, there are two options to provide that counterweight clearance. “You can increase the rod length, as SCAT does for its long-stroke crankshafts for 350 and 400 main bearings, or if using a shorter rod, cut the counterweight back to clear the piston’s skirt,” Lieb explains.

As GM designed it with the short 5.565-inch rod, the only way to add enough counter-weight to the small-block 400  was on each end of the crankshaft, at the damper and flexplate or flywheel. It was a compromise from GM that works well enough for torquey, lower-RPM applications, and it satisfied the bean counters.

“With our internal-balance cranks, you’ll notice that we use the longer connecting rod, which puts the weight back on the counterweight,” Lieb says.

Whether a builder specs, a 5.700-inch or 6.000-inch rod for a small-block Chevy, or a 6.135-inch or 6.385-inch rod for a big-block Chevy, the cost of the connecting rods is about the same, and the use of the longer rod reduces the need for Mallory metal when final balancing the assembly, Lieb points out.

Crank dampers can be neutral balance (left) for internally balanced engines or have a counterweight installed for externally balanced engines. Externally balanced engines will also have a counterweight on the flexplate or flywheel. The ’86 and newer one-piece rear-main-seal small-block Chevy has its rear counterweight attached to the flywheel or flexplate, so it is an unusual example of an internal/external-balance engine.

Externally balanced crankshafts can also be balanced by an engine builder or at SCAT, but the damper and flexplate or flywheel that will be used must also be bolted on during the balancing. If your application has an internally balanced crankshaft available, the only compelling reason to buy a new externally balanced crankshaft is the cost savings realized by reusing the damper and flexplate or flywheel, Lieb says. He recommends an internally balanced crank for any application that spins the engine more than 4,500 to 5,000 rpm.

Unlike internally balanced assemblies, external-balance weight is located a significant distance outside the support of the main bearings, which means it can create several issues. “It’s like leverage,” he explains. “If you go to break a bolt loose and you use a little end wrench, and you can’t get it, what do you do? You get the breaker bar and socket.”

“Well, that’s kind of the same thing that happens with the weight on the end of the crank. The further out you go, that weight has more leverage, which wiggles the crankshaft and puts more stress on it. When you internally balance, everything that is balanced is supported by the main bearings.”

The crankshaft balancing machine (left) spins the crankshaft to about 750 rpm, indicating where to remove weight with a drill bit; adding weight requires drilling into the side of the counterweight and pressing in a Mallory metal slug, which is a tungsten alloy, twice as heavy as steel for the same size.

Comparing the crankshaft forces to the childhood game, “Crack the Whip,” Lieb notes how forces multiply with speed on the outer axis to create additional stress on the crankshaft. “When we were kids, the guy in the center would hang onto four people. He would wiggle around, and the guy on the edge was running his backside off. And finally, the guy on the edge fell down. That’s what happens in the engine.”

“On the end of the crankshaft, you have this weight that starts out as two pounds or whatever, and all of a sudden it’s 40 pounds, 50 pounds, or 100 pounds. That rotating mass is trying to leave, and of course, it wants to take the crank with it. And that causes crank breakage.”

It is this rotational mass and engine speed — not horsepower levels — that factor into deciding to run an internally balanced crankshaft, says Lieb. He also points out that there is more stress on the block at higher RPM when an externally balanced crank is used.

Tom Lieb, the owner of SCAT Crankshafts, recommends internally balanced crankshafts, where available, for any application turning 4,500 to 5,000 RPM and higher.

“It’s just by virtue of the fact you’re on the end of the crankshaft with this weight, trying to wiggle it up and down, and the block has to support it,” Lieb says, adding that external balancing can result in other issues, such as small-block Chevys breaking the crankshaft nose.

Lieb pointed out a quick mathematical exercise. An externally balanced SBC uses an 8-inch damper. Squaring the distance from the centerline (or radius) is 4×4=16. And with a typical external SBC damper weighing 14 pounds, multiplied by the squared radius (14 x 16) that equals a whopping 224 pounds of rotating mass.

SCAT offers a broad selection of cast, forged, and billet cranks, for a number of different engine families. It also offers rotating assemblies with an almost unlimited number of options for pistons, connecting rods, rings, and bearings. Scat sells them as either unbalanced, externally balanced, internally balanced, or for one-piece-rear-main small-block Chevys, balanced internally for the front and externally for the rear.

We try to make the crank forgings with enough material on them that we can do just about anything that the application will allow us to do. -Tom Lieb, SCAT Crankshafts

Why Late Small-Block Chevys Are Both Internal- and External-Balance

Now that we’ve discussed the reasons for internal and external-balancing, we need to address an anomaly. Why is the 1986-newer small-block Chevy a combination of internal and external balance? Lieb explains it’s simply because the older, two-piece rear-main seal small-block Chevys had a rear counterweight on the irregularly shaped crank flange. With the round, one-piece, rear-main crank flange, this removed the weight. So engineers put the weight on the flexplate or flywheel.

Starting From Scratch, There’s No Price Difference

If you’re building an engine from the crank up, it costs no more (in most cases) to select an internal-balance crankshaft because they start as the same raw part. This practice is similar to how manufacturers of lightweight pistons or connecting rods often begin with the same forging and then differences in machining account for the differences in the final product.

“Because of the cost of the forging dies and so forth, we try to make the crank forgings with enough material on them that we can do just about anything that the application will allow us to do. We cut the counterweight on the forging based on the stroke and the rod length the customer is using,” Lieb explains.

In addition to a wide variety of cast, forged, and billet crankshafts, Scat can put together a rotating assembly with a multitude of options for pistons, connecting rods, and crankshafts. SCAT offers them unbalanced, externally balanced (for certain applications), and internally balanced (right.)

“The fact of the matter is, with the crank, as well as the rod, you’ve already bought the forging. This may sound a little silly, but we give you as much as you want, and the rest of it, we throw away.” So for the engine builder or hobbyist who is starting with a clean sheet, he can often have his cake and eat it, too.

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About the author

Jay Sicht

Since childhood, Jay has been fascinated by planes, trains, and automobiles, and all things mechanical. He's been in the automotive aftermarket for 25 years, having written about it for 15 of those years.
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