The Reagan administration, at the Commerce Department's urging, is reconsidering an earlier decision to prohibit export to South Africa of sophisticated metallurgical equipment that could be used in making critical components for nuclear weapons, government sources said yesterday.

The equipment is a large hot isostatic press, used to mold powdered metals at high temperatures and under great pressure into special shapes such as solid and hollow spheres.

While information about the exact use of such presses in the U.S. military program is classified, a knowledgeable source said yesterday that they could be very useful to a country seeking to build nuclear weapons.

The United States in recent years has rejected efforts by a half-dozen countries, including Israel, India and Taiwan, to buy these large presses. It also has successfully urged Sweden, the only other nation that manufactures a comparable press, not to export it to countries that might be developing nuclear weapons, sources said.

An attempt by South Africa to purchase one of these presses was rejected by the administration earlier this year. But now the Commerce Department, which would license such a sale, has resubmitted the South African export application for a new interagency review.

"This is a longstanding item over which there has been concern," said Archelaus Turrentine, deputy assistant director of the U.S. Arms Control and Disarmament Agency. "It's obviously something that could be used in a nuclear weapons program, and you don't want to give somebody a key capability that they are missing that might let them move ahead."

The presses, however, are considered "dual use" items because smaller presses are widely used in a variety of commercial applications, and large hot isostatic presses, which are manufactured only by three American comapnies and the one Swedish firm, also are used in this country in the aerospace industry.

The seller would be Autoclave Engineers Inc. of Erie, Pa.

The South African company attempting to purchase the press has said it plans to use the equipment in manufacturing tungsten carbide drill bits for oil rigs, an application that experts say appears legitimate.

"But our policy, dating back to the previous administration, has been that we would not export the large hot isostatic presses to countries of proliferation concern," a State Department official said yesterday. "When this latest application came before the interagency Subgroup on Nuclear Export Coordination (SNEC), that policy was maintained."

But now the Commerce Department, sources said, is pressing for a new meeting of SNEC within the next two weeks to reconsider the rejection, and is arguing that large hot isostatic presses may not be as important to a country seeking to manufacture nuclear weapons as has been believed.

The State Department and the arms control agency, however, which traditionally have played the senior role in approving exports of nuclear-related items, are taking an extremely skeptical view of the Commerce Department's position.

"Our policy has been that we do not approve these kinds of things, and I can only assume the Department of Energy and their technical experts made the right decision when they reviewed this the first time," a State Department official said.

"The people here are pretty firmly opposed to this sale," an arms control agency official added. "Other countries have agreed that not exporting large hot isostatic presses to countries of proliferation concern sounds like a good policy, and no one has fallen off the boat yet. There is sort of an agreement we will not undercut each other on this."

State Department officials expressed particular concern over the reaction of Sweden if the United States, after repeatedly urging the Swedish government not to permit export of these presses, now permitted an American firm to make such a sale to South Africa.

"If we in fact change our mind on the policy, we are going to have a certain amount of embarrassment in connection with the Swedes," a State Department official said.

But State Department officials noted that, while the Commerce Department has the authority to grant a license for the export of the hot isostatic press to South Africa, this license requires interagency concurrence. Commerce's recent approvals of electric police batons to South Korea and small jets to Iraq did not require such concurrence.

"Commerce is going to have to do one hell of a selling job on this one," an official said yesterday.