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Scat Enterprises Important Engine Balancing Basics

Balancing Act

Jim Smart Nov 9, 2018
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When you consider the crazy monkey motion of reciprocating internal combustion engines it is remarkable they work as well as they do. You have eight pistons and connecting rods doing a “do-si-do” around a spinning crankshaft at high speed and it’s a wonder how it all stays together. In the end, smoothness at high revs is all about dynamic balance—the art of getting the pistons, rods, and crankshaft counterweights on the same page weight wise to where they operate smoothly. As they swirl and reciprocate around each other, they must weigh exactly the same or suffer the penalty of vibration and shake.

Vibration can be annoying. It is also very destructive. Factory dynamic balancing, known as Detroit balance, operates on an average across hundreds of pistons, rods, and crankshafts, which gets balancing in the ballpark, but not perfect. When you’re building a high-performance engine, whether it’s a vintage small-block or modern LS, you want spot-on smoothness and durability. Vibration is destructive from what it does to bearings and journals. It can jar fasteners loose and even break them. It has been known to break crankshafts and throw connecting rods through the block.


002 SCAT Crank Balance LS Stroker 2/28

Dynamic balancing an engine’s bottom end begins with an experienced balancer and the best balancing equipment. It also begins with a stroker kit from Scat Enterprises. Here, Scat is setting up the balancer with an LS 4.000-inch stroker crank, ensuring the crank is level and ready to go.


Tom Lieb of Scat Enterprises stresses the importance of dynamic balancing whether you’re building a box-stocker or a high-revving screamer. Proper dynamic balancing equates to smoothness and durability no matter what you’re building. Because Lieb has been in this industry longer than most of us have been alive, we pay very close attention to what he has to say about the importance of dynamic balancing.

Scat tells us crankshafts are a series of main and connecting rod journals flanked by large counterweights engineered to “counter” the weight of the connecting rod and piston along with keeping the momentum going around the crankshaft’s centerline. The flywheel and harmonic damper also assist the counterweights with momentum. The piston-and-rod weight combination affects the size and weight of the counterweights. When we get into strokers we’re also talking stroke combined with a heavier piston, pin, and ring package, which requires a larger counterweight (increased mass) to balance against the higher reciprocating weight. The goal with balancing is consistency and uniformity of the rotating assembly.

Most American V-8 engines use large counterweights toward the front and rear of the crankshaft, leaving the center without counterweights. This approach divides the engine into fore and aft halves. It can be safely said counterweight positioning on all 90-degree V-8 crankshafts is the same. The height of the counterweight as measured outward from the crankshaft centerline is limited by both cylinder block dimensions and by placement of the cylinder bores. A counterweight placed farther away from the crank centerline has more balance effect, but it is limited by the width of the block. Weights placed toward both ends of the crank also have a greater effect and don’t need to be as large to effectively balance the engine. This keeps crankshaft weight light.


003 SCAT Crank Balance LS Stroker 3/28

Here’s a set of Scat 6.125-inch Chevrolet H-beam rods with important balance information written on the box. The large end of each rod weighs 452 grams. The small ends tip the scale at 191 grams each for a total weight of 643 grams per rod. Of course, Scat is going to put them on the scale to show you what they weigh exactly.


Internal vs. External Balance
Internally balanced means all of the counterweight mass is contained within the crankshaft counterweights and is independent of the harmonic damper and flywheel/flexplate. There’s no need to add counterweighted mass beyond the crankshaft with an internally balanced engine. It is when we start adding reciprocating weight that an engine can wind up externally balanced.

Small-block Chevys, as one example, have traditionally been internally balanced, which means they don’t require external balancing assistance from the flywheel/flexplate and harmonic damper. When Chevrolet took the small-block out to 400 cubic inches, it had no other choice but to externally balance this engine with an offset-balanced flywheel and harmonic damper. Small-block Chevys beyond 1985 and most crate engines are externally balanced as a result of additional reciprocated mass—heavier rods and pistons typically in high-performance applications. Big-block Chevys have been traditionally externally balanced. LS engines are all internally balanced.

Dynamic Balancing
The first order of business in dynamic balancing is to weigh each piston and connecting rod, then take the lightest of the set and lighten the rest to match the lightest. Scat lightens the heavier rods and pistons with a belt sander or a drill to get the weight down to that of the lightest piston and rod combo. Once these items are matched up, Scat gathers the rings, clips, and rod bearings for each piston/rod combo to get the total weight of each set, which is known as the bob weight.

Bob weights are mounted on each rod journal to simulate the weight of each piston/rod assembly. Brass bob weights are added to or subtracted from each rod journal to get the exact weight of each piston/rod assembly. Once the bob weights are calculated and mounted, the crankshaft is spun up on a balancing machine much as you would spin a tire and wheel assembly on a balancer.

Where this gets tricky is when you opt for heavier H-beam rods or wind up with a low-quality off-shore crankshaft that could use a little help. This is when it becomes time-consuming for the balance shop. This is why we prefer to opt for Scat Enterprises crankshafts and connecting rods because the quality is very consistent and these guys do an incredible job of dynamic balancing. What this means for you is easy engine assembly because your Scat stroker kit arrives balanced and good to go.


004 SCAT Crank Balance LS Stroker 4/28

To give you an idea of how much one gram weighs, we placed a one dollar bill on this very sensitive scale. This is the scale Scat uses to weigh reciprocating mass consisting of the piston, connecting rod, bearings, rings, and the pin clip. Reciprocating mass is weighed one piece at a time, then the total weight is determined to get the bob weight. As you can see, one gram, is not very much.


When the engine balancer winds up with a heavier set of rods, especially with a stroker kit, metal (weight) has to be added to the crankshaft counterweights to get the crank in balance. This is performed with Mallory metal, which is actually tungsten/nickel—heavy metals that add weight to the counterweights. By the same token, there are also segments of the counterweights where metal has to be removed, which calls for the drilling of counterweights to get the weight down.

We’re working with Scat Enterprises on a 416ci LS3 stroker for a Pro Touring 1968 Camaro project. We’re talking a whopping 4.070-inch bore with the long arm of a 4.000-inch stroke via Scat H-beam rods and a forged steel crank. What we expect to get from this engine with its bore and stroke combination is brute torque along with high-end horsepower when it’s time to get it on.


005 SCAT Crank Balance LS Stroker 5/28

Scat has taken this steel stroker crank and added Mallory metal (tungsten/nickel) to the counterweight to get the weight closer to the reciprocating mass consisting of the pistons, rods, rings, clips, and rod bearings.

006 SCAT Crank Balance LS Stroker 6/28

When the connecting rods are weighed each end of the rod is weighed to get the total weight. The Scat box markings match the weight indicated on this scale—452.2 grams for the big end.

007 SCAT Crank Balance LS Stroker 7/28

The small end of the rod tips the scale at 190.4 grams for a total rod weight of 642.6 grams.

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When we weigh each end of the rod and add up the two weights we get 642.6 grams. Lay the rod on the scale and get the true total weight of 643.2 grams.

009 SCAT Crank Balance LS Stroker 9/28

We’ve opted for a set of forged JE pistons (PN 311981) for the LS3 designed for a 4.070-inch bore and a 4.000-inch stroke—virtually square with equal bore and stroke. These slugs sport thin low-tension 1.2mm and 1.5mm rings, which frees up power while giving us exceptional cylinder sealing.

010 SCAT Crank Balance LS Stroker 10/28

The lightest piston weighed in at 392 grams. Each corresponding piston should be lightened to 392 grams.

011 SCAT Crank Balance LS Stroker 11/28

The rod bearings weigh in at 43-44 grams each. Here’s where that 1-gram dollar bill comes into play. There really is a different between 43 and 44 grams—the weight of a one dollar bill.

012 SCAT Crank Balance LS Stroker 12/28

Each ring package weighs in consistently at 34 grams.

013 SCAT Crank Balance LS Stroker 13/28

Scat provides a complete balance sheet with every balanced stroker kit.

014 SCAT Crank Balance LS Stroker 14/28

Scat takes the total weight of each reciprocating mass assembly and builds a bob weight to exactly the total weight of 1,728 grams. Bob weights consist of brass weights where weight can be added or subtracted to get the correct bob weight. One bob weight should weigh the same as two reciprocating assemblies because two opposing assemblies ride on the same journal.

015 SCAT Crank Balance LS Stroker 15/28

Here, a quartet of bob weights await installation on the LS3’s four rod journals.

016 SCAT Crank Balance LS Stroker 16/28

The crankshaft has been placed on the Hines electronic balancer and checked for level setup. It must be perfectly level and spin freely on the fixture.

017 SCAT Crank Balance LS Stroker 17/28

All of the bob weights have been installed on the rod journals. The first order of business will be to spin the crank and determine where metal needs to be added or subtracted from the counterweights.

018 SCAT Crank Balance LS Stroker 18/28

This is the monitor that shows the balancer where weight needs to be added or subtracted as the crank is slowly turned.

019 SCAT Crank Balance LS Stroker 19/28

When metal needs to be removed from a crankshaft counterweight, the counterweight is slowly drilled as shown until we get the weight spot on. If a large amount needs to be removed the amount can be split between two holes.

020 SCAT Crank Balance LS Stroker 20/28

This gives you some idea of how deep the lightening holes can be drilled to remove excess metal in the counterweight. Lightening holes are drilled where we need to remove weight, yet there are places in the same counterweight where heavy metal needs to be added.

021 SCAT Crank Balance LS Stroker 21/28

Once the necessary balancing steps have been made at each counterweight Scat spins the crankshaft again to ascertain proper balancing. Most of the time, there’s the need to return and add or subtract metal to the counterweights.

022 SCAT Crank Balance LS Stroker 22/28

In the wake of this spin-up, we learn where metal still needs to be removed at five-degrees and at 234-degrees. The process of spin-remove-spin is repeated until the crank is within the accepted parameters for the particular application.

023 SCAT Crank Balance LS Stroker 23/28

More metal is removed from the crank counterweight to get it more in line with reciprocating mass weight.

024 SCAT Crank Balance LS Stroker 24/28

The crankshaft shows correct balance with no further drilling or adding metal necessary. The beauty of the Hines electronic balancer is it shows the balancer exactly how much metal to add or remove and how deep to drill. The goal isn’t to get to zero, but to get within an acceptable rage, in this case we got down to 0.23 and 0.28 grams.

025 SCAT Crank Balance LS Stroker 25/28

This counterweight shows where metal has been removed (black arrows) and where it needed to be added (white arrow).

026 SCAT Crank Balance LS Stroker 26/28

Our Scat LS3 crank has been precision balanced for a perfect match with the H-beam rods and JE pistons. What this means for our Pro Touring Camaro is smoothness and long-term durability. Smoothness means less wear and tear.

027 SCAT Crank Balance LS Stroker 27/28

Finally, our LS3 crank gets a quick polish to ensure journal surfaces are free from scoring, which can happen during the installation of bob weights. And with that our LS crank it ready for installation along with the rest of the rotating assembly.

028 SCAT Crank Balance LS Stroker 28/28

Photography by Steven Rupp

Sources

JE Pistons
Cypress, CA 90630
714-898-9763
http://www.jepistons.com
Scat Enterprises
310-370-5501
scatenterprises.com

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