Nick F. Stames, longtime chief of the FBI's Washington field office and an innovative tactician in such law enforcement gambits as the "Sting" phony fencing operation here in 1976, was named an assistant director for the FBI yesterday.

He will be succeeded in the Washington office by C. Roy McKinnon, currently special agent in charge of the FBI's San Francisco field office. Both men expect to start their new jobs in early May.

Stames, 44, whose appointment was announced by FBI Director William H. Webster yesterday, will become one of 10 assistant directors in the bureau's hierarchy and will head its identification division. The largest of the FBI divisions, it maintains the agency's vast fingerprint files and employs hundreds of technicians skilled in fingerprint and other specialized identification procedures.

A 27-year veteran of the FBI, Stames took over the 400-agent Washington field office four years ago, threading his way through politically delicate fraud, bribery and espionage investigations in the nation's capital.

Investigative work by his office contributed to the espionage convictions of former U.S. Information Agency employe Ronald Humphrey and Vietnamese expatriate David Truong last May, and the conviction five months later of Rep. Charles C. Diggs Jr. (D-Mich.) of mail fraud and congressional payoff kickbacks.

Stames also achieved new levels of cooperation among traditionally rivalrous law enforcement agencies when he helped put together the "Sting" operation in 1976, jointly handled by the FBI, D.C. police and the Alcohol, Tobacco and Firearms Bureau of the Treasury Department under the U.S. attorney's office here.

In that operation, scores of thieves were lured into a fake fencing "business" run by undercover officers. Many of the suspects later were arrested and convicted.

The "Sting" operation has since been emulated by other law enforcement agencies in the country, and Stames and other principals in the Washington "Sting" were feted in the White House by then president Gerald Ford.

In becoming an assistant director of the FBI, Stames will be in charge of the same division where he started his career as a fingerprint clerk "when I was fresh out of high school" 27 years ago, he said in an interview yesterday.

"I have a warm place in my heart for the identification division," he said. He succeeds Assistant Director Robert E. Kent, who retired March 23.

Born in New York City, Stames served in the New York, Miami and Tampa field offices of the FBI, and as a fluent speaker of Spanish, was special agent in charge of the San Juan, Puerto Rico, office. He also was assigned to the American Embassy in La Paz, Bolivia, in the early 1970s.

Stames described McKinnon, his successor in the Washington field office, as "an innovative . . . outstanding man."

McKinnon, 48, is a 25-year career agent who has worked in numerous field offices and was assigned to FBI headquarters here in the early 1970s as an inspector.

In returning to Washington, he takes over the third-largest field office of the FBI. Only the New York and Los Angeles offices are bigger. The San Francisco field office has about 325 agents compared to Washingon's 400.

McKinnon could not be reached yesterday for comment. James Ahearn, his deputy in San Francisco, said in a telephone interview, "Your gain in our loss."

McKinnon's job here will be significantly different from what it was in San Francisco. There, his jurisdiction embraced all of norhtern California. Here, he will be responsible only for the 69 square miles of the District of Columbia.

In addition to investigating government fraud, robberies of federally insured banks and other routine federal crimes occurring throughout the country, the Washington field office also devotes a large portion of its manpower a counter-intelligence work because of the large diplomatic and foreign population here. CAPTION: Picture 1, NICK F. STAMES . . . 27-year veteran; Picture 2, C. ROY McKINNON . . . from San Francisco office