The Arlington County Board's Republican majority has agreed to depart from tradition Tuesday by reelecting Stephen H. Detwiler board chairman, making him the first person in 35 years to head the board two consecutive years.

If the board stuck with its practice of rotating the chairmanship among members of the majority party, the chair this year would have gone to board member Dorothy Grotos, who last served in 1979--the year Republicans took control of the five-member board.

Grotos this week said she has agreed to extend Detwiler's term so he can continue heading the board's search for a new county manager. The search effort, begun after the Republicans fired W. Vernon Ford last fall, is expected to continue until summer.

Extending the term of the chairman was one of several recommendations made recently by an Arlington Committee on Improving Local Government. In its report, the committee noted that the annual rotation of the chairmanship was a function of tradition, not of law. It suggested it might be "beneficial" for the chairman to serve more than one year "from time to time as circumstances may warrant."

"The committee made some recommendations and they are worth trying," said Grotos yesterday. "I'm a traditionalist and I would have been happy to be chairman but I'm also happy to try new things."

Detwiler, a savings and loan executive who is running for reelection to the board this year, discounted the political advantage of serving as chairman in an election year. "I've had as many people tell me it would hurt me as much as help me," he said, noting that the chairman is often the focus of unpopular board decisions.

At its reorganization meeting Tuesday night, the board also plans to reappoint Grotos as its representative on the regional Metro board and Republican Walter Frankland as the Arlington member of the Council of Government's board of directors.