As its sales to U.S. customers began to decline this year, Transcisco Industries Inc. picked up the slack with orders from overseas. The San Francisco-based rail car maintenance and manufacturing company is selling its heating systems for railway tank cars, made in Kansas, to railroads in the Soviet Union and Poland.

"We are exporting, and I am glad of it," said Transcisco Chairman Mark C. Hungerford. "I told the mayor of Topeka the other day that the Soviet orders were responsible for 20 jobs in his city. If we didn't have business overseas, we'd be letting people go. American companies need to export to insulate themselves from market swings here."

Hungerford said exports accounted for about 10 percent of the company's $42 million in revenue last year and he anticipates that overseas sales will increase to about one-third of company revenues in the next 10 years.

Transcisco is no isolated example. U.S. industrial exports are booming -- everything from giant jets to nuts and bolts. More efficient factories are one reason. A weak dollar is certainly another, making American-made products less expensive overseas. Continued strong economic growth in a number of key foreign economies has also been a factor.

What it all means, according to the National Association of Manufacturers (NAM), is that exports kept the American economy out of recession during the first nine months of this year and will cushion any downturn in the future.

"Manufactured exports kept the United States out of recession in early 1990 and will be critical to continued U.S. growth," said Jerry Jasinowski, NAM president.

Jasinowski said that exports have accounted for more than one-third of the growth in the U.S. economy since 1986 and more than 80 percent of overall growth in the first half of 1990. Since 1987, exports have run above $200 billion a year -- the highest levels ever -- and the NAM predicted they would reach $300 billion sometime in the 1990s.

A study released yesterday by NAM showed that the U.S. export boom is now in its third year and covers a full range of products. Western Europe and Canada form the strongest markets for U.S. products, NAM said. Overseas sales of jet planes, chemicals and advanced scientific and measuring instruments increased by a whooping 70 percent since 1985.

NAM figures showed that exports of manufactured products reached $287 billion last year, an increase of 70 percent since 1985.

With the U.S. economy turning downward and a recession expected by next year, Jasinowski said U.S. companies will find their best opportunities outside of U.S. borders between now and 1995.

"Export orders have held up better than domestic demand," NAM reported.

A NAM survey of 120 American companies showed that the firms, on average, expect to double their export business by 1993, to 20 percent of total sales.

Some U.S. companies, such as Boeing Co., have always turned to global markets, but now their international business is even more important.

Boeing delivered more planes this year to foreign-based airlines than domestic-based ones. "We believe that it will continue that way for the foreseeable future," said Larry Clarkson, senior vice president, who credited overseas sales for increased profits in the July-September quarter.

"Because our business is global, we are not as affected by the current domestic economic downturn," he said.

Others firms, newcomers to exporting, are prospering in a tough economic climate because of their overseas sales.

In Rockville, Fusion Systems Corp. President Don Spero reported receiving several recent orders from Japan and South Korea that he believes came in because of the weaker dollar.

Fusion Systems makes machines used to produce semiconductors, automobile parts, flooring, optical fibers and printing. It had $35 million in sales this year, with 35 percent of them going overseas.

"Exports are an important factor to us. We sell to 20 or 30 major industries. It is unlikely that all those industries will go down globally at the same time," said Spero.

Luctite Corp. of Hartford, Conn., does 60 percent of its business overseas, selling specialized sealants and adhesives that are largely produced in facilities spanning the globe.

Among its top customers are automobile makers, and Chairman Kenneth W. Butterworth said sales have remained strong because "we do business with foreign manufacturers in their home countries."

"The auto business is strong. It is just that business in Detroit is down. Our company would be in sad shape if we depended on sales to U.S. automakers," Butterworth said.