An arbitrator in New York City has ruled that the San Francisco Giants must honor a contract they offered to Joel Youngblood and then withdrew when the outfielder hesitated to accept a drug-testing clause, The New York Times reported yesterday.

The newspaper said the testing clause will be included in Youngblood's contract, but that arbritrator Thomas Roberts said his ruling would not serve as a precedent for a class-action grievance by the Major League Baseball Players Association.

The union contends drug-testing clauses in several hundred major league contracts are invalid and testing should be settled by collective bargaining, not in individual contracts.

Youngblood, 34, hit .270 with four home runs and 24 RBI for the Giants last season. His lawyer, Richard Moss, and the Giants agreed in November on a contract that would pay Youngblood $450,000 this year and next with an option for a third year at the same salary.

The Giants later asked Youngblood to accept a drug-testing clause. At first, he said no. Later, when he changed his mind, the Giants said they had withdrawn the contract . . .

A coalition of public and private investors has completed the purchase of the Pittsburgh Pirates for $21.8 million, ending months of uncertainty over the future of the National League club. Pirates President Malcom (Mac) Prine said the purchase of the team from the John Galbreath family of Columbus, Ohio, was finalized in expectation of a $20 million loan from the city of Pittsburgh, which approved a $21 million bond issue to help the buyers.

Pittsburgh Associates includes 13 investors: Aluminum Co. of America, Carnegie-Mellon University, Eugene and Raymond Litman, John McConnell, Mellon Bank, National Intergroup Inc., PNC Financial Corp., PPG Industries Inc., Ryan Homes Inc., Frank J. Schneider, U.S. Steel Corp., Harvey M. Walken and Westinghouse Electric Corp. . . .

The Arizona Industrial Commission has ruled that inadequate training of a plumber, who was attempting to purge a natural gas line in an inadequately ventilated room when gases were ignited by a heater, was responsible for the clubhouse explosion last month at the Milwaukee Brewers' new spring training complex in Chandler. The plumber, Jeff Sutton, and Brewers coaches Tony Muser and Larry Haney were seriously injured in the blast.