Sergio Scaglietti, a sports car designer whose hand-shaped Ferraris of the 1950s and 1960s were regarded as the most elegant forms of transportation to grace pavement and the most valuable collector cars in the world — commanding multimillion-dollar sums at auction — died Nov. 20 in Modena, Italy.

He was 91. Ferrari Chairman Luca di Montezemolo announced Mr. Scaglietti’s death in a statement, noting that he was “a man who had his name forever connected to the Prancing Horse,” the Ferrari logo. No cause of death was reported.

Mr. Scaglietti, the son of a carpenter, was regarded as the Italian automotive industry’s modern-day Michelangelo — a sculptor whose medium was metal. He was known as Ferrari’s “maestro of aluminum.”

Hammer in hand, he shaped his designs by pounding sheets of the light metal over bags of sand. Many of the classic cars he built are said to still bear lumps on the bodies from the swing of his “martello.” Blemishes to some, the imperfections often increase a car’s value, said David Gooding, president of the Gooding & Company collector car auction house.

In August, the 1957 Ferrari 250 Testa Rossa prototype that Mr. Scaglietti designed was sold at auction for $16.4 million. Known by its chassis number, 0666, the car was thought to be the most valuable ever to be sold at auction.

Mr. Scaglietti’s designs are “revered by historians and collectors as some of the most beautiful cars ever created,” Gooding said in an interview.

The Testa Rossa, Gooding said, is the “Sophia Loren of automobiles. It’s curvaceous. It’s voluptuous. It’s quintessentially Italian — wild, extreme, beautiful, of course beautiful, but also unique. It really looked like nothing else from the time.”

Compared with American cars of the day, Gooding said, the Testa Rossa was “so radical, so wild, and yet had such dramatic impact that it had tremendous influence on everything” designed since.

Sergio Scaglietti (pronounced Ska-yeh-tee) was born Jan. 9, 1920, in Modena, Italy. He was forced to drop out of school at 13 after his father died. To support his family, he went to work in a local garage, where he learned to repair by hand the bent and dented bodies of cars banged up in accidents.

It was by chance that the shop where Mr. Scaglietti worked was near the entrance to the Scuderia Ferrari factory in Maranello. Soon, Mr. Scaglietti’s handiwork was noticed by the race-car company’s imperious founder, Enzo Ferrari, who asked the young mechanic to repair a mud flap.

That seemingly trivial test led to more Ferrari assignments for Mr. Scaglietti, who opened in 1951 his own custom coach-building business, Carrozzeria Scaglietti.

Shunning pencil and paper, Mr. Scaglietti worked out his designs in his head and with his hammer.

In 1954, Mr. Scaglietti was commissioned by Italian movie director Roberto Rossellini to design a custom Ferrari 375 MM for his then-wife, actress Ingrid Bergman.

Mr. Scaglietti’s other well-known Ferraris include the 250 GTO, of which only 36 are said to exist, the 500 Mondial and the California Spyder.

His repertoire “embodied not just Ferrari beauty, but extreme beauty,” Leslie Kendall, curator of the Petersen Automotive Museum in Los Angeles, said in an interview.

“They weren’t just low slung, they were impossibly low slung. They weren’t just sexy,” he said, “they were impossibly sexy.”

Some of Mr. Scaglietti’s design had value beyond aesthetics. His famously bulging “pontoon” fenders on the Testa Rossa, for example, helped cool the car’s drum brakes.

How the Testa Rossa accidentally got its name was a tale of Ferrari lore that Mr. Scaglietti enjoyed telling.

As he explained to the Allentown (Pa.) Morning Call in 2000, the company’s chief of production told Enzo Ferrari that the company had to stop building the proposed car because it had run out of black paint to coat the engines’s cam shaft covers.

“Enzo asked, ‘What color do we have?’ ” Mr. Scaglietti said. “The chief of production said ‘red.’ Ferrari said, ‘Paint the engines red, and we’ll call it the Testa Rossa’ ” — red head, in Italian.

Mr. Scaglietti sold his business to Ferrari in the 1970s and retired in the mid-1980s. In 2004, Ferrari named in his honor its four-seater sports car, the 612 Scaglietti.

A list of Mr. Scaglietti’s survivors could not be determined.

Mr. Scaglietti’s clients for his high-end vehicles included many of the world’s richest people, including royalty.

In 2004, the London Mail on Sunday reported that members of European aristocracy had once given Mr. Scaglietti two precious racing pigeons.

Mr. Scaglietti, apparently unaware of the birds’ competitive qualifications, inquired how best they should be prepared for eating.