The Metro board yesterday voted to abandon its practice of buying buses through competitive bidding, in an attempt to avoid purchasing defective vehicles similar to those recently taken out of service because of hazardous cracks in their undercarriages.

Under the new strategy the transit authority, which plans to spend more than $40 million to buy up to 330 buses in the next three years, will negotiate with competing companies to seek the "most advantageous" buses. Metro may agree to pay a higher price if it can obtain sturdier, more trouble-free buses.

"This will tighten up the process and we think we will get a reliable bus out of it," said Vernon K. Garrett Jr., Metro's engineering director.

The shift was triggered by a series of problems stemming from Metro's controversial 1983 purchase of 76 buses from Neoplan U.S.A. Corp., a West German-affiliated company. Four months ago, the authority halted use of the buses after detecting hairline cracks described as "a potential safety hazard."

Officials warned that the cracks, found near the vehicles' front axles, could result in a loss of steering control. The $12.5 million purchase had been vehemently opposed by one Metro board member, Fairfax County Supervisor Joseph Alexander, who cited defects in other Neoplan vehicles.

In a related development yesterday, Metro officials announced that Neoplan has started carrying out extensive repairs on 15 of the 76 faulty buses at a plant in Pennsylvania. The first of these vehicles may be ready to go back in service as early as next week, officials said.

If no further flaws are detected, all 76 Neoplan buses are expected to be repaired by next fall. Metro officials warned, however, that the buses must undergo inspections to ensure safety before being returned to service.

The repairs, which resulted from protracted negotiations between Metro and Neoplan, include replacing defective components, reinforcing steel frames to prevent cracking, and modifying faulty electrical and other systems. The overhaul is expected to cost the company more than $1 million.

Under Metro's new strategy for buying buses, the authority will solicit proposals from manufacturers and evaluate both price and quality. Metro then will negotiate with competing firms to seek the most favorable agreement.

Officials said that they intend to consider a range of factors, including the soundness of buses' steel frames and other components, reports from other transit systems, maintenance requirements, plant capabilities, responsiveness by company employes and warranties.

In a further attempt to minimize risks, the board voted to seek a contract for an initial purchase of 150 buses for about $22.5 million, with options to buy up to 50 more buses next year and as many as 130 additional buses later.

If the first 150 buses prove satisfactory, officials said, the agency may exercise its options to buy more buses. But if the first vehicles are found defective, they added, Metro would drop its options and seek to buy buses from another manufacturer.