The engine block is the foundation everything bolts to, and the choice between cast iron and aluminum alloy fundamentally shapes an engine's weight, thermal behavior, durability, and cost. Neither material is universally superior — the right pick depends on the application.

Cast Iron (Grey Iron / Compacted Graphite Iron)

• Density around 7.2 g/cm³ — heavy, but that mass brings excellent vibration damping and noise suppression.
• Superior wear resistance. Iron cylinder bores can run directly against piston rings without additional treatment.
• Higher tensile strength and fatigue limit, making it the default for high-cylinder-pressure diesel applications. The Ford 6.7L Power Stroke and Cummins 6.7L ISB both use compacted graphite iron (CGI) blocks to handle peak cylinder pressures above 180 bar.
• Thermal expansion coefficient ~11 µm/m·°C. This lower expansion rate means tighter piston-to-wall clearances can be held from cold start to operating temperature.

Aluminum Alloy (typically A356 or A319 series)

• Density around 2.7 g/cm³ — roughly 60% lighter than iron for the same volume. A typical aluminum 4-cylinder block saves 25-30 kg over its iron equivalent.
• Thermal conductivity about 150 W/m·K versus iron's 50 W/m·K, meaning aluminum sheds heat roughly three times faster. This reduces hot-spot risk and allows smaller cooling passages.
• Thermal expansion ~23 µm/m·°C — more than double iron's rate. This is why aluminum blocks almost always need cylinder bore treatment: pressed-in iron sleeves (Honda B-series), plasma-sprayed coatings (BMW N52), or silicon-rich alloy bores like Alusil (Porsche M96). Without protection, the soft aluminum would score within thousands of miles.
• Lower stiffness (Young's modulus ~70 GPa vs iron's ~100-170 GPa), so aluminum blocks use thicker webbing and more ribbing to match rigidity, partially offsetting the weight savings.

Rule of thumb for thermal expansion: An aluminum block with a 90 mm bore diameter will grow roughly 0.16 mm in diameter going from 20°C ambient to 100°C operating temperature (90 mm × 23 µm/m·°C × 80°C = 0.166 mm). An iron block under the same conditions grows only about 0.08 mm. Engineers set cold piston clearances based on these numbers — too tight on an aluminum block at cold start causes scuffing, too loose causes piston slap until warm.

Modern hybrid approaches are common. GM's LS-series V8 came in both iron (LM7 truck) and aluminum (LS1 Corvette) versions sharing the same architecture. The aluminum LS1 block weighed about 25 kg less. Ford's 5.0L Coyote uses an aluminum block with plasma-transferred wire arc (PTWA) bore coating, eliminating iron sleeves entirely for both weight and heat-transfer advantages.

For forced-induction builds making serious power on stock blocks, iron tends to survive higher cylinder pressures before cracking. The legendary 2JZ-GTE's iron block routinely holds 800+ hp on stock main caps. Aluminum blocks at similar power levels often need aftermarket girdles or billet replacements.