The introduction of Metro's extensive bus-train transfer plan will be stretched out over a two-month period rather than going into effect all at once on July 1 as planned earlier, the Metro board decided yesterday.

The transfer plan, which Metro calls an "interface," orginally was to go into effect with the scheduled July 1 opening of the transit system's second with 18 stations, runs from the Stadium-Armory station through downtown Washington to the Rosslyn and National Airport stations in Arlington County.

It had been planned, starting on that date, to terminate many bus routes at outlying subway stations, where passengers would transfer - often but not always at higher fares - to the trains to get to and from downtown.

However, technical and political problems prevented that schedule from being met, officials said.

yesterday, Theodore C. Lutz, Metro's general manager, proposed the stretch-out, and said it would permit riders to get accustomed to the subway's new electronic fare collection system.

The new rail line will open July 1 as originally scheduled, under the revised plan. The first change of bus routes will come on July 17. It will involve all routes affecting Maryland riders, District routes chiefly in te Southeast part of the city and Virginia routes funneling through Rosslyn.

Remaining changes will be made Sept. 4, affecting all other District routes and the Virginia routes incolcing transfers at the Pentagon and National Airport stations.

As part of the District rearrangement, 50 buses now stored at the over-crowded garage at 14th and Decatur Streets NW - bringing frequent complaints from residential neighbors - will be transferred to the larger garage on Bladensburg Road NE.

In another matter, the Metro board ratified a labor contract with Teamsters Union Local 246, which represents the Metro transit police department. It provides for wages comparable to major city and county police forces in the area.