• Your shopping cart is empty!

Core Plug Materials Explained

What Are Core Plugs Made From?

Core Plug & Freeze Plug Material Guide

Core plugs (also known as freeze plugs or expansion plugs) are manufactured from different metals depending on strength, corrosion resistance, and engine application.

The most common core plug materials are:

  • Zinc-plated mild steel (CR4)

  • 403 stainless steel

  • CZ108 brass (63% copper / 37% zinc)

Each material offers different performance benefits. Below is a detailed breakdown to help you choose the correct type for your engine.

CR4 Zinc-Plated Mild Steel Core Plugs (BS1449)

What is mild steel?

Mild steel is a low-carbon steel (0.16–0.29% carbon) made primarily from iron. It is magnetic (ferromagnetic), strong, cost-effective, and widely used in automotive engine manufacturing.

Why is mild steel used for core plugs?

Zinc-plated mild steel core plugs are:

  • Strong and durable

  • Cost-effective

  • Suitable for most standard engines

  • Easy to manufacture and fit

  • Recyclable

The melting point of mild steel ranges from 1450°C to 1520°C, making it highly stable in engine operating conditions.

Corrosion Protection

Untreated mild steel can corrode. For this reason, our steel core plugs are typically supplied with:

  • CR4 silver zinc passivate plating (standard finish)

Zinc plating improves corrosion resistance and protects against rust during long-term use.

Additional protection can be applied during installation using:

  • Red oxide primer

  • Zinc primer

  • Engine enamel or protective metal coatings

Best for: Standard road vehicles where coolant is properly maintained.

403 Stainless Steel Core Plugs (BS EN 10088-3)

403 stainless steel is a heat-treatable chromium steel designed for higher strength applications.

Key Properties

  • Can be hardened by heat treatment

  • High mechanical strength

  • Good impact resistance

  • Resistant to scaling and corrosion up to 649°C (1200°F)

This grade is commonly used in:

  • Valve components

  • Fasteners

  • Turbine parts

  • High-temperature engineering applications

While stainless steel provides improved corrosion resistance compared to mild steel, it is typically used where additional mechanical strength is required.

Best for: High-stress or specialist engine applications.

CZ108 Brass Core Plugs (63% Copper / 37% Zinc)

CZ108 is a high-purity alpha brass known for excellent corrosion resistance and forming properties.

Why choose brass core plugs?

Brass core plugs offer:

  • Superior corrosion resistance

  • Excellent durability in coolant systems

  • Non-magnetic properties

  • Long service life

  • Ideal performance in classic and older engines

Because brass resists rust far better than steel, it is often preferred in:

  • Classic cars

  • Vintage engines

  • Marine environments

  • Engines prone to internal corrosion

Machining must be carried out at slower speeds with light feeds due to its material properties.

Best for: Maximum corrosion resistance and long-term reliability.

Brass vs Steel Core Plugs – Which Is Better?

Brass core plugs are more corrosion resistant.
Steel core plugs are stronger and more economical.

For most modern vehicles, zinc-plated mild steel is sufficient.
For long-term durability or restoration projects, brass is often preferred.

Stainless steel is generally chosen for strength-focused or specialist applications.

Quick Comparison Table

Material Strength Corrosion Resistance Cost Ideal Use
Zinc-Plated Mild Steel High Moderate £ Standard engines
403 Stainless Steel Very High Good ££ High-stress engines
CZ108 Brass Moderate Excellent ££ Classic & corrosion-prone engines

Frequently Asked Questions About Core Plug Materials

What material are most core plugs made from?

Most factory-installed core plugs are made from zinc-plated mild steel.

Are brass core plugs better than steel?

Brass offers better corrosion resistance, but steel is stronger and cheaper. The best choice depends on your engine and environment.

Do zinc-plated core plugs rust?

Zinc plating significantly reduces rust risk, but long-term durability depends on proper coolant maintenance.

Are stainless steel core plugs necessary?

They are not required for most road cars but are useful in higher-strength or specialist applications.