Jury selection began in U.S. District Court yesterday in the retrial of two anti-Castro Cubans charged with the 1976 car-bombing assassination here of former Chilean ambassador Orlando Letelier.

Judge Barrington D. Parker, who will preside over the trial, and lawyers in the case yesterday began the lengthy process of questioning 104 potential jurors to determine what, if any, reasons, might preclude them from deciding the case. The retrial is expected to take three to five weeks, during which time Parker said he will sequester the jury.

The defendants, Guillermo Novo Sampol and Alvin Ross Diaz, were indicted in August 1978 along with six others, including the head of the Chilean secret police (DINA), and charged with plotting and carrying out Letelier's murder. Ronnie Karpen Moffitt, an associate of Letelier at the Institute for Policy Studies in Washington, also was killed in the Sept. 21, 1976, blast, considered one of the most notorious acts of terrorism committed in this city.

In February 1979, after a trial marked by extraordinary security precautions, Novo and Ross were convicted of murder and other charges and were later sentenced to life prison terms. A third man, Ignacio Novo Sampol, was convicted of lesser charges at the same trial and is now awaiting a separate retrial. The Chilean courts have refused to extradite the DINA chief, his No. 2 man and a secret police operative named in the indictment. Two others, also Cubans, remain fugitives.

The convictions in the Letelier case were overturned last September by the U.S. Court of Appeals here, which ruled that testimony from jail house informants against Guillermo Novo and Ross was improperly admitted at their trial.