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Racing Rotating Assemblies Cranks, Rods and Pistons
The crankshaft, pistons and connecting rods inside an engine convert thermal energy in the cylinders into rotational energy that produces usable horsepower and torque at the flywheel.
By Larry Carley
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. Ba...
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