Vladimir Guerrero got angry. Then he got even.
Guerrero, whose game-winning shot came off reliever Scott Williamson (11-6), stood at home plate and exchanged words with Stan Belinda in the seventh after the Reds reliever who broke Guerrero's hand with a pitch in 1997 hit him on the left arm.
With Manny Martinez on first, Guerrero fouled off two 1-2 pitches before hitting a 445-shot over the left-field stands to give the Expos their fourth win in six games.
"He didn't get too many pitches to hit," Expos manager Felipe Alou said. "Finally someone hung him a breaking ball and he hit it. It's the longest ball I've ever seen hit here to that part of the field, especially with two strikes."
Guerrero came out for a curtain call his third in Montreal's last four games to acknowledge the cheers of the crowd of 11,238.
"He's a very dangerous hitter," Williamson said. "He had a 31-game hitting streak with a lot of home runs in there. He's a great hitter and I made a mistake."
Steve Kline (6-3) pitched a perfect ninth for the win.
Trailing 6-2, Montreal tied the game with four-run fifth, capped by Brad Fullmer's sacrifice fly.
Scott Sullivan relieved and Merced advanced to second on Rondell White's grounder to short. Wilton Guerrero tripled to make it 6-5 and then scored the tying run on Fullmer's fly to the right-field wall.
Tomko, who is 1-3 in his last 12 starts, allowed five runs and six hits in four-plus innings.
Casey, who also walked, is hitting .483 with 15 RBIs in seven career games at Olympic Stadium. He has 14 hits in 29 at-bats in Montreal, including two doubles and a homer.
Michael Barrett went 3-for-4 with an RBI double for Montreal.
Cincinnati took a 2-0 lead in the first on Aaron Boone's two-run single and could have done more damage, but Jason LaRue flew out to center with the bases loaded to end the inning.
The Expos tied it 2-2 in the home half on Fullmer's run-scoring single and Barrett's RBI double.
Casey hit an RBI single in the second to put the Reds ahead 3-2 and added a three-run double in the fourth.
Smith, who is winless in his last four starts, allowed seven hits and five walks in 3 1-3 innings.
"He made some bad pitches late in the count when he had two strikes on guys," said Widger, who caught the game. "He didn't make a couple of pitches to put them away but other than that, he pitched pretty well."
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