After three months of negotiations, Metro is close to an agreement with a West German-affiliated bus manufacturer on a series of repairs aimed at correcting what have been described as hazardous cracks and other defects in 76 Metrobuses here.

The transit authority halted use of the buses, manufactured by Neoplan U.S.A. Corp., in February after detecting hairline cracks near their front axles. Metro officials termed the cracks a "potential safety hazard" and warned that they might result in loss of steering control.

"There is a general agreement on what is to be done," said John S. Egbert, an assistant general manager for Metro. Neoplan has proposed starting the repairs in a few weeks, and the work likely will be completed by fall, according to R.N. Winston, a Neoplan vice president. Some buses are expected to be back in service this summer.

The repairs, along with previous modifications and other expenses linked to the defects, are likely to cost Neoplan more than $1 million, Metro officials said.

Plans call for installing steel reinforcements to stop cracks from forming in supporting frames near the front axles. Defective frames near the rear axles would be replaced for the second time. Repairs also are expected in structures that support the buses' engines and sides and in electrical and other systems.

John W. Fisher, an engineering professor at Lehigh University hired as a consultant by Metro, has endorsed the key proposals.

"I believe the primary safety issues of the A-frame near the rear axles and front suspension have been resolved," he said in a recent letter.

Neoplan, a Colorado-based company affiliated with Gottlob Auwaerter GmbH & Co., a West German bus manufacturer, has repeatedly contended that safety is not at issue.

In a May 3 letter, the company said there "is no safety-related basis for the vehicles currently being held out of service" and described Metro's actions as "unreasonable and unjustiified."

Metro's board of directors voted to buy the Neoplan buses for $12.5 million in June 1983 over protests from one board member, Fairfax County Supervisor Joseph Alexander, who had cited evidence of defects in Neoplan vehicles purchased by other transit systems. Neoplan had submitted the lowest bid.

Since then, the buses have posed recurrent problems. In addition to the cracks, Metro and federal safety officials have cited a range of defects, including faulty electrical circuits that were blamed for fires in two buses.

Under pressure from the National Highway Traffic Safety Administration, Neoplan agreed in March to recall 2,000 buses it had sold to transit systems throughout the United States to correct flaws. According to Metro officials, the company is to pay for all repairs under a long-term warranty.

The Neoplan buses had been assigned mainly to routes serving Southeast Washington, partly in an attempt to improve service in Anacostia and other low-income areas. Recently, Metro called in older buses from its reserve fleet to offset the out-of-service vehicles.

Metro officials have criticized Neoplan for failing to meet several deadlines. "They promise everything, and they don't come through," said Vernon K. Garrett Jr., Metro's engineering director, citing the company's failure to submit a schedule, due last week, for carrying out the repairs.

Neoplan's Winston said the delays stemmed partly from efforts "to do the job to Washington's satisfaction." Most outstanding isssues were settled in talks two weeks ago attended by Albrecht Auwaerter, board chairman of Gottlob Auwaerter.

The cracks near the front axles were found in steel plates and tubes and in welds in a section known as the axle subframe assembly. Under the proposals, Neoplan would install steel plates and V-shaped braces to prevent the frames from bending and cracking. The company would replace A-shaped steel frames near the rear axles with heavier models. When cracks were detected in welds last year, these frames were replaced. But the replacements later proved to be too weak and could crack again, officials said.

Steel plates also would be attached to welded joints in front of the left rear wheels to prevent twisting and cracking. Metal reinforcements are being considered to stop cracking in steel skeletons supporting the buses' engines and in frames behind the rear doors.

A computerized analysis of the frames is expected to be completed later this month and, officials said, additional reinforcements may be found to be needed.