The International Association of Machinists in Aerospace District 751 Wednesday approved a four-year contract with Boeing by a 74% margin that assures the union of continued job security on the company’s 737 MAX re-engining program.

An IAM official said there was “heavy turnout” among the 31,000 eligible voters, mainly in Seattle’s Puget Sound area, but also Oregon, Kansas and California. The IAM did not release the actual vote tally.

“This agreement represents a historic moment in changing the relationship between this union and the Boeing Co.,” said District 751 President Tom Wroblewski. “As a result of this vote, we have the strongest commitment to the future of aerospace jobs in Washington state that we’ve ever had.”

Boeing Commercial Airplanes President and CEO Jim Albaugh said the contract “reflects an effort on the part of the company and the union to find a better way to work together and achieve common ground.”

The IAM’s current four-year contract did not expire until September 2012, but Boeing proposed renewing it early with a commitment to keep production of the 737 MAX at the company’s current 737 factory in Renton, south of Seattle. The company had been considering moving the work away from that site, which dates to World War II, raising concerns that the work would move out of Washington.

When Boeing moved a second final assembly line for the 787 to its non-union factory in North Charleston, S.C., the IAM complained to the National Labor Relations Board (NLRB). The NLRB began hearings last June on a staff move to force Boeing to shift the 787 work back to Puget Sound.

Besides assuring that MAX will remain in Renton, Boeing also made work commitments on current widebody and military programs in Puget Sound.

In return, Union leaders, who strongly backed the new contract, vowed to drop their NLRB complaint if members ratified the contract.

The extended contract brings machinists a 2% pay increase each year, cost-of-living adjustments, a $5,000 signing bonus, an incentive plan tied to productivity improvements that will pay up to 4% each year, added pension payments for new and existing employees and continuation of the current company-match 401(k) program.

Union members will pay higher medical premiums starting in 2013.

Wroblewski acknowledged the long-time strife between Boeing and the union, which has struck three times in the past 15 years. He says ratification means the two sides will now work together.

“It’ll be a big shift; that we both need to embrace. But we must do this,” he said. “Because it’s obvious that we as union members cannot prosper if the Boeing Co. isn’t successful.”