Ecuadorean authorities have released eight American archeologists who spent more than two months in jail during an investigation into charges they cooperated with an assassination plot in return for dissidents' help in smuggling out gold Indian artifacts.

The lengthy investigation by military authorities failed to turn up any illegal political activities by the Americans, Ecuadorean officials said, and police also cleared them of charges they had begun an illegal dig in search of pre-Columbian art.

The eight, along with three Ecuadorean guides, were released from jail at Portoviego near the Pacific coast on Saturday. They said after arrival here that they took a bumpy public bus to the port city of Guayaquil and then made their way to Quito, the capital.

Much about the case remained cloudy, including the reasons for the dig or under whose auspices they were digging. Neither the released Americans nor Ecuadorean investigators would comment on the case. The charges stemmed from a tip to the Ecuadorean Embassy in Washington from Richard Abbey, an American who went to Ecuador with the group and then broke away and returned home.

Abbey told an Ecuadorean diplomat in the U.S. capital last October that the Americans had discovered millions of dollars in gold artifacts near a remote village in the steamy coastal province of Manabi. They had agreed with Ecuadorean political dissidents to smuggle some small arms into Ecuador, he charged, in return for the dissidents' connivance in getting the artifacts out of the country.

There were reports here at the time of an assassination plot against presidential candidate Jaime Roldos, which was never carried out, but the Ecuadorean government declared later that the Americans had no connection with it.

Ecuadorean authorities identified the Americans as:

Clyde Nickelsen, 57, of Sanford, Fla.; his sons Terry, 32, of Memphis, Tenn., and Tom, 24, of Sanford; Bill Scarborough, 33, of Memphis; Ned Mauldin, 27, also of Memphis; Suzanne Arpin, 22, of Jacksonville, Fla.; Beverly Holcomb, 48, of Sanford; and Charlotte Koehl, 48 or Akron, Ohio.

The elder Nickelsen said all are in good health and probably will leave the country after paying legal bills and trying to recover equipment confiscated by troops who arrested them at the dig Oct. 25.

"We went through nearly two weeks of hell during interrogation by military intelligence," said Clyde Nickelsen, "and then we had 11 more days of hell when we were transferred to a civilian prison that was a real pit."

Terry Nickelsen said he was kicked, punched about the head and put in solitary confinement for six days after a shoving match with a guard.

"They used psychologicial intimidation on us and physical intimidation on the Ecuadoreans," Clyde said, charging the Americans received little food or protection against the night cold.

The American men said they did not witness the tortures of the three Ecuadorean helpers, but saw the results when they were returned to their cells after interrogation sessions.