General Motors Corp. Chairman Thomas A. Murphy modified the automaker's position on air bags yesterday for the second time in 10 days by advancing the target date for making the front-seat safety devices extra-cost options on GM small cars to "probably. . . 1984 or 1985."

The new timetable contrasts with one in a statement GM made in March to the National Highway Traffic Safety Administration that the company "currently does not plan to offer inflatable restraints in medium -- or small size cars" in the 1982-1986 model years.

Murphy was questioned about air bags in an hour-long session with reporters at the National Press Club. Outside, up to 40 chanting, sign-carrying pickets -- most from organizations afffiliated with Ralph Nader -- protested GM's June 3 announcement that it was postponing the introduction of the optional air bags on its large cars to the 1983 models. Six months earlier, GM said it planned to offer the devices on the 1982s.

Under a Department of Transportation safety standard, protective passive restraints -- air bags or automatic lap and shoulder-belt systems requiring no buckling up or other action by frontseat occcupants -- must be provided on all large 1982 models, all medium 1983s and all small 1984s.

GM said in the June 3 announcement that it would comply with the safety regulation by equipping the 1982 models with automatic belts. At the time, a spokesman also said that bags would be offered either on large or medium 1983s, although GM had told DOT in March that the cushions would be unavailable on the medium 1983s.

In response to questions yesterday, Murphy:

Estimated that GM would save $20 million by delaying optional air bags from the 1982 to the 1983 full-size cars.

Said it would be "very wasteful" to make air bags available on 1982 large cars becaue they will be the last before a new "downsizing," which apparently was decided upon since March.

Disclaimed "a possibility that there are going to be any more fatalities" as a result of the decision to delay air bags, although NHTSA Administrator Joan B. Claybrook has calculated that up to half of the owners of cars with automatic belts will cut or otherwise circumvent them because they will consider them inconvenient.

Pleaded with motorists to use active and passive belt systems, crediting them with superior potential to prevent death and injury, particularly because they protect in side as well as frontal collision.