A not-so-funny thing happened to domestic auto makers on their way to lower production costs and increased efficiency.

In their rush to reduce parts inventories and streamline delivery procedures, they made themselves more vulnerable to labor strikes.

"Even if you look at their balance sheets, you'll see that they are carrying much less inventory than they were carrying two or three years ago," said analyst David Healy of New York-based Drexel Burnham Lambert Inc.

"The auto makers have almost deliberately made themselves more vulnerable to a walkout" in their efforts to improve production, Healy said.

That vulnerability is particularly great in this season of strong car sales, when automotive plants are working overtime -- pushing the industry's new "just-in-time" inventory operations to their limits.

"Just-in-time" operations, copied from Japanese auto makers, were widely adopted by U.S. manufacturers as a way to slash costs during the long sales slump of the early 1980s. It means just that: the right part at the right time at the right place. It means building parts as near to perfect as possible the first time so that no time is lost replacing or reworking defective parts, and no time is spent, or space wasted, stocking excess parts.

But excess parts have been one of the industry's traditional insurance policies against strikes.

The possibility of a strike against General Motors Corp., the nation's largest auto maker, looms large now, six days before the expiration of a GM contract with the United Auto Workers union.

At this point, the union and the company seem far apart on the important issues of job security and pay. Although both sides have expressed confidence that they can settle peacefully at the bargaining table, they also have begun preparations in case their differences lead to picket lines.

The UAW also has been negotiating with Ford Motor Co., GM's largest domestic rival. But the Ford talks were put on hold last week after the UAW chose GM as its single strike target.

That strike, should it come about, could be a general walkout closing all of GM's 151 facilities operating in 26 states and 90 cities in this country.

But it also might be a selective action aimed at strategic GM operations -- for example, the Hydra-Matic Division in Ypsilanti, Mich., which makes critical components such as automatic transmissions for passenger cars and trucks.

UAW officials have been considering both strike options. And GM and Ford officials say privately that the selective action could be just as effective as a general walkout, given domestic auto makers' growing dependence on just-in-time inventories.

That is because the streamlined inventory system eliminates redundant plants, thus leaving GM with only one internal source for many kinds of parts.

"The UAW could tie the whole operation up in knots by shutting down the Packard Electric Division," which makes electronic wiring systems for GM vehicles, said an auto industry executive, who asked not to be identified by name or company.

A wildcat strike last year against a Chrysler Corp. stamping plant in Twinsburg, Ohio, supports his point.

The six-day walkout at the stamping facility, which made front doors and steel underbodies, shut down six of Chrysler's eight assembly plants almost immediately. Chrysler officials said then that under traditional inventory procedures, and with "normal" consumer demand, they would have had enough parts on hand to continue operating for up to 10 days after the start of the strike.

But Chrysler, perhaps more than any other domestic auto maker, had stripped itself of fat and put itself on a strict just-in-time regimen. The Twinsburg plant then was one of three remaining Chrysler stamping facilities, each of which was the sole supplier of specific parts for Chrysler cars and trucks.

"The Twinsburg strike was the first real test" of the just-in-time approach to auto production in the United States, industry analyst Arvid Jouppi said.

"If anything, it proved that, if you're going to have a just-in-time inventory and a single-source supplier, you'd better have very good labor relations. Otherwise, the entire system could collapse."

That is not to say that the just-in-time system should be done away with. On the contrary, auto industry analysts and officials say that Japanese auto makers' dedication to that system (called "kanban" in Japan) is a major reason for Japan's manufacturing cost advantage over U.S. competitors -- estimated by some experts at $1,500 to $2,000 per car.

The just-in-time approach means U.S. auto makers and their employes must adopt a whole new labor-management relationship, say Alan Altshuler and Daniel Roos, authors of a report on the future of the automobile industry soon to be published by the Massachusetts Institute of Technology's International Automobile Program.

The traditional supply-production system developed by Ford and GM in the early days of the domestic auto industry "is now recognized to be in the process of transformation," Altshuler and Roos wrote.

Under the old system, the authors said: "It was assumed . . . that the assembly workers would not report on problems, would not repair their own machines, and would take no initiative in spotting or correcting faults.

"It seemed to follow that large inventories would be needed as buffers so that parts of the system could continue to operate while problems in other parts were being diagnosed or repaired," the authors said.

But the just-in-time system "asserts that large supply buffers hide problems rather than ameliorate them and that elimination of buffers makes it possible to identify and eliminate production bottlenecks in their order of priority," Altshuler and Roos wrote.

The new approach requires managers to rely more on the intelligence and skills of their workers, who must now prevent defects, instead of relying on inspectors somewhere down the line to detect them. "It holds that ideas for improving the production process can and should come largely from the front-line workers, who know the system best," the authors wrote.

Employes, on the other hand, must be willing to accept new work practices to make the system operate, according to Altshuler and Roos. In short, the new system is more efficient -- but also more fragile, which means it requires a higher degree of trust between labor and management, the authors said.

If that trust is developing, it may make some workers less willing to support a strike this time around. But whether that trust will manifest itself in the current contract talks is still open to question, industry analyst Healy said.

"The practical effects of a selective strike would be the same as a general walkout, and both would be costly," he said. "So my guess is that they're going to settle without a strike. But I wouldn't want to make a bet on how good that guess is."