Ford Motor Co. yesterday recalled 1.5 million Pinto and Mercury Bobcat cars, to make alterations that will reduce the risk of fuel tank fires in rear-end accidents.

The manufacturer's decision came in the wake of a federal investigation that showed that low moderate-speed rear-end collisions of Pintos had produced massive fuel leaks by punturing or tearing the fuel tank, and separating the filter pipe from the tank.

Involved in the recall are all 1971-1976 Pintos and 1975-1976 Bobcats, except station wagons. Government sources said letters to purchasers of teh next cars should be mailed within the next three months; by which time Ford will have to make corrections.

Ford spokesman, Sue Galvin said the recall campaign will cost the firm between $20 and $30 per automobile. That means the cost could reach $45 million if all the cars are returned for repairs. But in a typical recall about half the automobiles involved are brought in: at rate the cost to Ford would be under $25 million.

Last month, the National Highway Traffic Safety Administration said it had received reports of 38 fires, with 27 fatalities and 24 non-fatal burns, resulting from rear-end collisions with Pintos. No similar reports had been received on Bobcats, but Ford told the government agency that the Mercury line's fuel system is identical to the Pintos.

A jury in Santa Ana, Calif., recently awarded a record $28 million to a teenager who was burned critically in a Pinto accident that resulted in a fire and death of the driver. The youth agreeds to a reduced award of $3,5 million but Ford is appealing the verdict.

Yesterday's recall, one of the largest in automotive history, was not the first involving the Pinto - a car introduced in 1970 as a crossbreed of American and European technology that was designed to battle small imported autos. More than 2.5 million have been sold.

Soon after the Pinto was introduced, 26,000 were recalled because accelerators were sticking. Later, 220,000 Pintos were recalled for modifications to prevent possible engine compartment fires.

Consumer groups first raised complaints about Pinto fuel tanks last year. Ford replied with an eight-page statement saying the charges were exaggerated.

But NHYSA, a Department of Transportation agency, informed Ford on May 8 about results of the new investigation, which concluded that Pintos had a safety defect. A public hearing was sheduled for next week, at which time internal Ford documents related to the fuel tank situation were to be made public, according to consumer advocate Ralph nader.

A spokesman for NHTSA said that his agency and Ford began a "process of negotiation" after May 8 that led to Ford's announcement in Detroit yesterday. "They knew something would come out and there would be bad press . . . they decided to cut their losses," said Nader.

In a prepared statement, Ford vice President Herbet L. Misch said: "Ford informed NHTSA that it does not agree with the agency's initial determination of May 8 that an unreasonable risk of safety is involved in the design of these cars . . ."

Misch said Ford decided to offer the modifications "so as to end public concern that has resulted from criticism of the fuel systems in these vehicles".

Specifically, the repairs planned by Ford are replacement of the original fuel-filler pipe and seal with a longer pipe and improved seal, and installation of a polyethylene shield across the front of the fuel tank.

Nader termed these changes "inadequate technical corrections" and urged the government not to accept Ford's plan and to insist on double-lined fuel tanks and other corrections.

But the changes "will significantly reduce the possibility that a rear-end collision would cause the fuel-filler pipe to seperate from the fuel tank or that the front of the fuel tank would be punctured if pushed against the rear axle".