Wally Backman enabled the New York Mets to take a one-half game lead in the National League East tonight by doubling in the eighth inning to beat the Montreal Expos, 1-0.

Ron Darling, who had a toothache, and Roger McDowell held the Expos to four hits in the game, the only action in the league today. The game was added to the schedule to make up a game lost to the players' strike two weeks ago.

Danny Heep, hitless in nine tries as a pinch hitter this season, batted for Darling to open the eighth and doubled to right-center off Tim Burke (8-1), who had come in to start the inning.

Len Dykstra grounded out, moving Heep to third, then Backman doubled just beyond the reach of center fielder Herman Winningham.

"It was only a one-day trip to Montreal but it sure was well worth it," Backman said. "Right now we're a half of game in front in first place but it's still going to go down to September. It's between us, the Expos and the Cardinals. The team that is the best in September will win it all."

Burke, a rookie who had never lost in the majors, also lost his chance to set a team record for consecutive victories. Starter Bryn Smith had held the Mets to three hits in his seven innings before leaving for a pinch hitter.

Darling pitched seven innings, walking five and striking out three, then McDowell got six straight outs for his 11th save.

Darling, who had lost a filling in a tooth, said he slept in the clubhouse before the game.

The Mets squandered a scoring chance in the fourth as Dykstra led off with a single and went to third when Backman singled. Keith Hernandez flied to left fielder Tim Raines, who threw out Dykstra at the plate.

Andre Dawson tripled with one out in the Montreal sixth, but Darling struck out Hubie Brooks and got Terry Francona on a grounder.

New York's George Foster hit a fly to right in the ninth inning that Dawson played into a three-base error. But, when Foster tried to score on the play, Dawson threw him out at the plate.