When it comes to pianos, one name has stood above the rest for 135 years - Steinway & Sons - and if current sales are any indicator, that name will endure well into the future.

The staying power of Steinway, the world's most prestigious piano manufacturer, has been underscored in recent years by the disappearance of other American piano makers through mergers or bankruptcy.Gone are such familiar names as Chickering, Knabe, Fischer, Kohler & Campbell, and Aeolian, replaced by such Asian firms as Yamaha of Japan and Young Chang of Korea, leaders in the electronic keyboard field.

Piano sales in the United States declined from 1978 to 1986, then slightly rebounded, while sales of electronic keyboards, most of them portable, are approaching $1.5 billion, more than twice that of pianos.

"Sales of electronic keyboards probably are taking away from piano sales, but mostly from the sale of middle quality and low quality instruments," said Bruce Stevens, Steinway's youthful president, who still takes piano lessons.

In an interview at the company's elegant showroom in Manhattan, Stevens said Steinway has no intention of entering the electronic field, which includes player pianos based on laser technology, pianos that control other intruments through electronic interface, and synthesizers which imitate piano sound.

"Electronic keyboards are not detracting from the sale of quality pianos such as Steinway, and I don't think they have to be feared. If young people become interested in music through these instruments, it isn't all bad," said Stevens.

"We're optimistic about quality instruments, in line with the rebirth of interest in style and value. Our sales are extremely good, at an all-time high, in fact. We have back orders of four to six months on all our models."

Steinway furnishes pianos for most of the world's concert halls, but the majority of instruments are purchased by private individuals, some of them older than the average buyer used to be.

"Most private purchasers want to play alone at home," said John Steinway, great-grandson of Steinway's founder and a consultant to the firm. "It has great therapeutic value, especially for high-powered people. A lot of middle-aged persons are beginning to study piano or are returning to the piano after many years away from it."

Figures from the American Music Conference in Willmette, Ill., show that about 20.6 million Americans play the piano. In 1986 - the last year for which figures are available - 167,000 acoustic pianos were sold nationwide, compared to 282,000 sold in 1978, a 40 percent decline. Of the 167,000 pianos sold, 96,000 were made in America.

Steinway only produces about 5,000 pianos a year in its factories in New York and Hamburg, Germany, due to the amount of handcrafting involved. Some 4,000 are grand pianos, priced from $16,600 to $48,300. The rest are uprights, priced from $7,100 to $ll,5l0.

It takes two years to make a Steinway grand, 18 months to make an upright.

Most Asian companies using production line methods can turn out 5,000 pianos in just under four days. Grands made in Japan and Korea are sold for as little as $5,000 and spinets for $2,650.

"We can't hurry up our production because our standards are uncompromising, from the quality of the screws we use to the training of our staff," Stevens said.

A gala performance at Carnegie Hall, just down the block from Steinway, is planned June 2 to celebrate the firm's 135th anniversary and its founding by Henry E. Steinway, a German emigre piano maker. Several of the world's top pianists will perform on Steinway's 500,000th piano, a 9-foot concert grand with a case specially created by designer Wendell Castle.

"Our first two commemorative pianos, our 100,000th and our 300,000th, were presented to the White House, but our 500,000th is going to be a gift to the world," said Stevens.

"Our celebration will last one or two years and there will be gala concerts in other cities and in Europe to benefit a company foundation set up to nurture new composition of music for the piano and its performance."

Since 1880, concert pianists who use Steinways exclusively are known as Steinway Artists, and the firm makes a piano available to them for every public performance, charging only for delivery and tuning. There are now 800 such artists around the world.

Paderewski was the first Steinway Artist. The newest is Vladimir Feltsman, who emigrated from Russia and visited the Steinway showroom the minute he got to New York last year.

In a recent TV documentary, Vladimir Horowitz, another world renowned Steinway Artist, was shown choosing a piano at the Steinway showroom.

Richard Probst, worldwide director of Steinway's concert and artist department, said Horowitz turned to his wife after filming the scene and remarked:

"You know, Wanda, without me, Steinway is a big nothing."

Probst said Wanda Toscanini Horowitz replied:

"And without Steinway, Vladya, you are nothing."