Scott Hoch of Raleigh; N.C., and John Cook of Dublin, Ohio, scored 20-hole, semifinal victories yesterday and gained today's final of the 78th U.S. Amateur golf championship at Plainfield Country Club.

Hoch two-putted for par on the second extra hole and upset Bob Clampett of Carmel, Calif. Cook birdied the hole and eliminated Mike Peck of Kansas City, Kan.

Hoch, 23, who played for Wake Forest and won the 1976 North Carolina Amateur, three-putted the 17th hole, allowing Brigham Young All-America Clampett to pull even.

They halved the 18th. On the first extra hole, Clampett's second shot was stymied by trees but he played a sweeping hook around them and halved the hole in par.

Clampett again pushed his drive behind trees on the 20th hole. He punched a shot on the fairway, short of the green. Hoch hit the par-4 hole in two shots and won the match when Clampett took three to get down from the fairway.

Clampett was low amateur in the U.S. Open at Cherry Hills, N.J.

The gallery's largest roars were heard in the Cook-Peck shootout.

Peck knocked his approach shot into the cup on the par-4 10th hole for eagle duece to square the match. Peck had eagled the 13th hole with a drive and a six-iron in the morning quarterfinals. That ball landed in the hole on the fly, damaging the cup and forcing USGA officials to relocate the hole. Peck's eagle against Cook rolled into the cup after landing on the green.

On the 13th hole, Cook's drive was four inches out of bounds and he fell one down.

Cook, two-time Ohio State All-America, pulled even with a 10-foot burdie putt on the 15th hole. They halved the next four holes and Cook won with a 20-foot birdie putt.