Automotive; a piece of metal, generally "I"-shaped, that connects the crankshaft to the piston in an engine. Is put under incredible amounts of stress at high engine speeds; in some cases more than 24 tonnes of force are required to reverse the direction of a piston from its compression stroke to its power stroke.

It's not easy being a connecting rod. You've got to sit inside a blisteringly hot engine, move up and down at speeds of up to 11,000 RPM, endure rapid acceleration and deceleration, all without snapping at the beam. And you thought your job was tough!

Jokes aside, connecting rods, or rods for short, are one of the single most vital parts in an engine. They allow the up and down motion of the pistons to be synced with the reciprocal motion of the crankshaft, translating the combustion in the chamber to power at the the flywheel.

Manufacture

Connecting rods must be constructed to meet some pretty rigid standards. Among them:
  • Extremely high tensile strength
    A connecting rod that is 5.7 inches in length will endure up to 1500 G's of positive acceleration and 2600 G's of negative acceleration. That's enough to squish a fighter pilot into a ball of goo.
  • Extremely aerodynamic
    Due to the speed at which the rod whips around in the crankcase, it is important that it doesn't have too thick a profile. The slight resistance caused by air friction can add up when you have 4, 6, or 8 rods rotating at 3,000 RPM, and further drains power.
  • Extremely lightweight
    In addition to being strong as an ox, they must be lightweight enough so that they don't contribute to too great of a reciprocating mass. The greater the reciprocating mass, the more energy is lost trying to spin the engine components, and the less makes it to the wheels. This decreases fuel efficiency and horsepower.
  • Extremely clean
    By clean, I mean free of surface flaws, which may be left over from the manufacturing process. Excess metal that exists on the rods, poorly surfaced edges, or barbs left over from the mold are very bad things. These are known as stress risers, and have a tendency to act as weak spots, focusing pressure in their surrounding areas. This leads to cracks that will inevitably cause rod damage.
In the last 100 years, connecting rod engineering has weeded out all of the poor manufacturing techniques, materials, and designs. The following are the ones that have survived the culling:
  • Manufacturing Technique
  • Materials
    • Steel Alloys (such as 4340, also known as carbon steel/chro-moly)
    • Aluminum Alloys (such as 7075)
    • Titanium Alloys (though titanium is usually used least by quantity, it's one of the most expensive)
  • Design
    • H-beam
    • I-beam

Layout

As you can see by the diagram below, different connecting rods can look drastically different. However, they tend to be based around two constructs: H-beam and I-beam.

     I-beam                       H-beam


                                                                                                                                                                                                        
                                                                                                                                                                                                        
                                                                                                                                                                                                        
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First of all, let's go over the primary parts of the rod. Part 1 of the figure is the wrist-end of the rod. This is where the piston wrist pin goes, securing the piston to the rod. Part 2 is the beam portion. Part 3 is the journal end, where the rod connects to the crankshaft journals.

The letter in front of the word "beam" refers to the construction of the beam portion. Viewed from the top, a lateral cross section of the beams would look like this (tilt your head and look at them sideways):

     I-beam                       H-beam
  ....           ....                 ...................
  |  |...........|  |                 |......     ......|
  |  ............   |                 ......|     |......
  |..|           |..|                 |.................|
It's been found over the years that H-beam and I-beam both have their advantages and disadvantages, but on the whole, they're roughly equal for street and mild forced induction usage. Once you get beyond that though, you need to look deeper into the ability of the parts to tolerate lateral thrust and rotational forces. That's a whole other wu in of itself, so we won't go there.

Maintenance and Repair

Due to the fact that you can't exactly look inside your engine to examine its inner workings, you have to assume that your rods are in decent shape if your engine is still running.

However, that could change at a moment's notice. And there's not much you can do about it.

Change your oil when your owner's manual says you should. Change your coolant at the specified intervals. Try to avoid going WOT when you don't need to, and don't run the air conditioner if you can cool off by rolling the windows down. In other words, don't run your car into the ground. Treat it like an investment, and it'll treat you nicely back.

If something does go wrong with your rods, you'll know. Breaking a rod while your engine is in full stride would cause as much damage as your block cracking and beginning to spew coolant onto the crank - IE, your engine would be totaled. Expect to buy a new engine or scrap the car altogether.

Final Words

That pretty much concludes the relevant background information on connecting rods. If the information is requested, I may end up inserting a section here relating to engine rebuilds and proper rod preparation. But only if the people want it.

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