Thousands of motorists were stalled in an eight-mile-long traffic jam on the Capital Beltway during yesterday morning's rush hour after a huge girder destined for a future Metro transit line became stuck in a median near Rockville Pike.

Traffic was blocked up in the Beltway's westbound lanes from New Hampshire Avenue to the Rockville Pike exit, and on several other Montgomery County roads.

A Metro constractor. Expressway Construction, had a county permit to keep the Beltway closed between 1 a.m. and 5 a.m. The State Transportation Department granted an extension after a dolly carrying the girder became mired in a soft spot on the Beltway median, a Metro spokesman said. The westbound Beltway lanes were kept closed until about 9 a.m.

The Beltway at Rockville Pike is tentatively scheduled to be closed again between midnight and 5 a.m. next Tuesday, Wednesday and Thursday, according to Bill Keim of the country's division of traffic engineering.

The county executive's office was deluged with calls from angry commuters, some of whom threatened to move out of Montgomery County according to Charles Majer, aide to County Executive James P. Gleason.

Gleason himself said he was delayed 20 minutes driving to work because of the traffic jam.