Seattle ? A strike vote by The Boeing Co.’s largest union failed Friday, automatically imposing on union members a contract offer that most didn’t want to accept.

Three-fifths of Machinists union members cast ballots in favor of a strike and against the aerospace giant’s “best and final” contract offer. But the vote fell short of the 67 percent necessary to authorize a strike, meaning the company’s final offer goes into effect.

The vote was a blow to union leaders, who had urged approval of the strike.

The union represents 25,000 workers in Washington state; Wichita, Kan.; and Portland, Ore. It held the vote Thursday and Friday, and announced the results Friday night, less than an hour after workers at a Boeing helicopter plant in suburban Philadelphia walked out.

The vote comprised two questions: one on whether to approve Boeing’s contract offer, made Aug. 27, and another on whether to strike. Some 62 percent of the union’s members voted to reject the contract, but union bylaws dictate that unless two-thirds of members vote to strike, a contract offer is automatically accepted.

Members said the key sticking point was job security an especially sensitive point this past year. After the Sept. 11 attacks, which devastated the airline industry, Boeing slashed production and announced it would lay off 30,000 workers.

The layoffs claimed 25 percent of the union’s membership.

The Machinists had been working without a contract since Sept. 1, when the previous three-year agreement expired. The average wage under the old contract was about $50,000 a year.

The new contract includes an 8 percent ratification bonus; wage increases of 2 percent and 2.5 percent in the second and third years; and a 20 percent boost in monthly pension payments by the third year.

The contract increases health costs for many employees. But the biggest concerns for union leaders were what they called attacks on job security. Boeing refused to tie employment levels to indicators of the industry’s health.