Boeing Co. faces a quandary in final contract talks with union machinists that begin Thursday: It must avoid a strike that would cause delays on an already backed-up production line, yet be tough enough to make inroads against mounting pension and health-insurance costs.

The round-the-clock talks are aimed at averting a strike like the one that shut down the company’s jet production lines for almost a month in 2005. Going into the talks, both Boeing and the International Association of Machinists and Aerospace Workers are digging in on the same thorny issues — health care, pensions and job security — that led to the walkout three years ago. The union is scheduled to vote on Boeing’s final offer on Sept. 3.

Boeing has sought to limit its pension and health-care liability by paring who is covered by plans or by asking employees to pay part of the costs through payroll deductions and insurance co-payments. In successive contract negotiations, Boeing has pushed for limits to future retiree benefits and sought changes in job-security language that permitted the company to farm out work or bring in contract employees to save costs.

Union officials have already said they are disappointed with Boeing’s initial offers, predicting if things don’t improve, members will strike. “Boeing is negotiating like it’s in bankruptcy court,” said Mark Blondin, lead negotiator for the machinists union’s main offices. “I hope their game plan isn’t to stick with the approach they’ve taken so far. If it is, there will be a work stoppage.”