The Office of the U.S. Trade Representative is embarking on the tough task of bringing the United States' definition of products for customs purposes into line with the rest of the world's.

It is a time-consuming job that has been under way in highly technical negotiations in Geneva for a decade. The changes are not expected to be sent to Congress until 1985.

The trade office is holding hearings across the country this fall. Any revisions proposed as a result of them will have to go back to the international talks for approval.

"This is the first time since 1963 that the United States has contemplated such a major overhaul of its tariff and statistical classification systems," said U.S. Trade Representative William E. Brock. The number of classifications is expected to grow from 7,000 to more than 8,000. Also, many of the basic U.S. tariff rates are expected to change.

Most of the industrialized world follows a common system of customs classifications. The United States and Canada are the major holdouts. The U.S. effort coincides with a move to overhaul the international agreement, the Harmonized Commodity Description and Coding System.

The problems with going it alone on tariff classifications are manifold. Take a product such as Bounce, the paper sheets put in a clothes dryer to soften fabrics and reduce static. When it is exported, some countries consider it a textile, carrying one tariff; others classify it as a chemical, which carries a different duty.

The harmonized system, with its Customs Cooperation Council to resolve differences arising from new products, settles questions like that, according to Douglas Newkirk, an assistant U.S. trade representative. However, the United States does not participate in it.

Moreover, he said, being part of the harmonized system would help limit trade disputes, simplify trade documents and encourage the use of automatic data transmission.

Meanwhile, Deputy Trade Representative Michael Smith took off for Tokyo over the weekend for what seems to be a continual round of trade talks with the Japanese. This session will focus on the possible trade-distorting effects of Japan's industrial policies and charges that Japan, through government subsidies and trade barriers, has helped its machine-tool industry undercut American products, keeping them off the Japanese market long enough for the industry there to grow.

Both sets of talks arose from a trade complaint, rejected by President Reagan, by Houdailles, a Fort Lauderdale machine toolmaker. Smith, though, is said to be prepared to go beyond machine tools, and may discuss Japanese quotas on imports of U.S. beef and oranges.

There is speculation among trade sources that both countries would like to get those issues out of the way before Reagan visits Prime Minister Yasuhiro Nakasone in Tokyo in November.

But one U.S. trade official complained that the Reagan visit to Tokyo has assumed such importance that even routine meetings have become significant.

"I'm afraid that everything that happens with Japan will be couched in terms of the president's trip," he said.