Black & Decker Manufacturing Co. will close an unspecified number of plants and lay off more workers as part of a restructuring program undertaken recently to shrink costs, the power-tool and appliance giant announced yesterday.

Black & Decker, which is based in Towson, Md., said the restructuring plan cost $205.3 million for the 1985 fiscal year, which ended on Sept. 29, contributing to the firm's year-end net loss of $158.4 million ($3.11 a share). Even apart from this one-time charge, the company suffered a 51 percent decline in earnings, from $95.4 million ($1.95) in fiscal 1984 to $46.9 million (92 cents) for the latest fiscal year.

Black & Decker has been struggling in recent years to contend with a wave of imported power tools that have cut into its market share, as well as with high costs incurred from maintaining large production capabilities. Company officials said the restructuring -- plans for which they said began this summer -- would slim down the manufacturer to bolster its competitive position around the world.

"Significant gains in manufacturing productivity during the past few years, lower-than-expected sales growth, and forecasted future productivity improvements have resulted in excess plant capacity and unacceptably high fixed costs," Laurence J. Farley, the firm's chairman and chief executive, said in a statement.

Farley said the restructuring -- which also includes efforts to cut overhead and some tax-reporting adjustments -- represents a "bold plan to help us achieve our profitability goals in a difficult environment." He predicted the plan would enable the firm to regain profitability by the end of the calendar year.

As part of the reorganization, the company said it expects to raise activity in its remaining plants to 75 percent of their operating capacity from 50 percent now, and to cut overall production costs by between 4 and 6 percent. By the time the shrinkage is complete, the company's manufacturing capacity should be reduced by about one-third, a spokeswoman said.

The company would not specify which, and how many, further plants would be shut down, or how many of its 22,000 employes worldwide will be laid off. Barbara B. Lucas, the spokeswoman, said she expects this to be announced within the next two months, although she said that all of Black & Decker's 24 manufacturing facilities around the world would be affected by the plan because certain operations would have to be shifted between plants.

A plant in Allentown, Pa., was previously scheduled to close at the end of this month, and a power-tool-accessories manufacturing facility in Maryland was shut down in September. The company also already has announced that it will eliminate 630 positions in West Germany and the United States as a result of earlier restructuring measures.

The thrust of future changes will be to streamline company operations, for instance through reducing duplication of functions in the plants, as well as concentrating on products that can be marketed around the world, Lucas said.

Franklin Morton, an analyst who Black & Decker's effort to scale back production capacity comes as the firm also has been expanding into the hotly competitive appliance market. follows the company for Alex. Brown & Sons, predicted the restructuring would enable the company to cut $50 million a year in overhead costs and help it regain its profitability.

"This year, the operating earnings were under pressure from sluggish sales. That made them look long and hard at underlying profitability," Morton said. "While [the restructuring] is a painful step in the short run, I think it adds a lot to the earnings potential of the company over the long run."

Sales for the year totaled $1.73 billion, an increase of 13 percent from $1.53 billion in 1984, although unit volume declined, B&D said.

Black & Decker's effort to scale back production capacity comes as the firm also has been expanding beyond its traditional power-tool market into the hotly competitive appliance market. Last year, the company paid $300 million for the small appliances division of General Electric Co. and has been moving to replace the GE logo with its own, a transformation that Lucas said should be complete by the end of 1986. The firm particularly has stated a desire to expand its overseas operations, and recently began offering a line of home appliances in Great Britain.