The Pittsburgh Pirates got a shutout pitching performance from Nelson Briles to salvage a doubleheader split with the Montreal Expos and remain a half game behind the Mets, who were idle yesterday.

Montreal blew what was probably its last chance to stay in contention as the Pirates scored a 3-0 victory paced by Willie Stargell's 43rd homer. In the first game of the twinighter played before a standing room only crowd in Montreal the Expos gave their fans something to cheer about as they chased Pittsburgh starter Bob Moose for a 5-4 victory. Steve Renko got his 15th win against 11 losses.

The split means that the Pirates and Mets each have lost 77 games this year.

The other contenders--St. Louis and Chicago--were idle yesterday.

In the National League Western division the Cincinnati Reds clinched the championship with a 2-1 victory over the San Diego Padres yesterday. The Reds were led by the powerful bats of Tony Perez and Andy Kosko, who each hit solo home runs. It was the third division title in four years for Cincinnati. Dick Baney, making his first National League start, earned the victory with late help from relievers Tom Hall and Pedro Borben.