NASHVILLE, Tenn. (AP) _ Workers at a Peterbilt ''big rig'' tractor plant voted 284-84 Saturday to end a three-month strike after the company threatened to hire replacement workers.

The workers approved the same contract they had rejected a week ago. The pact, which includes a 3 1/2 percent raise, requires, for the first time, that employees pay part of their health insurance costs.

''I consider it probably the worst contract that's ever been negotiated in so far as the differences between the old contract and the new contract,'' said Mike Brown, president of the United Auto Workers Local 1832.

But officials at Peterbilt's parent company, PACCAR Inc., of Bellevue, Wash., warned that replacement workers would be hired if strikers didn't return to work by Aug. 17. Union officials said workers would return Monday.

Under previous contracts, Peterbilt paid for employee and family health insurance. Now employees must pay up to $60 a month initially, escalating to $80 a month in the third year, for family coverage.

Workers at the plant make the ''big rig'' tractors used to pull 18-wheel truck rigs. About 400 people are employed there.