Connecting Rods 101 Design,Material,Modifications,and Machining

Posted by spoonman, Mar 22, 2010

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  1. spoonman

    spoonman Supporting VIP

    Joined Jul 20, 2007
    Omaha, Nebraska
    The "bottom end" of an engine is composed of three main components, the Piston, the Connecting Rod, and the Crankshaft. Connecting rods can be manufactured from many different materials and designs, each type of connecting rod is designed for a certain application. When building an engine it is important to choose the correct connecting rod, not everyone can do this though because the proper information is not provided. This article will give you the information you need to properly decide which connecting rod is ideal for your engine build.

    What is the engine's intended application?

    Before you can decide what connecting rod to use in your rotating assembly you need to have an idea of what the engine's power goals will be and how the engine will be used. Will the engine be a high revving horsepower machine? Will it be a daily driver with low end torque? Is the engine destine for drag racing, auto cross, or time attack? Higher power levels place increased compressive force on the connecting rods. Higher RPM's increase the tensile strain on the connecting rods. Both go hand in hand, more power requires a beefer rod. With every rotation of the crankshaft the rod will be alternately stretched and compressed. Connecting rod failures are usually caused by over revving the engine, which places a tremendous amount of stress on the rod bolts. The rod bolts will come apart as the piston travels up on the exhaust stroke, the force and inertia of the rotating assembly rips the rod cap off of the connecting rod. If the connecting rod fails because it bends this is usually due to engine knock from a bad tune, not a bad set of rods.

    Connecting Rod Design

    Connecting rods are produced in two major designs, the "I" Beam and the "H" Beam.
    The "I" Beam design is used in almost all production engines. An "I" Beam rod can handle high compressive loads while also providing good tensile strength. The design of the "I" Beam allows for the rod to have better tensile strength and is the choice for high RPM engines.

    An "H" Beam rod has two flat sides that are perpendicular to the piston pin, with a thin center section in the middle. The "H" Beam design is very stiff so it can handle higher compressive loads than an "I" Beam without bending. In high RPM operation the "H" Beam connecting rod should not be used, the design limits the amount of material that can be added and will lack the tensile strength required. Most "H" Beam rods are lighter and stronger than stock rods because they use less material combined with better engineering. "H" Beam rods are good for low level power production.


    Connecting Rods can be made from various grades of steel, aluminum, and titanium. Steel rods are the most widely produced and used type of connecting rods. Their applications are best used for daily drivers and endurance racing due to their high strength and long fatigue life. The only problem with using steel rods is that the material is extremely heavy, which consumes more power and adds stress to the rotating assembly. Performance steel rods can be made from 4340 and even 300M grade steel. The tensile strength, yield strength, and hardness of 4340 steel depends on the temperature at which the steel is forged, and how the steel is heat treated. Variations in the tempering temperature and quenching procedure can produce extremely different results with tensile strength and yield strength. This is why it is important to buy your rods from a reputable manufacturer. 300M steel alloy is stronger than 4340 and can be used to manufacture steel rods that are lighter than their 4340 counter parts by using less material. 300M rods rated at the same strength level as 4340 rods can be 10% to 25% lighter.

    Aluminum rods are used widely in high horsepower, high revving drag engines. Aluminum rods can be manufactured to the same strength level as a steel rod due to large amount of material that makes up the rod but still be 40% lighter than a steel connecting rod. Reducing the weight of the rods reduces the mass of the rotating and reciprocating parts and allows the engine to rev faster and rev higher safely. In addition to better throttle response than steel, aluminum's lighter weight can reduce vibration and stress on the crankshaft. This reduced stress from inertia loading results in connecting rod journal bearings looking "good as new" after thousands of miles. The rods can stretch, and they may fatigue and fail, but unlike the aluminum rods from the 70's modern aluminum rods are much stronger and can be used in daily drive engines. Depending on the use and power levels aluminum rods can last 50,000 miles on an engine that is street and strip, the secret to making aluminum rods last long is to not constantly be over revving the engine when you drive to work. I am a very strong supporter of using aluminum rods in every engine I build.

    Titanium rods are both lighter and stronger than steel. Unfortunately titanium is extremely expensive so they are only used in drag racing and endurance racing applications in the most expensive engines. Unless you are building the ultimate in endurance racing engines use a steel rod. For drag racing, using an aluminum rod is both cheaper and lighter than using titanium.

    Connecting Rod Machine Work

    When you hand your new connecting rods off to your machinist there are only a few procedures performed before they are ready to be popped into your engine. The machinist will first stamp the number of the designated cylinder that the rod will be place in on each connecting rod, both at the base of the rod and on the rod cap. On the 4G63 I number the rods 1,2,3,4 from the flywheel to the harmonic damper side of the engine. The machinist will then component balance each rod. Connecting rods are balanced on both the "big" end and the "small" end of the rod. This is because the top half of the connecting rod (small end) is considered a reciprocating weight, while the bottom half (big end) of the connecting rod is considered a rotating weight. The machinist will weigh all the "small" ends of the rods and then grind from the top of the rods to match the weigh of the lightest rod. Then the machinist follows the same procedure on the "big" end of the connecting rod.

    Suggested Connecting Rod Choice Cased on Material, Application, and Design.

    Steel H Beam - Often used in low horsepower (600 max) and low RPM (8000 max) racing and daily driving. For DSM engines I suggest Eagle H beams for there price and known durability.

    Steel I Beam - To be used in moderate to high horsepower (Forged 900 max, Billet 1200 max) and moderate RPM (9500-10,000 max). Suitable for daily driving, endurance racing, and drag racing. Pauter and Crower makes the highest quality steel I beam connecting rods for the DSM.

    Aluminum - To be used in high horsepower (1,200 max) and high RPM (14,000 max) applications. Due to low fatigue life, when used under high stress situations drag racing only. Can be used safely with daily driving. Groden and Magnus aluminum rods are the best choice for DSM's, strong and light.

    Titanium I Beam - To be used in high horsepower (1,400 max) and high RPM (14,000) applications. Can be used in daily driving, endurance racing, and drag racing.

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    If you have any question or suggestions please feel free to PM me any time!

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    Last edited by a moderator: Jun 5, 2014

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