Federal Aviation Administration investigators are studying evidence that damage in the engine mounting pylons of about 30 DC10 jumbo jets is related to improper installation of bolts during assembly, The Washington Post has learned.

The problems were not a factor on the plane that crashed in Chicago May 25 and killed 273 people but were discovered during inspections of oter DC10s after that crash.

Two highly placed federal sources said today that the series of cracks in a critical area of the engine pylon apparently were caused by less-than-perfect installation of special bolts. "We think we have a quality control problem during assembly," one of the sources said. Both stressed that the finding was preliminary and could change.

The special bolts on about 30 of the 138 DC10s in the U.S. fleet were found to have cracked or sheared, the sources said. That in turn led to some metal fatigue cracking in the spar web, a large plate that connects structural supports in the pylon.

The most dramatic case was discovered on a United Airlines DC10 inspected in Chicago. The fatigue cracking in the spar web was so severe that the engine and pylon could be moved laterally by hand. A United mechanic noticed the problem when he was inspecting another area of the pylon, one that is thought to have failed on the crashed plane, American Airlines Flight 191.

"One thing is for sure; we have to decide definitively what caused that spar web cracking before we let the plane fly again," another FAA source said.

Officials had thought earlier that the spar web cracks might be related to vibration caused when an engine shut down in flight, as happened in the case of the United Airlines plane.

However, engine tests and other evidence have led them to discard that theory. "We are unable to correlate engine stoppage with the damage," said Charles Foster, the FAA's associate administrator for aviation standards.

A spokesman for McDonnell Douglas, manufacturer of the DC10, said that all data on the problem had been turned over to the FAA. A service bulletin - a manufacturer's advisory to users of the DC10 - was issued recently suggesting a chect of the bolts on some planes, the spokesman siad.

FAA Administrator Langhorne M. Bond, Foster and other key headquarters officials are here to press the investigation into the DC10 problems and find solutions for them in hopes of returning the planes to service. All domestic DC10s have been on the ground since June 6.

McDonnell Douglas has appealed the grounding to the National Transportation Safety Board, and a once-postponed hearing on that appeal is scheduled here Monday. FAA officials are seeking another postponement on the grounds that the hearing would occupy the people trying to resolve the DC10 problems. The FAA is also pushing hard to find solutions to those problems, perhaps in hopes of making the hearing moot, sources said.

Bond said in a brief interview that McDonnell Douglas has submitted all the data the FAA has requested, and that his agency's experts, plus consultants from industry and academe, are studying it. He declined to put a timetable on the effort but said, "We are moving at a good pace."

Bond's order grounding the DC10 came because of problems found in the pylon. The safety board, which is officially charged with determining the cause of the Chicago accident, has found that there were failures in two of the three points that held the pylon to the left wing on the crashed plane. The third point could not carry the load alone, the pylon came loose and ripped out hydraulic and electric controls.

This happened just as the plane was taking off from O'Hare International Airport. After climbing to an altitude of 600 feet, the plane rolled to the left and crashed.

FAA officials are leaning heavily toward the conclusion that damage in the attachment points - particulary the aft pylon bulkhead - is related primarily to maintenance practices by some airlines. However, that maintenance problem is exacerbated by a pylon that is difficult to inspect and difficult to service, aviation experts say.

Before the DC10 can be permitted to fly again, a rigid inspection program for the pylon area will be established. The inspections will have to be repeated frequently unless a newer, less-maintenance-intensive design is approved and built for the DC10, officials said.

Safety board and FAA experts started experiments today in the McDonnell Douglas lab to determine whether a DC10 damaged as American Airlines Flights 191 was could conceivably continue in flight. A number of senior test pilots are taking turns in the simulator to check various possibilities. Nobody is criticizing the actions of Flight 191's pilot, who accordingto all experts performed exactly as he was trained.