Pedro Guerrero and Mariano Duncan each hit a grand slam last night in Montreal to give the Los Angeles Dodgers an 8-4 victory in a game in which the Expos lost starter Joe Hesketh for the season with a broken leg.

In the second inning with Montreal leading, 1-0, U.L. Washington singled and Hesketh walked. Tim Raines hit a double to shallow center off the glove of Candy Maldonado. Washington scored and Hesketh tried to score all the way from first. But catcher Mike Scioscia blocked the plate and Hesketh tripped over Scioscia's foot, landing hard on his left leg. He was carried off the field on a stretcher and taken to a hospital where it was determined that he fractured his left shin bone. Hesketh, a rookie left-hander, is 10-5.

It was Montreal's fifth straight loss.

Orel Hershiser (13-3) won his fifth straight and his 16th in his last 19 decisions over two seasons. He worked only five innings and gave up eight hits. Tom Niedenfuer notched his 13th save.

The Dodgers trailed, 3-0, in the sixth, when Tim Burke (8-2) walked Scioscia leading off. Steve Sax and Duncan singled to load the bases. Enos Cabell's fielder's choice forced Scioscia at the plate, but Guerrero hit his 30th home run, over the center field wall, to make the score 4-3.

In the seventh, singles by Greg Brock and Ken Landreaux and a walk to Scioscia loaded the bases. Pinch hitter Len Matuszek forced Brock at the plate before Duncan hit a pitch from reliever Gary Lucas over the left field fence to make the score 8-3.