Investigators into the nation's worst aviation disaster said tonight that they are looking for a bolt as they strive to learn why an engine, apparently in perfect working order, fell off American Airlines' flight 191 here Friday afternoon 38 seconds before it crashed.

While that search continued, the National Transportation Safety Board's vice chairman, Elwood Driver, said the death toll in the crash could exceed 273. "We have found infants' clothing in the wreckage, and infants are not usually carried on flight manifests," Driver said. The plane crashed on takeoff from O'Hare International Airport. It was headed for Los Angeles.

The 273 toll was reached today when workers found two bodies in a charred pickup truck parked on the field.

The missing bolt is the first major clue in the accident investigation. Probers have learned that they can expect to obtain little information from the crew's last recorded words.

Driver said that the cockpit voice recording, which was reviewed today in Washington, showed perfectly normal conversation until, suddenly, one crew member said, "Damn." "We don't know what 'damn' refers to," Drivers said. "Then all the electrical power went out, and the recording stopped."

As the aircraft accelerated down O'Hare's Runaway 32 Right, the crew members went through their check list. "V-1," a crew member said, then "V R - damn."

When a plane reaches V-1, it has passed the point of no return, and the pilot must continue to attempt the takeoff because he is going too fast to bring the plane safely to a halt on the runway. V R stands for velocity rotation, the point at which the plane is going fast enough for the nose to lift off the ground.

The flight data recorder, a computer-like device that records 32 specific functions of aircraft performance on a DC10, is in good shape and will give the board much helpful information, Driver said.

The engine, which fell off the left wing just as the plane was lifting from the ground, was examined carefully on the runway by investigators today. They plan to move it to an American Airlines hangar tomorrow to begin a careful disassembly in hopes of finding clues.

Driver, a retired Air Force command pilot, said the three-engine plane should have had enough power to continue taking off even with the loss of one engine "but we don't know what else might have gone wrong." Various control surfaces and hydraulic systems could have been damaged when the engine fell.

The impact area was an abandoned airfield one-half mile from the end of Runway 32 Right. It is surrounded by small commercial and residential areas. The remains of the plane itself cover remarkably little terrain, probably no more than 100 yards square. This indicates that the plane came almost straight down.

Teams from the Cook County sheriff's office and the medical examiner's office and investigator walked slowly through the rubble, carefully making notes or tagging fragments of metal. The airplane was nothing but bits and pieces. The largest visible sections were one engine and is flap from the tailing edge of a wing. Everything was charred black.

Witnesses to the crash said the heat from 79,500 pounds of burning and exploding jet fuel was so intense that rescuers had to wait hours before they could move onto the site.

Bruce, Arbeit, a fireman from nearby Norwood Park and one of the first on the scene, said, "There wasn't one body intact. Heads, arms, hands were all over the ground. There were shoes with legs in them. Small bodies that I thought were children may just have been people who were badly burned. They were all so badly burned . . ."

Dr. Robert Stein, Cook County medical examiner, said the bodies were so badly mangled that "we may never get positive identification on all of them."

At about the same time the crew member said "damn," one of the nine air traffic controllers in the O'Hare tower shouted, "Look at that! Look at that!He blew an engine!" At that instant 18 eyes were riveted on flight 191.

Driver said, "We have unusually good witnesses. They said that the climb attitude was about normal, but that the climb rate was less than normal." The plane rose to about 400 feet, took a slight turn to the left then rolled sharply to the left and crashed.

Driver said the investigators can tell from a trench dug in the field that the nose struck first followed by the left wing.

The missing bolt was described by Driver as "more of a positioning bolt," not a structural one. Nonetheless, he said, its absence "could cause problems."

Another bolt - similar to the one being sought - was found near the area where the DC10 had been parked prior to takeoff, Driver said. However, he said, "that is not the bolt we are looking for."

Driver said metal detectors will be used in a thorough attempt to find the bolt in the field where the plane crashed.

The missing bolt is called a nacelle mounting bolt, Driver said, "not a bolt that holds the pylon to the wing."

The pylon, the large supporting structure that connects wing and engine, is still attached to the engine.

When engine and pylon separated from the wing, Driver said, his experts believe that they went up and over the wing before falling.

Driver appealed through Chicago television stations and newspapers for anyone having photographs of the stricken plane in flight. "We need to take a closer look at the wing structure," Driver said, "particularly the area where the pylon separated."

The number one engine, built by General Electric, came to rest just right of the center line of the runway. Small peices of it were scattered along the runway for almost 2,000 feet. The runway remained closed today, somewhat hampering operations at the world's busiest commercial airport.

Langhorne Bond, administrator of the Federal Aviation Administration, told a morning press conference that he had "no plans" to ground DC10s, as was demanded today by consumer advocate Ralph Nader.

"There is simply not enough information to warrant that," Bond said. If such information is developed, he said, the FAA will move quickly. The safety board has responsibility for the investigation, but only the FAA can ground a plane.

American Airlines spokesman Art Jackson said a few relatives of the victims were flying into Chicago, "but there's really not much they can do."

American Airlines has featured in its television advertising the fact that passengers can watch their own takeoffs and landings on closed-circuit television. The camera is located over the pilot's shoulder and looks out a cockpit window.

This presents the gruesome possibility that the passengers of flight 191 could have been watching the ground rush up as the plane plummeted. "Of course we don't know whether the captain used the camera, but mostly of them do," Jackson said.

It has been widely reported in Chicago that the same American Airlines DC10 took off from O'Hare on Tuesday, lost power in one engine and returned to the airport. The incident happened, and on the same numbered flight, Jackson said, but not on the same plane. Jet engines do occasionally "flame out" in flight, but in almost all cases pilots are able to cope.

Driver said it was his experience that if a plane lost power in a left-wing engine, the plane would "yaw to the left," as engines in the center and on the right would continue at power. That would create an "asymmetrical control problem," Driver said, and "the lower the air speed the greater the control problem would be." CAPTION: Picture 1, and 2, Seen from Chicago terminal, DC10 has lost its left engine in a trail of white smoke across the wing and has banked more than 90 degrees counterclockwise toward its flaming crash just north of O'Hare., copyright (c) 1979, Chicago Tribune via UPI; Picture 3, Lost left engine of American Airlines three-engine jet awaits transportation from O'Hare to hangar. UPI; Picture 4, Jim King, chairman of the National Transportation Safety Board, shears shield from jet flight recorder. AP