SEATTLE (AP) _ Machinists who stayed out 48 days on the picket line for a better offer from Boeing voted overwhelmingly Monday night to approve a three-year contract and return to work.

The pact that could become the standard for the aerospace industry was approved by 81.4 percent, the Machinists union announced.

Strikers will return to work Wednesday morning, said Tom Baker, International Association of Machinists and Aerospace Workers District Lodge 751 president.

''They can do that with pride in themselves and in what they've done,'' he said.

Union members voted in Seattle, near Wichita, Kan., and in Portland, Ore., on the agreement reached Sunday with the world's biggest manufacturer of commercial jets.

The company's second-longest strike virtually halted production, delaying deliveries to airlines at a time of growing passenger loads and rising concern over aging jets.

The Machinists union represents about 43,300 workers in Seattle, 12,000 in Wichita, 1,700 in Portland and a few hundred more in scattered sites, including California, Hawaii, Utah, Wyoming, Montana, North Dakota and other states.

Reporters were barred from the meeting at the Kansas Coliseum north of Wichita. Dozens of people stayed only long enough to pick up a seven-page explanation of the offer.

There was no clearcut sentiment among union members who left early.

''I voted to go back to work,'' said Rodney Nobles of Wichita. ''I think it's a good contract. They gave us our lump-sum improvements.''

Galan Smith, 22, of Wichita, said he voted against the contract.

''We're wanting a larger wage increase in the general wage,'' he said. ''We've been out long enough, we feel like they should make up for some of what we've gone through.''

Baker said the proposed contract substantially improves two earlier Boeing offers, including boosting the total dollar value.

Other union leaders were less satisfied. Justin Ostro, an international union vice president who coordinates aerospace industry negotiations for the Machinists, said Boeing workers were entitled to ''a great deal more.''

Boeing officials said they were pleased with the pact.

According to a handout distributed at the Kansas Coliseum, the offer included four major improvements from the Oct. 3 proposal machinists overwhelmingly rejected before going on strike the next day:

- Lump-sum bonuses of 10 percent of gross pay the first year, 5 percent the second and 4 percent the third.

- A 60-cent-per-hour cost-of-living increase immediately, with quarterly increases thereafter.

- An increase in the cost-of-living adjustment base pay by 45 cents an hour to make up for a 1986 adjustment workers didn't get.

- A cut in mandatory overtime to two consecutive weekends and 144 hours per quarter, with double-time pay after 160 hours, down from 200 mandatory hours of overtime per quarter and four consecutive weekends in the previous contract.

Machinists have said that after six years of accepting yearly bonuses rather than wage increases, Boeing workers were entitled to a healthy chunk of the company's prosperity.

Boeing had record earnings in its most recent quarter and has a backlog of orders for more than 1,700 jets worth about $85 billion, with delivery dates stretching beyond the turn of the century.

A settlement with Boeing machinists traditionally has set the pattern for other aerospace companies, especially Lockheed Corp. and McDonnell Douglas Corp., as well as for other unions at Boeing.

The Seattle Professional Engineering Employees Association, which represents about 28,000 engineers and technical workers, has agreed to extend its contract with Boeing, which expires Dec. 1.

Boeing Helicopters in Philadelphia and United Auto Workers Local 1069, which represents about 3,000 production workers, extended their pact beyond its Oct. 4 expiration.

Union contracts between machinists at McDonnell Douglas in Torrence, Calif. and Lockheed Corp. at Burbank, Calif., expired in October, but negotiations have been extended. Those talks are separate from the Boeing negotiations, and the outcome of the Seattle talks had no direct effect.

Don Nakamoto, spokesman for Machinists District Lodge 727 in Burbank, acknowledged the Boeing developments have been closely watched.

''I'm sure it will have some impact,'' Nakamoto said. ''It's hard to say what it will be but I think both parties will be taking a hard look to see what it means to our talks.''

During the strike, Boeing, using supervisory and non-striking workers, delivered 22 planes, but all were virtually complete and out of the factories before the walkout.

Boeing has said the strike will have a serious financial impact during the last quarter of the year, but has not given specific forecasts.

The last Boeing strike, in 1977, lasted 45 days. There was a 19-day strike in 1965, and a 140-day walkout in 1948.