What are car connecting rods made of?

Most car connecting rods are forged steel, designed for strength and durability. Lighter engines may use aluminum rods, while older or certain applications still use cast iron or even titanium in limited contexts.

Common materials used in connecting rods

Understanding the main materials helps explain why rods weigh as they do and how they endure the stresses of operating an engine. Below is a look at the primary materials in use today.

Forged steel

By far the most common material for mass‑market engines, forged steel rods are typically made from high‑strength alloy steels such as 4340 (chromium‑molybdenum) or similar families. They are heat‑treated to maximize fatigue life and can withstand high torque and RPM before failure. They balance strength, ductility, and cost, making them suitable for gasoline and diesel engines across a wide range of applications. The rods are formed by hammer‑forging or closed‑die forging and then machined and sometimes nitrided or shot‑peened for extra surface hardening.

Powder-metallurgy steel

Powder‑metallurgy (PM) rods are produced by pressing and sintering steel powders into near‑net shapes, followed by finishing machining. They offer good consistency, reduced weight, and cost advantages for certain engine programs. PM rods can meet the needs of many modern engines while simplifying manufacturing, but may not match the absolute strength of the best forged steel in extreme high‑rpm or high‑torque duty, so their use is typically guided by engine design and torque targets.

Aluminum alloy

Aluminum connecting rods are significantly lighter than steel, which helps reduce reciprocating inertia and can improve throttle response and efficiency in high‑revolution engines. They are commonly used in some high‑performance or sports cars and racing engines. Aluminum rods use high‑strength alloys (often 7000‑series) and are carefully engineered to withstand heat and wear; they can be more expensive and have different fatigue characteristics than steel, making them more sensitive to lubrication and piston/ring choices.

Cast iron

Cast iron rods — typically gray iron or ductile iron — appear mainly in older or very low‑cost engines. They are heavier and generally less fatigue‑resistant at high RPMs than forged steel, but can be robust for low‑speed or low‑torque applications. In modern passenger cars, cast iron rods are uncommon but can still be found in some older designs or specialized industrial engines.

Titanium

Titanium rods offer a superior strength‑to‑weight ratio but at a much higher cost and with stricter material compatibility requirements. They are used in some limited‑production or racing applications where weight savings justify the price and complexity. For typical street cars, titanium connecting rods are rare and generally found only in exotic or race‑spec builds.

Manufacturing methods

Connecting rods can be manufactured using several methods, with the choice guided by performance targets, cost, and durability requirements. The main methods are forging, casting, and powder metallurgy.

Forging

In forging, a billet is shaped under high pressure to align the metal grains and improve strength. Closed‑die or hot‑forging is common for forged steel rods, followed by precision machining and surface treatment. Forged steel offers excellent fatigue resistance and durability, making it the default choice for most engines.

Casting

Cast iron rods (and some aluminum rods) involve pouring molten metal into a mold. Casting is usually cheaper and can be sufficient for low‑to‑moderate performance applications, but cast rods typically exhibit lower strength and fatigue life compared with forged rods, especially at higher RPMs or heavy loads.

Powder metallurgy

Powder‑metallurgy rods are formed from compacted steel powders and then sintered. This method enables tight tolerances and consistent material properties, often reducing weight and cost for mass‑market engines while maintaining adequate strength for many duty cycles. Finishing heat treatment and machining complete the process.

Performance considerations and durability

The material choice affects weight, stiffness, fatigue life, and heat tolerance. Heavier rods add reciprocating mass, impacting efficiency but increasing inertia tolerance; lighter rods reduce inertia and can improve high‑rpm performance but may require more aggressive lubrication and precise balancing. In diesel engines, rods also have to handle significant torque and heat, influencing material selection and design.

Summary

In short, the standard in most cars is a forged steel connecting rod, with aluminum, cast iron, titanium, or PM steel used in niche cases to balance weight, cost, and performance. The exact material depends on engine type, duty cycle, and the manufacturer's design goals.

Are titanium rods worth it?

Titanium is an incredibly stable material to work with and another benefit found is that the set-up with Titanium rod is one and done. Unlike aluminum, Titanium will not grow so there is never a difference from the first bang to the last.

What material is used for connecting rods?

Materials. The materials used for connecting rods widely vary, including carbon steel, iron base sintered metal, micro-alloyed steel, spheroidized graphite cast iron. In mass-produced automotive engines, the connecting rods are most usually made of steel.

Why are connecting rods cracked?

Connecting rods fracture due to mechanical failure from factors like metal fatigue, over-revving, or hydro-locking, which can cause extreme stress on the material. Other causes include insufficient lubrication, leading to heat buildup from friction, and manufacturing or installation errors. 
This video explains how connecting rods are manufactured using a fracture-split method: 58sJim's Automotive Machine Shop, Inc.YouTube · Dec 27, 2020
Causes of connecting rod fractures

• Mechanical stress and fatigue:
  • Over-revving: Operating the engine above its designed limits puts excessive stress on the rod. 
  • Abnormal combustion: Detonation or pre-ignition can create extreme, sudden pressures inside the cylinder. 
  • Metal fatigue: Repeated stress cycles over time, especially under high load, can weaken the metal and lead to failure. 
• Lubrication issues:
  • Oil starvation: A lack of proper lubrication causes metal-on-metal contact, generating excessive heat that weakens the rod and can lead to seizure. 
  • Incorrect bearing clearance: Both too tight (causing friction) and too loose (causing oil pressure loss) bearing clearances can lead to failure. 
• Hydrolocking:
  • When a cylinder fills with a liquid, like water from a leaking head gasket or coolant, the piston cannot compress it. This results in a sudden and immense force on the connecting rod, often causing it to bend or break. 
• Manufacturing and installation errors:
  • Defects: Material flaws or poor design in the connecting rod itself can make it more prone to failure. 
  • Improper assembly: Incorrectly torqued bolts or poor alignment during engine assembly can create stress points that lead to fracture. 
  • Incorrectly assembled fracture-split rods: Using a fracture-split rod cap backward during installation can flatten the fracture surfaces and cause deformation, leading to bearing damage and eventual failure. 
• Component failure:
  • A broken piston pin can transmit force in a way that breaks the connecting rod. 

This video demonstrates how connecting rod failure can be caused by a lack of lubrication: 44sCP-CarrilloYouTube · May 4, 2017

Are connecting rods forged or cast?

Connecting rods, usually used in internal combustion engines, can be manufactured by casting or sintering forging. Choosing one of them largely depends on the application, performance requirements, and cost.