Remember when automakers changed a car's body every year? General Motors' 1959 Chevrolet Bel Air looked nothing like the 1958 model it replaced or the 1957 version before that. A drive for efficiency ended all that, as carmakers tended to bring out multiple cookie-cutter models. It didn't make sense for manufacturers to retool factories for short-lived designs or for low-volume vehicles that didn't command a royal price.

GM is about to reengineer that theory. This year it will roll out two $20,000 roadsters and hopes it can make money on them even at low volumes. The automaker borrowed parts from other GM cars around the globe and used digital tools for engineering. But the biggest cost-cutter is a manufacturing process called hydroforming. It uses intense water pressure to shape a car's steel body panels on half a stamping die. At half the cost of traditional metal stamping, hydroforming may well be the secret to low-volume production on models that might appeal to a narrow group of buyers. With less money at stake, says Lori Queen, GM's small-car engineering chief, "I can be daring. I can be risky. I can be trendy."

Combined sales of the roadsters, the Pontiac Solstice and the Saturn Sky, are expected to be short of 40,000 vehicles a year. That's less than $1 billion in revenue for the world's largest automaker, with estimated revenues of $185 billion in 2004. They won't generate fat margins, Queen concedes. But at a 20% lower development cost, neither will they lose money. That would be a feat. GM hasn't made money on small cars for years. And the Solstice and Sky could polish the image of the struggling Pontiac and Saturn brands.

GM used hydroforming in the 1960s for forming things like a headlamp's mounting, known as a bucket. More recently it's been used to bend steel truck frames or to produce beer kegs. GM is believed to be the first maker to use the process to shape a car's body.

In the traditional stamping process, flat sheets of steel are shaped into body panels when placed between a pair of custom dies, worth as much as $1 million each, that are slammed together in presses. Hydroforming uses water pressure (up to 15,000 pounds per square inch) to force the flat steel against a one-sided tool, achieving the same result for half the cost. Its ability to shape more curves and sharp creases than a stamping press makes it ideal for expressive vehicles like the Solstice and Sky.

Hydroforming is too slow for high-volume vehicles. But if it saves as much money as GM hopes, the maker may build at least three other inexpensive low-volume cars.