Negotiators for the nation's major coal companies, a month after walking out of contract talks, resumed bargaining yesterday with the 160,000-member United Mine Workers of America in an effort to avert a strike before the union's Sept. 30 deadline.

The UMW and the Bituminous Coal Operators Association are attempting to break a costly cycle of seven consecutive strikes in the past 20 years of negotiations by reaching a settlement.

"Things are going fine," Bobby R. Brown, the BCOA's chief bargainer, said during a break in the afternoon talks. "As long as talks are going on, things are fine." Brown and UMW spokesmen have declined comment on the specifics of their bargaining stances.

Prime issues include the BCOA's goal of reducing labor costs, possibly through changes in work rules, pensions, and health care costs. UMW President Richard Trumka, whose members are suffering an unemployment rate of more than 25 percent, has said that the union wants guarantees of job security and will resist company efforts to reduce benefits.

BCOA, whose miners produce 40 percent of the nation's soft coal, left the talks at the Sheraton Carlton Hotel here Aug. 21, with Brown saying that "no basis exists" for fruitful discussions.

In case of a strike, coal producers and their major customers have been mining and stockpiling record-high inventories, according to coal industry analyst Jack Kawa of Dean Witter Reynolds in New York.

"I don't know whether there'll be a strike, but if there is, it looks to me like the industry can take a 90-day strike without too many problems, and probably a 120-day strike . . . with a little discomfort," Kawa said. Coal production was a record 90 million tons in August, enabling electric utilities to amass an estimated 98-day supply, he said.

Representing 40 percent of U.S. miners, mostly in eastern and Appalachian states, Trumka has authority to call selective strikes, but the BCOA said a strike against one mine would be considered a nationwide strike against the association, which might close the other mines.