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General Motors Corp. plans to close as many as 13 factories, four more than it previously said, within the next four years if members of the United Auto Workers approve a tentative contract that union leaders have said will save jobs.

An engine plant in Livonia, Mich., near Detroit, would close in 2010, according to the text of the contract, which UAW members began to review over the weekend. Other plants vulnerable, according to the contract, are a small-engine plant in Parma, Ohio; a metal-stamping plant in Indianapolis; and a stamping plant in Flint, Mich.

Together, the four plants have about 2,500 employees. Many of their workers are eligible to retire with full benefits, and the contract states that GM is to offer more buyouts, which would ease the process of cutting jobs.

Anyone laid off, however, would be subject to new restrictions on the length of time they could continue receiving pay and benefits under a longtime job-security provision known as the Jobs Bank.

The plant closings are among the negative aspects of the contract that workers are learning about since union leaders unanimously approved the deal on Friday. All 73,000 of GM’s hourly workers are scheduled to vote on the deal by Oct. 10.