Alkaline Earth Metal
Magnesium
The lightest structural metal used in engines — chosen when weight matters more than anything else.
- Atomic Number
- 12
- Atomic Mass
- 24.305 u
- Melting Point
- 650°C
- Density
- 1.74 g/cm³
Overview
Magnesium is the lightest metal ever used as a structural material in engineering — roughly two-thirds the density of aluminum and about a quarter the density of steel. In an engine, that weight advantage makes it the material of choice for non-structural covers where every gram matters but load-bearing strength doesn’t.
Pure magnesium is rarely used on its own; like most engine metals, it’s alloyed with small amounts of aluminum, zinc, or manganese to improve strength and corrosion resistance while keeping most of the weight savings.
Atomic Structure & Properties
Magnesium’s electron configuration is [Ne] 3s², with two loosely-held outer electrons available for metallic bonding. Its low atomic mass combined with a relatively open crystal structure is what gives magnesium its exceptionally low density among structural metals.
Hexagonal Close-Packed (HCP) Lattice
Magnesium atoms stack in hexagonal layers arranged in an A-B-A-B pattern, with three interior atoms (highlighted in blue) sitting in the gaps between layers. Each atom touches 12 neighbors — the same packing efficiency as aluminum’s FCC structure, just arranged differently.
That A-B-A-B stacking sequence is the key difference from aluminum’s face-centered cubic structure, which stacks in an A-B-C-A-B-C pattern instead. The practical consequence: HCP metals like magnesium have far fewer independent “slip systems” — directions atoms can shift relative to each other without breaking — which is why magnesium is noticeably less ductile than aluminum at room temperature and more prone to cracking if bent or stressed the wrong way.
This same structural limitation is why magnesium parts are typically diecast (poured molten into a mold) rather than stamped or formed like sheet aluminum — diecasting sidesteps the need to deform solid magnesium at room temperature.
Why Engines Use Magnesium
Magnesium shows up specifically where a part needs to be as light as physically possible and isn’t carrying structural or combustion loads — valve covers being the clearest example. A magnesium cover can weigh roughly half what an equivalent aluminum cover would, and about a quarter of a steel one.
The tradeoff is corrosion resistance: magnesium reacts more readily with moisture and road salt than aluminum does, so magnesium parts typically need a protective coating or careful alloy selection to avoid long-term surface corrosion — one reason it’s used selectively rather than throughout an engine.
Where You’ll Find It
On the Toyota 2AZ-FE 2.4L, magnesium appears in one part:
- Valve Cover Magnesium Alloy Diecast
As more engines are added to the site, every part using magnesium will link back here.
Common Questions
Is magnesium the lightest metal used in cars?
Among metals used structurally in vehicles, yes — magnesium is the lightest, at roughly two-thirds the density of aluminum. Lithium is technically lighter as a pure element, but it’s far too chemically reactive to use as a structural metal.
Why is magnesium less common in engines than aluminum, if it’s lighter?
Magnesium’s hexagonal close-packed structure gives it far fewer ways to deform under stress than aluminum’s face-centered cubic structure, making it more brittle and harder to form into complex load-bearing shapes. It also corrodes more readily. Both limitations restrict it to lightweight, non-structural parts like covers rather than blocks or heads.
What’s the difference between HCP and FCC crystal structures?
Both pack atoms at the same 74% efficiency with 12 touching neighbors, but they stack in different repeating patterns — HCP repeats every 2 layers (A-B-A-B), while FCC repeats every 3 layers (A-B-C-A-B-C). That difference in stacking sequence is what gives HCP metals like magnesium fewer directions to deform in, making them generally less ductile than FCC metals like aluminum or copper.
See where Magnesium sits on the Periodic Table
View all 118 elements and explore the ones used across every engine on this site.