Racing Rotating Assemblies: Cranks, Rods and Pistons - Engine Builder Magazine

Racing Rotating Assemblies: Cranks, Rods and Pistons

Building a performance engine requires assembling the optimum mix of rotating components that are compatible with the block and heads, properly matched with each other, and balanced to precise tolerances.

The easiest way to get the right combination of parts is to buy a complete rotating assembly from a supplier who offers such kits.  Most suppliers offer a wide range of rotating assemblies for street, strip or circle track applications. A complete kit takes the guesswork out of matching the rod lengths and piston configurations with a stroker crank, or matching piston and rod weights with the counterweights on a crank (particularly lightweight cranks). For an extra fee, many suppliers will balance their rotating assemblies for you, which they say reduces the risk of balancing errors that sometimes occur when cranks are incorrectly balanced or over-drilled to correct a heavy spot.

Though crankshaft, connecting rod and piston kits are often marketed directly to racers who want to assemble their own engines in their garage, kits are certainly an option for professional engine builders who are working within time and/or budget restraints, or who lack their own balancing equipment. Buying a complete rotating assembly (which may also include bearings and piston rings) versus sourcing the crank, rods, pistons, rings and bearings from different suppliers reduces the risk of mismatched parts that can cause assembly problems, durability and balance problems. It’s one-stop shopping – and only one supplier to deal with if there are any problems.

For example, it makes no sense to spend big bucks on a lightweight crankshaft, then mate it with a set of relatively heavy connecting rods and pistons. The advantages of using a lightweight crank (faster throttle response and more rapid rpm changes) would be reduced because of the heavy pistons and rods. A lightweight crank must be used with lighter pistons and rods to take full advantage of the reduced rotating mass of the crank.

Mismatched parts can also create balance problems if the mass of the counterweights on a crank don’t closely match the reciprocating mass of the pistons and rods. A lightweight crank has smaller counterweights because it is designed for lighter pistons and rods. If you try to use pistons and rods that are too heavy for the crank, balancing the crank will require adding slugs of expensive Mallory (heavy metal) to offset the added mass. That adds weight back to the crank and undermines the advantages of buying a lightweight crankshaft to reduce weight.

It’s a Balancing Act

A precision balance job is absolutely essential for any high revving performance engine, but it’s also recommended for street performance engines, too. Balancing reduces loads and vibrations that stress metal and can eventually lead to component failure. What’s more, a smoother running engine is a more powerful engine. Less energy is wasted by the crank as it thrashes around in its bearings, which translates into a more usable power at the flywheel.

If a rotating assembly is put together without balancing all of the individual components, it’s impossible to say whether or not the assembly will be within acceptable tolerances for balance.  The counterweights on stock cranks are sized for stock connecting rods and pistons. Replacing the stock rods and/or pistons with aftermarket performance parts will likely upset the balance because the new parts will usually have a different weight (usually lighter, but not always).

A difference of a few grams may not seem like much, but it all adds up. A few grams here, a few grams there, and pretty soon you’ve created an imbalance that can produce noticeable vibrations and harmonics at various engine speeds.  Imbalance usually gets worse the higher the engine revs due to centripetal forces that multiply exponentially with rpm. Double the rpm and you quadruple the force of the imbalance.

Many crankshaft suppliers publish “target bobweight” specifications for their cranks. This allows engine builders to choose rods and pistons that more closely match the design bobweight, or to estimate how much effort it will take to balance the crank if the bobweight of the rods and pistons vary significantly from the target bobweight.

The counterweights on the crankshaft are supposed to offset the reciprocating and rotating mass of the pistons, rings, wrist pins, rods and bearings. The mass of each counterweight should equal 50 percent of the reciprocating weight (the piston, wrist pin, rings and small end of the connecting rod), and 100 percent of the rotating weight of the big end of the rod and rod bearings (which you have to multiply times two on V6 and V8 engines because each throw on the crank is connected to two rods and pistons).

Though many suppliers publish factory weights for pistons and rods, the only way to know for sure how much these parts actually weigh is to weigh them on an accurate gram scale. The same for the bearings and rings. A special support must be used when weighing the small and big ends of the rod to determine the weight of each end.

Once the reciprocating and rotating weights have been measured on a scale and calculated, a bobweight that equals 50 percent of the reciprocating weight and 100 percent of the rotating weight can be assembled and mounted on the crankshaft journal before the crank is spun on a balancing machine. The balancer will then detect any imbalance and show you where weight needs to be removed or added to achieve proper balance.

Most of the piston suppliers we spoke with said their off-the-shelf performance pistons and custom pistons sets are within plus or minus one or two grams of each other, though some piston sets can vary up to 3 or 4 grams or more. One gram is the approximate weight of a dollar bill, and it takes 28 grams to equal one ounce.

The basic idea behind matching pistons is to weigh each piston, note all their weights, then match the entire set to the weight of the lightest piston. Some engine builders say they weigh and match pistons to within 0.5 gram or less when balancing an engine. Others say plus or minus a gram is close enough, so there’s usually no need to check or match piston weights provided you are sourcing your pistons from a quality manufacturer.

All the piston manufacturers we spoke with cautioned against trying to lighten performance pistons significantly either for balancing purposes or to further reduce weight because doing so may weaken the piston or create stress risers that could cause a piston to fail. A lightweight piston has already had most of the “unnecessary” metal removed to minimize its weight. Drilling or machining away additional metal in the pin boss area or under the crown could weaken the piston to the point where it might crack, collapse or pull apart under high load or speed.

If you have to remove weight from a piston, the safest areas for doing so are usually behind the oil ring or along the edges of the pin boss towers. Avoid drilling or cutting near the pin boss radius, or under the piston crown. If you don’t know where to remove metal, contact the piston supplier for their advice.

An alternative method of matching piston weights is to also weigh the small ends of all the connecting rods, then mix and match the rods and pistons to equalize the total reciprocating weight of each piston and rod assembly as much as possible (combining lighter rods with heavier pistons, and vice versa). This may eliminate the need to drill or grind altogether.

Rod weights tend to vary more than piston weights, as much as 4 to 5 grams in many instances, though some rod manufacturers say their rods are within plus or minus one gram. Rod weights on the large and small ends are matched by grinding away metal until the weights are equalized to within 1 gram or less. Rods should always be ground in a direction perpendicular to the crankshaft and wrist pin, never parallel as this can leave scratches that may concentrate stress causing hairline cracks to form.

If you’re building an engine with a stroker crank, the rod length will obviously be different than stock rods to accommodate the longer stroke. This will also change the rotating and reciprocating weights of the assembly.  Longer rods are heavier, but not as much as you might think because the counterweight only has to offset 50 percent of the reciprocating mass. What’s more, a longer rod moves the pin up higher in the piston, which usually means the piston can be lighter (which helps offset the added weight of a longer rod). For higher output applications, shorter rods may be the way to go. A taller piston is heavier, but it can also have thicker and stronger ring lands.

With longer strokes, it may be necessary to reduce the outside diameter of the counterweights so the pistons will clear the crank at bottom dead center. Smaller counterweights mean lighter pistons and rods are necessary to achieve proper balance (or you have to add heavy metal to the counterweights).

Internal and External Balancing Facts

On “internally balanced” engines, the counterweights are balanced to the pistons and rods. On “externally balanced” engines, additional weights on the flywheel and/or harmonic damper assist the crankshaft in maintaining balance. An engine may have to be externally balanced if the counterweights are too thin or too small to achieve internal balance by themselves (as is the case with small block Ford engines and Chevy LS engines).

The main advantage of internal balance is that once the rotating assembly is balanced, it will stay in balance. You can change the flywheel, clutch and/or harmonic damper without affecting the engine’s internal balance. These external parts should also be balanced separately to make sure they don’t cause any vibrations.

On an externally balanced engine, the flywheel and damper must be mounted on the crank prior to balancing. Once balance is achieved, the index position of the flywheel has to be marked so it can be reassembled in the correct position to maintain proper balance. If the flywheel is later removed for resurfacing and is replaced without indexing it back in its original position, balance is lost. The same holds true if the flywheel is replaced with a different one. The whole engine will have to be rebalanced with the new flywheel.

Racing cranks also require a good harmonic balancer. The balancer helps dampen torsional vibrations that may cause a crankshaft to fail or the nose to crack. One crankshaft manufacturer recommends using the lightest and smallest diameter dampener, and balancing the crankshaft with the damper bolted in place. Not balancing the crank with the dampener in place is like trying to balance a wheel with a flat tire.

For racing applications, the same crank manufacturer also recommends using a dampener with an elastomer ring rather than a fluid filled dampener or one with moving parts. They say engine speed changes too quickly in a racing engine for dampeners that are “self-balancing.” A conventional elastomer dampener, in their opinion, eliminates any chance of cold start vibrations and reduces the risk of nose failure on the crankshaft.

Balancing Tips

One crank manufacturer we interviewed said some shops don’t know how to balance cranks correctly, don’t check the accuracy of their equipment often enough, and don’t use the best procedures for correcting imbalances. A precision balancing shop, we were told,  should use an engine lathe rather than a drill press to remove metal when balancing cranks.

When a crankshaft is spun in a balancer, sensors detect wobble that reveal the amount and approximate index location of any imbalance. The machine then shows the user where metal either needs to be added or removed to achieve proper balance. It usually takes several spins to narrow down and correct the crank to the point where it is within the desired range of balance (plus or minus a few grams or less).

If the crank has a heavy spot, metal is removed from the counterweight by drilling or machining. Drilling is quick and easy, and can usually be done while the crank is still mounted on the balancer provided the balancer is set up with a drill press. But if a lot of metal needs to be removed, the crank can end up looking like swiss cheese. Holes create turbulence and may also create stress risers that could lead to cracking and failure down the road. 

If more than two holes per counterweight are required to correct an imbalance, the counterweights should be machined in an engine lathe. Machining the counterweights to trim weight requires extra labor, but is a cleaner, safer approach to balancing that also helps to reduce windage inside the crankcase and the risk of fatigue failure.

If an engine is externally balanced, and is heavy on one end, one crank manufacturer says it’s better to take the weight off the flywheel or damper than the crank. In cases where a crank needs extra weight added (as may be the case with some stroker cranks), counterweights have to be drilled so slugs of heavy metal can be inserted in the holes.

For information on suppliers of complete rotating assemblies, see the March issue of Engine Builder magazine or click on the "Buyers Guides" tab. Particular thanks goes to Callies, CP Pistons, Diamond Pistons, Eagle, Federal-Mogul, Lunati, MAHLE Clevite, Racetech Pistons, Ross Pistons, Scat, United Engine & Machine, Wiseco and Wossner Pistons for their assistance with this article. the easiest way to get the right combination of parts is to buy a complete rotating assembly from a supplier who offers such kits. most suppliers offer a wide range of rotating assemblies for street, strip or circle track applications.a precision balance job is absolutely essential for any high revving performance engine, but it
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