Slow, tedious, and costly does the job at Rolls-Royce. Forget the term `assembly line'; reenter bygone era of coach-building

The sound of the hammers is deafening. The workmen in their faded uniforms are busily pounding sheet metal into place, using hacksaws to trim off the scrap.

``You're stepping back a century,'' says a Rolls-Royce executive as he welcomes a visitor to the aging, four-story building, hidden within an industrial park on the edge of the city.

Here on the fringe of modern London, Mulliner Park Ward is an anachronism. Forget the familiar term ``assembly line''; this is a throwback to the days of gaslight and royalty, and the nearly forgotten art of coach building.

Mulliner Park Ward is one of two main factories operated by British automaker Rolls-Royce, used to produce some of the company's lowest-volume products, including the Corniche convertible and the Phantom, the huge carriagelike sedan favored by kings and plenipotentiaries. It's a slow and tedious process, and a costly one, with the factory's products commanding from $150,000 to more than $750,000 each.

The first floor at Mulliner Park Ward is used primarily to assemble the Corniche and its Bentley counterpart. There are no moving lines here, just row upon row of vehicles in various stages of assembly. Hammers, hacksaws, and welding guns mix into a cacophony of sounds as sparks play in the air.

Although the company is trying to increase its presence in the United States with new Bentley models (see below), very little has changed for the Rolls-Royce.

A typical Detroit assembly plant would be rolling these cars off the line at the rate of one a minute.

This week, Mulliner Park Ward may ship eight convertibles out the factory door. Yet, this is mass production compared with what goes on upstairs.

It is quiet on the second floor, teatime, and several veteran craftsmen relax in a corner telling stories. Suddenly, a sirenlike warble cuts through the air. The break over, the workers wander back to their stations.

At one station, a wizened old man guides a piece of sheet metal through a press, slowly shaping it to match the pattern of a 50-year-old jig, or form. Carrying the heavy metal across the cavernous room, he carefully mates it to the bare-metal frame that in 16 months will become a Phantom, the world's most expensive automobile.

Another flight up and a middle-aged woman is busily stitching sheets of fine, cream-colored leather, using a Singer sewing machine older than she is. As she finishes each sheet, she turns them over and marks them with an identification number, a tag then recorded in a thick and growing folio.

There is a history, so to speak, of each Rolls-Royce, a logbook of parts and procedures. Since every car is built by hand, there may be subtle differences between vehicles. This record book will be maintained indefinitely, and should an owner damage a fender or wear out a seat cover, it will provide the exact dimensions for a replacement.

The last decade's worth of logbooks are stored at Rolls-Royce's headquarters at Crewe in the Midlands. Another 90,000 books are filed at the Rolls-Royce museum a few hours away, a record of every vehicle built by the company since Sir Henry Royce and Charles Rolls agreed with a handshake to begin building cars together more than 80 years ago.

Over the course of those years, the company has built a total of 108,000 cars, barely half the annual output of a typical assembly line in the US or Japan. Volume has slowly been on the increase, and this year sales should hit a record of nearly 3,000 units.

The vast majority of those vehicles will come from the main plant in Crewe, which, compared with the Mulliner Park Ward factory in London, is a modern wonder.

Based in northwestern England, the main factory once built the Merlin engines used to power fighter aircraft such as the British Spitfire and the American P-51 Mustang. To some, then, the plant helped save the island nation.

Surprisingly little has changed in the four decades since the end of the war.

A ``major'' investment program is slowly introducing the tools of a modern era, adding a few numerically controlled, or computer-programmed machines. But in fact, much of the old machinery is still in use.

As is the same critical attention to detail. Every Rolls-Royce will be run on a test track, usually clocking more than 100 miles. Every 100th engine is torn down to the smallest pieces, carefully checked for any deviation from tolerance, and then reassembled.

Even here at Crewe, it will typically take four to six months before a car is ready for delivery. And if the customer wants any custom work, the process can take still longer.

For instance, in a small out-building used to complete unusual modifications, a workman is fitting a videocassette recorder into the back of a sleek, black Rolls for a British antiques dealer living in Paris.

``Every time we think we're finished, the customer changes his mind,'' he says with a mixture of frustration and awe. ``Now he wants a bank of switches put in just behind the sunroof. He doesn't even know what he wants the switches to do, but he says that by time we get done, he'll figure out something.''

``This thing was supposed to be delivered last August,'' he says, shaking his head and reaching for a screwdriver. ``I'm not sure we're ever going to get it done.''