THE SMALL FLEET of 76 Neoplan buses that the Metro system bought for $12.5 million in 1983 is plagued by a new and potentially serious defect. Cracks have become visible on the undersides of 23 of the buses, where the main frame connects to the front axle. Closer inspection has revealed that cracks were beginning to form on other buses in the fleet as well. Metro officials have responded by pulling all of the Neoplan buses off the streets until they can be fixed. Metro has pressed reserve buses into service, so there has been no reduction in the number of buses on the streets.

Officials at Neoplan U.S.A. Corp. say that Metro officials have "overreacted." Have they? Metro fears that the cracks could cause steering problems. Cracks had been found on the rear of the subframes on the buses last March. By May, Neoplan, which to its credit responded to this and other problems quickly, had reinforced the frames. But that did not hold up, and Neoplan ultimately had to replace each of the frames. Electrical problems and a couple of fires then occurred, necessitating other alterations.

Given such a track record, it is difficult to argue with Metro's decision to take the buses off the streets until some sort of solution is found. It would be senseless to wait and find out on the streets whether the cracks can cause a loss of steering, resulting in accidents. Metro's passengers have a right to know that this costly system is safe.

One point should be remembered, however: Voices were raised on the Metro board before the Neoplan buses were purchased; examples were cited of other jurisdictions' discovering structural defects. But Metro bought the buses anyway, apparently convinced that the manufacturer had solved the problems. That was a mistake, and Metro evidently realizes it; the transit authority is considering ways to revamp its purchasing practices. In the meantime, officials want to hire an independent firm to do a complete structural analysis of the Neoplan buses.

That brings us to a final observation. American firms are crying out against competition from foreign businesses, but they might find an explanation for that competition in the quality of their own workmanship. For example, this land of know-how seems to find it difficult to perform so conventional a task as producing a durable bus.