Tony Pena hit a three-run homer in the sixth inning and Larry McWilliams pitched 5 1/3 innings for his 12th victory last night as the Pittsburgh Pirates defeated the Expos, 6-3, in Montreal.

McWilliams (12-6) allowed two runs on five hits and one walk; he struck out one. Cecilio Guante finished for his fifth save of the season. Steve Rogers fell to 14-7.

Pittsburgh took a 1-0 lead in the first inning when Marvell Wynne singled, was sacrificed to second by Johnny Ray, and scored on Bill Madlock's single to center.

The Expos tied the score, 1-1, in the fifth when Gary Carter hit his 13th home run of the season. But Pittsburgh scored four times in the sixth off Rogers to take a 5-1 lead.

Ray led off with a double and moved to third on a fly ball to center by Madlock. After Dave Parker was intentionally walked, rookie Doug Frobel hit a bloop double to left to score Ray. Pena followed with his sixth home run of the season, over the left field fence.

"Rogers gave me just what I wanted," Pena said. "When I came to the plate, with two men on base, I prepared myself for a fast ball and when he delivered one, I hit it over the wall.