The amazing thing, it now appears after more than four years of planning and restoration on the Statue of Liberty, is not that her torch arm had been installed incorrectly (it deviated 1 1/2 feet from the original plan) or that some of her iron framework was half rusted through (the corrosion was hastened every time rain leaked in and created a weak battery where the iron touched her copper shell).

The amazing thing is that so much of the century-old colossus -- the world's tallest structure when it was built -- was in excellent shape and that the original engineering designs required only one significant change: The rust-prone iron was replaced with stainless steel, nonexistent 100 years ago.

Virtually all other work in the $31 million project was to remedy problems introduced, mainly in the 20th century, when "improvements" turned sour or to make tourists comfortable as they poked about inside the hollow, green giant.

The statue's copper shell, about half again as thick as a penny, was almost as good as new. A century of weathering had eaten away only 5 percent of its thickness and, protected by its patina of copper oxide, it is said to be good for several centuries more.

Although Frederic-Auguste Bartholdi created the sculpture, establishing Liberty's outward appearance, it was Alexandre-Gustav Eiffel, a French bridge engineer, who designed the internal framework that keeps it standing. Eiffel's tower -- the one inside Liberty, not the naked one he built in Paris -- has seen the statue through 100 broiling summers, 100 freezing winters, nearly continuous buffeting by the wind and countless lightning bolts.

"Eiffel's superb calculations withstood both the test of time and our subsequent computer-aided analysis," Richard S. Hayden and Thierry W. Despont, the American and French architects who oversaw the job, said in their book, "Restoring the Statue of Liberty."

Eiffel's problem was how to support a hollow copper vessel 151 feet tall and 35 feet across at its widest, a vessel made of 310 contoured copper plates a few feet on each side, riveted together at the edges. The shell weighs 179,200 pounds and could not support itself, much less withstand wind, without crumpling on the spot.

Eiffel's genius lay in inventing a concept -- later to be known as the "curtain-wall" method of construction -- that would support the copper and allow it to flex in wind, expand in heat and shrink in cold without breaking the link between shell and framework. The weight of each plate of copper shell is transferred to the skeleton inside by iron bars.

The juncture of such dissimilar metals as iron and copper would cause one of Liberty's most dangerous corrosion problems, one that Eiffel anticipated and tried to prevent. But he had no other choice. Iron was the strongest building material available then.

Eiffel designed the main load-bearing support as a massive, four-legged iron tower that tapers as it rises 97 feet from the stone pedestal to Liberty's shoulders. Atop this is a set of arches lining the inside of her head. From the tower's side, a narrower tower juts out and curves sharply upward to support the uplifted right arm.

Surrounding the main tower is a secondary, triangulated web of iron beams that reach out at varying distances from the central tower and bring the load-bearing capability to within a foot or so of the shell. From this secondary web, individual iron bars angle up or down to touch still another set of iron bars that are Liberty's ribs.

There are 1,830 ribs, flat straps of iron bent and twisted to follow every curve of the shell, including the intricate folds of Liberty's gown. The ribs are held loosely against the shell at intervals by copper "saddles," straps riveted to the shell at each end but raised in the middle to hold the ribs.

The idea was to allow the ribs to slip within the saddles without wrenching loose from the iron framework. The statue could "breathe" and still stay strong. Although new methods of supporting the statue were considered, nobody could think of a better way than Eiffel's.

Eiffel knew that when dissimilar metals touch, they can produce a galvanic reaction, acting as a battery if there is moisture between them. Electrons from the iron atoms will flow to the copper, leaving the iron in a state in which it will combine with oxygen atoms from the water to form iron oxide, or rust.

To prevent this, Eiffel insulated the ribs with a layer of asbestos coated with pitch, but decades of flexing crumbled the insulation away.

Rusting was hastened by moisture inside the statue. The water came at first from tiny, unavoidable leaks and from the condensed exhalations of countless thousands of visitors trooping up and down the spiral staircase in the middle of the tower. As the ribs rusted, they swelled so much that they seized inside the saddles. The swelling popped many of the saddle rivets, opening sizable holes in the shell that let in still more rain.

The rusting was so bad that in 1936, during the 50th-anniversary restoration, many of the ribs were replaced -- again with iron. The 100th-anniversary solution, and one of the restoration's biggest jobs, was to use rust-resistant stainless steel to replace each rib, more than 30,000 rivets and the 325 bars that link the ribs to the secondary framework.

Some engineering consultants wanted to use copper ribs to avoid the galvanic reaction but, like Eiffel, concluded that no known copper alloy was as strong as iron or steel. To match iron's strength, the copper would have had to be much thicker, and the extra weight was a worry.

Stainless steel is also more like copper in galvanic terms than is plain iron, and engineers estimated that stainless would corrode as much in 1,000 years as iron in 100 years. Still, the stainless ribs were insulated from the copper with Teflon tape, which is unlikely to crumble with age and friction.

The choice was narrowed to two stainless alloys. One, ferralium, was more resistant to galvanic corrosion but became brittle when hammered into shape. Temporary heating, called annealing, would remove the brittleness but would also destroy the corrosion resistance. Moreover, ferralium had "elastic memory" and, after being shaped, would tend slowly back to a straight bar. The other alloy, 316L, was more vulnerable to corrosion but kept its new shape well.

The decision was to use 316L for the ribs and ferralium for the bars linking the ribs to the support structure. Ferralium's springiness was an advantage here, allowing flex.

At first, the American and French teams disagreed on how to shape the ribs. The plan was to remove the old ones, four at a time, copy them in stainless steel and install them before taking out another group.

The Americans -- P.A. Fiebiger art restoration company and Nab Construction Corp., a steel contractor -- tried various high-technology, computer-aided methods to ensure accurate reproduction. None worked as well as the trained eyes of the French artisans, who would simply look at an old rib and hammer the new one until it looked as if it matched.

Before the ribs could be attached, however, the workers had to strip the seven coats of paint and two coats of tar inside the statue. These layers, added over the years to prevent corrosion, abetted the problem by hiding it and, in some cases, blistering into pockets that held water.

Chemical strippers were ruled out because the flammable vapors might explode. Then a National Park Service conservator hit on the idea of spraying supercold liquid nitrogen (about 350 degrees below zero) on the paint. The chilled paint shrank into brittle chips that fell away.

The tar remained. The solution here was a form of sandblasting, without the sand that could erode holes in the copper shell. The restorers experimented with various materials: Ground walnut shells were too hard, removing too much copper. Ground corn cobs were too soft. Baking soda was just right.

About 40 tons of baking soda were blasted against the tar, eventually removing all of it. Some of the soda leaked outside and, reacting with rainwater, turned parts of Liberty's green patina blue. Washing and chemical treatment restored the familiar color.

Once cleaned, the inside was painted with a water-based zinc primer paint developed by the National Aeronautics and Space Administration.

Then there was one of the restoration's biggest mysteries: Why was the arm's installation so unlike Eiffel's plan?

The arm's 40-foot-long framework juts from the central tower near the shoulder, 18 inches to the statue's right of the place where Eiffel intended it to be. As a result, the frame's outboard edge dangled freely at the armpit, braced by what the restoring engineers considered a very clumsy arrangement of ironwork.

Eiffel's drawings showed a much more elegant attachment -- but who changed it? The statue had been assembled in Paris before being disassembled and shipped to New York, and photographs taken in Paris revealed that the change was made there.

One theory was that when sculptor Bartholdi first saw the statue assembled, he found the arm esthetically misplaced and ordered it moved. There is no record of what Eiffel thought of the awkward rearrangement.

The arm has long been known to be weak, and tourists were excluded from it many years ago. The restorers debated taking it down and reattaching it as Eiffel specified, but historians, preferring the traditional and esthetically more pleasing version, prevailed. The arm was strengthened with new ironwork.

The most visible part of the restoration, aside from the 300 tons of aluminum scaffolding that surrounded it for most of the last two years, was the replacement of the flame in Liberty's torch.

The flame's history is a sorry one. As designed by Bartholdi, it was solid copper sheeting covered with gilt to simulate fire when illuminated with floodlights from the torch's platform.

A week before the dedication in 1886, however, the U.S. Army Corps of Engineers complained that the gilt would dazzle the eyes of ship pilots navigating into New York harbor. They proposed instead to cut portholes in the copper and put electric lights inside.

At the dedication ceremony, Bartholdi was severely disappointed. The lights could barely be seen from Manhattan, and he observed that it gave off "the light of a glowworm." It hardly fulfilled the statue's original name, "Liberty Enlightening the World."

Six years later, larger windows were cut around the flame's base, and a skylight was added. On a visit in 1893, Bartholdi was again disappointed at such distortion of his creation.

In 1916, Gutzon Borglum, the sculptor of Mount Rushmore, was commissioned to try his hand. He resculpted the flame into a lantern with 250 panes of yellow glass and put even brighter lights inside. The poorly sealed windows leaked from the start.

The only solution, the restorers decided, was Bartholdi's original gilded copper flame with external lighting. A new flame was built in a temporary workshop at the statue's base. French artisans skilled in the ancient method of repousse hammered new copper sheets into replicas of the originals, riveted them together, smoothed the joints and pounded gold leaf onto the surface.

The old torch will be on display in a new museum inside the pedestal. Also for visitors, the 171-step staircase to the crown has been refurbished with new rails and rest stops; screens that obscured views of the statue's interior have been removed, and a new heating and ventilation system has been installed.

For all of the new comforts inside and the high-tech realization of Eiffel's revolutionary engineering, the view from outside remains as it always has. But now, when President Reagan switches on the lights at the rededication ceremony, Liberty's torch will shine, for the first time, as Bartholdi intended.