Despite recurring instances of engine failure due to missing oil seals on Eastern Airlines L1011 jetliners, the two mechanics who serviced the jumbo jet that lost power in all three of its engines over the Atlantic Ocean did not check whether the seals were in place, they testified today.

That May 5 flight, carrying 172 persons from Miami to Nassau, made a dramatic emergency landing here on one restarted, crippled engine and triggered a major federal investigation that has turned up startling news about airline maintenance.

Eastern mechanics Lynn Burris and James Sunbury told National Transportation Safety Board investigators that they are responsible for ascertaining whether the oil seals, known as O-rings, were installed.

"Sometimes you visually check, sometimes you don't," Burris said. This time, they were working outdoors at night in poor light and did not, they said.

Federal officials said today that preliminary checks with other airlines flying the L1011 domestically, including Trans World, Delta and Pan Am, found no similar pattern of engine failure.

As recently as March 8, Eastern mechanics' general foreman, Joseph Pacheco, had posted a warning on the maintenance department bulletin board here that missing O-rings had resulted in an engine power loss on another Eastern L1011 a day earlier and four or five other similar occurrences. Sunbury said he saw the note, Burris said he did not and both said no one brought it to their attention.

Pacheco, who has charge of about 30 mechanics, testified that Eastern's procedure on handling oil seals has not been changed since the May 5 incident.

But his boss, Kenneth Wilson, testified later that changes have been ordered, and Eastern spokesman James Ashlock said they have been implemented. "I can't explain the inconsistency," he said.

Federal sources said that, counting the May 5 incident, 13 examples of Eastern L1011s losing an engine because of oil problems have been documented, dating to 1981. Only one incident has involved more than one of the Rolls-Royce RB211 engines, they said.

Despite an Eastern training directive issued in November, 1981, to alert maintenance personnel about such incidents, Burris and Sunbury said they had not heard about two incidents earlier that year.

The source of the drama is a two-inch-long bolt with a magnetized tip that is screwed by hand into a fitting in the engine's oil line. It is replaced daily on each L1011 engine, and the tip is checked to see if metallic chips have collected on it, a sign of engine wear. The replacement bolt is to be installed with two rubber O-rings.

Safety board officials are concentrating on who knew what about the seal problem, when it was known and what was done. The same types of questions were raised after 273 persons died when an engine fell off an American Airlines plane on takeoff in Chicago in 1979. That crash was traced to a maintenance problem previously discovered but not widely disseminated throughout the industry.

A major concurrent question is whether Federal Aviation Administration airline inspectors, who have access to information on such incidents, took any action to ascertain whether Eastern adequately informed its mechanics of problems or told other airlines flying L1011s of Eastern's difficulties.

On March 8, Pacheco wrote and posted his warning, saying in part, "Yesterday we dispatched an L1011 with a master chip detector bolt with no O-rings installed. This caused another in-flight shutdown due to loss of oil." The memo urged extra care by mechanics.

Regarding that letter, NTSB investigator Edward Kraus today asked Burris: "Do you recall any discussion about chip detectors?"

Burris: "No, sir."

Kraus: "Did you receive any special training on changing the chip detectors?"

Burris: "No, sir."

Kraus: "How did you learn how to change the detectors?"

Burris: "Strictly from the work assignment cards."

Kraus: "Were there mechanics' meetings where the chip-detector problems were discussed?"

Burris: "No, sir."

The May 5 incident occurred because Burris drew the chip-detector bolts from the stockroom when they were not available in the supervisor's area, the normal procedure. The parts he drew did not contain O-rings, although they bore a tag indicating they were "serviceable."

Wilson, manager of terminal aircraft maintenance for Eastern here, testified later that the tag should have said "unserviceable." Since the incident, Wilson said Eastern requires that O-rings be stored with the bolts.

Burris and Sunbury were questioned about their engine-testing procedure after installing new bolts. Without seals, a proper test should produce an oil leak, experts say, when the engine is set in motion with compressed air, which pressurizes the oil system.

Federal officials have learned that it takes about 20 seconds under pressure to produce a leak. Burris and Sunbury said they thought a 10-second test was long enough. Eastern now requires at least a 30-second test, Wilson said.

Changing the bolts, Wilson testified, "is a procedure so routine, so simple, I don't think anyone realized the magnitude."

Wilson said Eastern officials were unaware of the airline's 12 previous bolt-related engine failures because they were "system-wide" and "so scattered, most of us weren't aware of them."

The board, in the first of two days of interviewing witnesses, is also asking why, after the first engine failed, the flight crew elected to return to Miami instead of continuing to Nassau, which was closer.

Capt. Richard Boddy explained that a thunderstorm was building in Nassau, which has no radar, so he returned to Miami. That was before he knew power would be lost in all engines. He has also noted that Nassau lacks maintenance facilities such as those Eastern has in Miami.

Flight attendant Christine O'Steen described frightened passengers who inflated their life vests despite her instructions and screamed in panic, "especially when the pilot said ditching was imminent."