Darrell Waltrip beat Bobby Allison by half a length in a Buick duel yesterday to claim his 12th NASCAR Grand National victory this season, the rain-delayed American 500 at North Carolina Motor Speedway.

It was the fourth consecutive victory for Waltrip and gave him a sweep of the three fall races in North Carolina.

Harry Gant was third in a Pontiac, Richard Petty finished fourth in a Buick and Joe Ruttman placed fifth in a Pontiac.

The 34-year-old Waltrip averaged 107.399 mph in the race, which featured 33 lead changes among 11 drivers on the 1.017-mile track.

In Martinsville, Va., Sam Ard raced to an easy record-breaking victory in the Late Model Sportsman half of the Cardinal 500 Classic doubleheader, ruining Geoff Bodine's bid for the third sweep in the history of the event.

Driving an Oldsmobile, Ard averaged 71.353 mph on the .525-mile Martinsville Speedway track, wiping out the track mark of 70.670 by Mike Porter and the old Cardinal 500 record of 68.971 mph by Morgan Shepherd.

Bodine, who earlier had raced to a two-second victory over Carl (Bugs) Stevens in the Modified half of the twin bill, was second to Ard in the Sportsman race.