Under threat of possible legal action from the Federal Trade Commission, the Ford Motor Co. announced yesterday that it would voluntarily contact the owners of some 1.8 million cars and inform them that they may have prematurely worn engine parts.

In addition, Ford said that the parts in question would be replaced at the company's expense if found to be worn.

Ford said in a statement by Chester Barion, general manager of its Parts and Service Division, that the cars involved are 1974-1977 models and early 1978 models equipped with 2.3-liter, four-cylinder engines.

Barion said that "a small percentage of these engines may experience premature wear of the camshaft and rocker arms if the engines have been lubricated during periodic maintenance with oils containing insufficient wear protection."

He said letters would be mailed shortly to car owners advising them that the company would repair "any camshaft/rocker-arm wear that occurred within 36 months or 36,000 miles, whichever comes first, from the time the vehicle was first put into service."

Ford owners who already have paid for such repairs will be reimbursed, Barion said.

At the same time, Ford filed a brief in with the FTC opposing staff motion asking the commission to force Ford to take the action it has just announced it will take.

The staff also had asked the FTC to approve a federal court suit against Ford if the auto maker did not act on its own.

The FTC staff had contended in its brief that Ford was secretly reimbursing dealers who were fixing the problem, but that the dealers were not performing the work because the reimbursement didn't cover the cost of replacement.

The staff also charged that Ford did not notify car owners of the problem after it became known. In some cases, the camshaft and rocker arm assembly had worn out in less than 200 miles, the staff said.

The Ford brief argued that the improper wear is not the result of any "design defect in our engines and is, in fact, not limited to Ford Motor Co. products. We have determined the condition as it applies to the 2.3-liter engine results from the use of some . . . oils containing insufficient anti-wear [WORDS ILLEGIBLE].