More than half a million Ford Pinto owners have been sent letters by the Ford Motor Co. instructing them to bring their cars into Ford dealers to be retrofitted with new equipment to protect the gas tank against catching fire in case of rear-end crashes.

About 1 million more Pinto and Mercury Bobcat owners will receive similar letters by Oct. 15, the auto-maker reported yesterday.

Ford spokesman George Trainor said that although the recall covers all Pintos and Bobcats - except for station wagons - built between 1971 and 1976, only model years through 1974 have been recalled thus far.

The 1975 and 1976 models have a slightly different gas tank configuration becasue of the switch to unleaded gasoline. Trainor said, and the kits needed to fix those models have not been shipped to dealers yet.

He said 546,000 letters have been sent to owners thus far, and 408,000 repair kits have been sent and are either in the dealers hands or in the distribution system on route to the dealers.

Kits and letters pertaining to the later models will begin being sent during the first week of October, he said.

The letters are being sent out as soon as possible, the Ford spokesman said, indicating that it took time to locate many cars that had been resold one or more times.

Still, local consumers who had bought used Pintos have reported receiving the letters with instructions on the repairs. Ford stressed that Pinto and Bobcat owners need not wait for the letter to before bringing in their cars.

The repair kits, which Ralph Nader will claims provide "inadequate protection", cost between $20 and $30 each.It would cost the company up to $40 million to repair all of the cars in question, but recalls generally result in about half of the cars actually being returned.

Local dealers report that the time needed to repair each vehicle is usually about an hour, and that business has been brisk since the first repair kits arrived on Sept. 11.

"We've done about 40 or 50 of them in the past two weeks," said Jeff Neal, service manager at Academy Ford in Laurel.

But Bob Plummer, service manager at Northeast Ford, said the recall has brought him "between four and eight cars a day."

Plummer said that he doesn't mind that the repair work has "put a burden on my shop, because we find that once people bring their car in, they have us do other work at the same time."