Ever since primitive man learned how to smelt iron about 3,000 years ago, the metal has been synonymous with durability and strength.
For most of this century, automobile engines have been made with cast-iron blocks, the heart of any engine.
The block contains the cylinders, coolant jackets and shaft passageways around which the hundreds of engine parts will be assembled.
A good block is the starting point of a good engine, capable of having tight specifications that will provide for long wear on the crankshaft bearings, piston rings and rods.
Although cast-iron blocks and cylinder heads still dominate, aluminum is increasingly being used by designers to reduce weight and improve fuel efficiency.
But are aluminum-block engines as durable as iron-block engines? Are the aluminum blocks more susceptible to metal warpage under high heat? Can they handle contaminated oil and coolant, resulting from owner neglect, as well as iron-block engines?
There is a lot of disagreement on these issues.
Jim Walker, chief engineer for General Motor's base engine technology, said the decision to substitute aluminum for iron is purely economic. Aluminum makes for lighter and more fuel-efficient engines.
Although iron still accounts for 95% of the blocks and 83% of the cylinder heads GM turns out, aluminum is showing up more and more.
The majority of Cadillacs have either an aluminum head or an aluminum block, for example.
One limitation to aluminum is cost: Iron costs about 25 cents a pound; aluminum is about $1 a pound, Walker said.
But the weight savings of aluminum can be enormous.
A four-cylinder aluminum block weighs about 50 pounds less than the same block made of iron.
That savings allows 50 pounds to be trimmed elsewhere from the car.
For a typical V-8 engine, the difference is 75 pounds for the block and double that for the entire car.
Because aluminum weighs only half as much as iron, you might think the weight savings would be even greater.
But aluminum loses one-third of its strength at 200 degrees, so aluminum blocks must have thicker structures to compensate.
If properly designed, an aluminum engine should be every bit as strong and durable as an iron one, Walker said.
The cylinder bores, in which the pistons move, are lined with cast-iron sleeves, so the wear characteristics are no different than those of a cast-iron block.
"There is not a longevity issue," Walker said. "You don't hear of spacecraft being made of cast iron, do you?"
Walker was a key engineer on the Chevrolet Vega engine, the aluminum-block disaster that GM introduced in the early 1970s. "The Vega got a bad rap," he asserts.
In some quarters, however, aluminum engines still have a dubious reputation.
The engine remanufacturing industry, which annually rebuilds three million failed engines, finds that aluminum blocks are more susceptible to failure from overheating and dirty oil than cast-iron blocks, according to experts at the Automotive Engine Remanufacturing Assn., a trade group.
Many aluminum-block engines require sophisticated cooling systems to ensure that the block is not damaged. In some cases, the engine oil must be run through a special radiator cooling system.
If the block does overheat, the oil can become contaminated with coolant or the coolant contaminated with oil. In some cases, blocks are subject to warpage. And corrosion is a greater problem in aluminum water jackets.
The point is not that aluminum engines are all bad but that buyers need to be careful about the reputations of particular engines, and more careful than ever about maintenance of oil and coolant levels.
Next week: Examining the record of one aluminum-block engine that has had more than its share of problems.