Jay Leno, the gearhead’s favorite funny man, was somewhat bedazzled recently while watching a Nissan GT-R engine assembly.

“An engine like this is so labor-intensive, so it’s fun to see the mixture of computerized technology with the old hand-eye stuff,” says Leno, who dropped by the the factory in Yokohama where the twin-turbo, 545-horsepower 3.8-liter V6 is built. “It’s very quiet in there. It’s like being in a library, everybody tiptoes around and each worker does his job. Nobody’s looking at a clock. They only build 13 engines a day – and that’s a whole group, so it’s quite methodical. They take their time, and it’s a fascinating process to watch just to see how it’s done.”

Mercedes AMG has a one-man, one-engine philosophy when building engines for upscale Mercedes models.

Watching work at most engine factories is not very exciting. The long assembly lines and almost robotic workers are commonplace in high-production factories, and action is rather routine. But high-performance engines get special treatment, as in GM’s Wixom plant where just over 3,000 engines are hand built for special Camaro and Corvette models. And then there’s the European specialty plants like Ferrari, Bugatti, McLaren and Mercedes AMG. All boast of traditional craftsmanship with time-consuming, non-automated assembly processes that promote the “hand-built” attention to detail. Bugatti claims it takes a full week to assemble more than 3,500 parts into its W16 engine

NASCAR car owner Rick Hendrick builds his own Corvette engine at GM’s Wixom plant.

Buyers of the Z06 and ZR1 Corvettes can pay about $5,800 extra to assemble their own 7.0-liter and supercharged 6.2-liter V8 engine, respectively, at the GM Performance Build Center in Wixom, Michigan. It’s all done under the supervision of expert technicians. The owners get their own personalized plate attached to the engine, and  they still get the 5-year warranty 

Hand-built specialty engines are also good for advertising and marketing, as witnessed by the Leno visit. It’s doubtful Nissan would have promoted Leno following the build of a 1.6-liter Versa engine. But anything over 500 horsepower is certainly celebrity eligible.

“I like mechanical things like mechanical watches,” sums up Leno, “and just the mechanical nature of the process–though we live in an age where everything is computer controlled–how mechanical it is, is fascinating to see.”