Balance Rotating Assemblies

Why Getting Every Engine Balanced Is Important

Jake Amatisto Jan 13, 2014 0 Comment(s)
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We recently got to hang out with the guys from West Coast Balance in Santa Ana, California, to learn about the procedures they use to meticulously balance rotating assemblies—everything from high-powered drag racing monsters to high revving endurance combos and everything in between. In these days of stroker cranks, lightweight rods and pistons, engine balancing is even more important. And while the aftermarket cranks from overseas are strong and affordable, they require balancing if you want your engine to last.

Shop operator Andy Maron has been balancing rotating assemblies since the late 1970s and got his start on diesel engines, but these days, his SoCal shop is geared towards high-performance and racing engines. The procedure starts with simply weighing the connecting rods and then the pistons to find the lightest one of each bunch. Then, the remaining rods and pistons are shaved on a belt sander to match the weight of the two lightest components. At this point, a set of rings, the pin, and the locks are weighed, as are the rod bearings. The idea is to get to total bob weight of the rod assembly. Once that number is known, brass bob weights are stacked and bolted to the crankshaft and spun up on a crank balancer. The bob weights mimic the presence of the rod and piston assembly and the crank balance machine senses where the crank either needs weight added or needs weight taken away. This becomes tricky when a customer wants to balance a stock crank with lightweight rods, or when a lower quality crank is used. It also gets tricky when you want to convert an engine with light rods to heavier ones. Maron explained, "Guys will bring in LS7 engines to convert them over to H-beam rods (likely because they want to boost them). Well, you have to compensate for that added weight in the crank, which is substantial on those, but customers ask for it."

The cost of your balance job relies mainly on not how much metal needs to be removed, but rather how much needs to be added. "We sometimes have to add a lot of weight to a crank in order to make it right, and that can get expensive." Heavy metal, or in West Coast Balance's case, Tungsten/Nickel, is inserted into the crank counter weights in order to achieve proper balance. "When we have to add heavy metal, we freeze the slugs before pressing them in," Andy said. By freezing them, they shrink by a few thousandths and can easily be pressed into the side of the counter weight. When the slugs reach ambient temperature, they expand, locking them in place; even at 10,000 rpm.

We learned that small-block Chevys can be confusing when it comes to balancing depending on if you have an early or late-model engine. Because of the one-piece rear main seal on late model SBCs, you could say the engine is sort of half-internal, half-externally balanced. The rear seal on late small-blocks rides on a large concentric journal, so there's no offset flange like on early motors. To compensate for the added weight, an externally balanced flexplate must be used, even though your harmonic balancer may be a zero balance.

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1. If you do the math, a ¼-ounce of unbalance 4-inches from the center of rotation creates crazy amounts of forces: 7 lbs at 2,000 rpm, 28 lbs at 4,000, 63 lbs at 6,000 and well over 110 lbs at 8,000 rpm. This can be detrimental to the bearings and severely reduce your engine’s lifespan.

Each Component Weighed And 3/17

2. Starting with the rods, each component of the piston assembly gets weighed and recorded.

All Pieces Match Weight Of Lighest 4/17

3. Once the lightest rod is found, the other seven pieces are matched by removing material with a belt sander.

Areas Okay To 5/17

4. There are a couple areas on the rod where it’s safe to grind: the side of the big end and the very tip of the small end. It’s incorrect to grind on top of the rod cap since that area needs to be as strong as possible in order to stay perfectly round under load.

Pins Locks Rings And 6/17

5. The pins, locks, rings, and bearings also get factored into the final weight, but remember there are two bearings for each journal, so the weight of the bearing is typically just doubled. West Coast Balance also factors in half of a gram for the oil that will stick to the crank.

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