Top officials from American Airlines and McDonnell Douglas, the owner and the builder, respectively, of the DC10 that crashed in Chicago May 25 and killed 273 people, spent yesterday morning in Congress pointing fingers at each other.

John Brizendine, president of the Douglas Aircraft Co. division of McDonnell Douglas, said the crack in the aft bulkhead of the engine support pylon that is believed to be the origin of the Chicago crash "could only have occurred when the pylon and wing were not connected and the aircraft was not in service."

In other words the crack occurred during maintenance at American Airlines.

Donald J. Lloyd-Jones, senior vice president for operations at American, testified that "it may be that we did cause the crack. It also may be that the existence of shims in the aft bulkhead created [a situation] that made the creation of crack inevitable . . ."

In other words, there might have been a manufacturing error, because shims were not part of the design.

Shims are small pieces of metal used to fill a gap and make parts fit. Sometimes they are designed into a structure, and sometimes they are used to fill unplanned gaps.

While this discussion was going on in the Senate Commerce aviation subcommittee, the Federal Aviation Administration was continuing its inspections of the grounded DC10 fleet. As of late yesterday, more than 50 of the 138 U.S.-operated DC10s had been inspected, and six had been found with cracks in the center spar - a support in the pylon that is described as nonload-bearing.

If the FAA decides that is not a critical problem, the way will be cleared for restoring the DC10 to service, perhaps as early as this weekend. The planes have been grounded since June 6.

When the planes are permitted to fly again, the FAA will require frequent reinspections of the pylon areas, inspections that will add considerably to the plane's down-time and to pressure on McDonnell Douglas to make the pylon more maintenance-free.

Brizendine said he considered the pylon "sound," and added that "we will examine any aspect of design" that the FAA wishes. As for shims, he said, "They are not unusual in aircraft construction."

Lloyd-Jones said that in his view the DC10 pylon does not have sufficient tolerance for maintenance abuse and that "a major review [of the pylons] should take place."

Nonetheless, he said, American Airlines will fly the DC10s if they are released because "we believe those inspection procedures . . . are adequate."

Brizendine also said Douglas had elected not to use locks that would hold out the DC10's high-lift wing slats in case of hydraulic failure because "mechanical locks are an added factor that can cause problems."

Douglas demonstrated during certification, he said, that the DC10 could be safely flown even with the slats extended on one wing and retracted on the other.

In Chicago, however, investigators have found that the American Airlines pilot did not know he had "asymmetrical slats" because the electrical warning systems that would have told him were shorted out when the pylon and engine fell off the left wing at takeoff. Therefore, he could not take actions necessary to fly through the asymmetrical-slats condition.

Lloyd-Jones said that American was studying engine-out flight procedures on takeoff to determine if more speed should be used. At that point in flight, however, more speed could only be gained with a reduction in the angle of climb and, Lloyd-Jones said, "there are some negatives in terms of obstacle clearance."

In a related matter, FAA spokesman Fred Farrar said yesterday that there are no plans to ground the widebodied Boeing 747 fleet just because one Pan American Boeing 747 was found with cracks in the engine-to-wing struts on two pylons.

The cracks were found during an FAA-ordered inspection of the pylons on all wide-body planes, Farrar said, and so far the Pan Am plane - one of the older 747s - is the only one of 94 inspected to be found with the problem. "We regard it as an isolated incident," Farrar said. CAPTION: Picture, DOUGLAS' JOHN BRIZENDINE . . . blames maintenance practices