Decoding LS Head Casting Numbers: What They Mean and Why They Matter

If you spend any time around LS engines, you quickly discover that not all LS cylinder heads are created equal. General Motors produced dozens of variations across cars and trucks, aluminium and iron blocks, cathedral and rectangle ports. To make things more confusing, engines with the same displacement often left the factory with different heads altogether.

This is where LS head casting numbers become incredibly useful. Those small raised digits on the top or side of the head can tell you a lot about chamber size, port design, potential airflow, and even which vehicles the head originally came from. For anyone planning an LS swap, performance build, or matching parts to a specific goal, understanding these numbers is essential.

At Malex Motorsports, we speak to customers every week who want to know whether they should keep their stock heads, upgrade to a different factory casting, or move to aftermarket performance options. The first step is always identifying exactly what you already have.

What is a casting number?

A casting number is not a part number. It is the number GM used to identify the mould used to manufacture the head. The number appears raised in the metal because it was part of the sand casting process.

Casting numbers help identify:

• Port style, such as cathedral or rectangle
• Chamber volume
• Valve size
• Likely flow characteristics
• Intended engine family
• Whether it came from a car or truck application

They are usually found under the valve cover or near the rocker area. Some are also visible externally once grime is cleaned away. When buying second-hand heads, always verify the casting number yourself rather than relying on a seller’s description.

Common LS cathedral port casting numbers

Here are some of the better known cathedral port heads LS enthusiasts encounter.

241 heads
These are early LS1 heads found mainly on VT to VY Commodores and C5 Corvettes. They are aluminium, have cathedral ports and are often considered the baseline casting. They have smaller chambers than many truck heads but generally flow less than later 243 and 799 castings in stock form.

243 heads
Very popular with builders. Found on LS2 engines and LS6 applications. They feature improved port design and better flow than 241 castings. They also have smaller combustion chambers which can bump compression slightly when replacing 241 or some truck heads on 5.7 and 6.0 engines.

799 heads
These are often described as the “truck version” of the 243. In reality, the port design is extremely similar and flow potential is almost identical. They are found on many 5.3 litre alloy truck engines, which makes them common, affordable, and a great upgrade for budget LS builds.

For Malex Motorsports customers building responsive street cars or mid-level performance engines, 243 and 799 castings are very popular choices when paired with the right camshaft and supporting hardware.

Common LS rectangle port casting numbers

Rectangle port heads are associated with later high output engines and larger displacements.

823 heads
These appear on LS3 and L92 engines and are among the most common rectangle port heads in Australia. They feature large valves and significantly more airflow potential than cathedral heads in factory form. Their larger chambers are well suited to 6.0 and 6.2 litre combinations.

821 / 0821 heads

You may also see 821 or 0821 casting numbers, depending on how the digits are stamped/raised on the head. These are very similar to the 823 rectangle port casting and are commonly grouped together in the LS community because their port layout and overall performance potential are comparable. If you’re shopping second-hand rectangle port heads, it’s worth knowing that 821/0821 can deliver much the same benefits as 823 when matched with the right intake, camshaft and supporting hardware.

373 and 356 style supercharged heads
These appear on LS9 and LSA engines in factory blown applications. They support significant airflow and are designed to work with forced induction. When combined with stronger valvetrain components and correct tuning, they support very high horsepower levels.

Rectangle port heads like the 823 are the go-to choice for high rpm naturally aspirated builds, stroker engines, and boosted applications where maximum airflow is the goal.

Why casting numbers matter in real builds

Casting numbers matter because they influence almost every other decision in an LS build.

They affect:

• Compression ratio
• Camshaft selection
• Intake manifold compatibility
• Piston to valve clearance
• Flow characteristics
• Valvetrain strength requirements

For example, swapping 243 heads onto a 5.7 litre bottom end can increase compression compared to 241 heads because of the smaller chambers. Installing 823 rectangle port heads onto an engine that still runs a cathedral port intake manifold is a mismatch and will hurt performance unless changed together.

Malex Motorsports often helps customers select supporting parts such as:

• Intake manifolds matched to port shape
• Correct gasket sets for the casting used
• Appropriate camshaft to suit rpm range and port design
• Upgraded trunnion or valvetrain kits
• TR6060 and T56 shifter solutions for swap applications
• Conversion mounts and brackets for engine fitment

Matching parts correctly always delivers better results than simply chasing the “best flowing head” in isolation.

Stock heads vs aftermarket options

Factory heads can achieve impressive performance when ported, but there is a limit based on the casting. Aftermarket heads introduce:

• Revised port designs
• Thicker deck surfaces
• Larger valves
• Stronger materials
• Chambers optimised for performance

For many customers, factory castings like 243 or 823 are ideal foundations. Others prefer to upgrade straight to aftermarket units paired with supporting cam, valve springs and high quality installation components. The correct choice depends on budget, goals and how the vehicle will actually be used.

The smartest next step

The most important thing you can do before choosing heads, cam or intake is simple. Identify exactly what you already have.

Find the casting number, confirm whether it is cathedral or rectangle port, and then define your goals:

• Street drivability
• Track performance
• Drag racing
• Boosted power levels
• Towing or load applications

Once your goals are clear, Malex Motorsports can help you choose the right combination of components to suit. Understanding casting numbers is the key that unlocks smarter decisions and better performing LS builds.

Whether you are building a budget 5.3, refreshing an LS1, installing a TR6060 shifter behind a 6.0, or planning a high output 6.2, the number on top of the head is a small detail that makes a very big difference.