IMPACT OF THE TRADE AGREEMENT : Accord Could Help Some U.S. Stocks : Securities: But experts say not to expect an immediate windfall in many cases.

The proposed North American Free Trade Agreement, by breaking down trade barriers among the United States, Canada and Mexico, could boost stock prices of U.S. firms expected to benefit from the accord.

But investors should not look for a quick buck, experts say. The agreement, announced Wednesday but facing a ratification battle in Congress next year, is likely to have a subtle effect on U.S. firms the first few years, the pros say. As Mexico’s economy improves and its citizens gain spending power, the impact should increase.

Also, stock prices of companies expected to benefit the most may already reflect the expected gains, some analysts say. Investors will have to do a lot of homework to make sage investments in this market, the experts contend.

“You don’t want to get too carried away� when talking about the impact of the trade agreement on individual firms, said Gary Schlossberg, senior economist at Wells Fargo Bank.

The first beneficiaries of the agreement are expected to be construction companies, engineering firms and producers of heavy equipment, such as Caterpillar, Ingersoll-Rand and Fluor, market analysts say.

The Mexican government is already building up the nation’s infrastructure. That has created tremendous demand for the services of companies that build roads, power plants, telecommunications equipment and office buildings, said Salvador Villar, president and chief executive of California Commerce Bank in Los Angeles and president of Banamex U.S.A. Bancorp.

Meanwhile, U.S. car companies, which already have production facilities in Mexico, are expected to win similar advantages. The current tariff system dramatically limits supply and inflates prices of U.S. cars in Mexico. If the agreement is ratified, Mexican citizens should be able to buy a U.S. car for about half the current price.

Does that mean that it’s time to buy shares in a Caterpillar or General Motors? Not necessarily.

“It might be time to ‘average in’ to Caterpillar or Ingersoll-Rand,� said Michael Metz, market strategist for Oppenheimer & Co. in New York. (Investors who “average in� buy a small amount of stock at a time on the assumption that the price may fluctuate; thus they obtain an average price over time.) “But I am not that optimistic about the market as a whole, so I think there’s plenty of time to think about it.�

Meanwhile, other analysts note that engineering firms and car companies saw their shares rise significantly during the first six months of 1992, partly on expectations that these companies would do well when trade barriers eased.

Now that share prices are at relatively high levels, investors should wait for more definitive signals before buying more stock, said Eugene Peroni Jr., market analyst at Janney Montgomery Scott in Philadelphia.

“At least for the short run, I think these stocks are in a no-man’s-land. The initial speculative drive has already happened, and now the values are not compelling,� Peroni said.

Investors who already have shares in healthy auto, construction and engineering firms should probably hang on to them, he noted. Those who don’t should probably wait to buy until share prices fall or corporate profits start to rise substantially, Peroni added.

A wide array of companies that make everything from soft drinks to washing machines are expected to benefit eventually from the trade agreement, analysts added.

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