A judge here yesterday dimissed the federal conflict-of-interest indictment against the former head of the Bureau of Engraving and Printing, James A. Conlon.

Conlon had been indicted here by a grand jury on Aug. 9 on charges in connection with alleged favoritism toward a New York printing firm that hired him almost immediately after he resigned his job at the federal agency that prints U.S. currency.

U.S. District Judge Louis F. Oberdorfer's ruling said the charges against Conlon were faulty because they did not provide enough specificity about the exact time that the jury found that Conlon was beginning the alleged conflict of interest.

Oberdorfer's ruling, believed to be the first legal interpretation of the portion of the conflict-of-interest statute under which Conlon was charged, does not block the government from attempting to have another grand jury indict him on a more narrowly drawn charge.

Justice Department attorney's handling the case could not be reached for comment. A Justice Department spokesman said it was too early to determine whether a new indictment would be sought.

The ruling by Oberdorfer came on the eve of Conlon's expected two-week trial on the sole remaining count against him from the Aug. 9 indictment.

Oberdorfer earlier had thrown out three other charges against Conlon that alleged he lied to the grand jury that was investigating the charges against him. In that ruling last month, Oberdorfer also said the government had been improperly vague in drawing up its criminal counts against the former engraving bureau chief.

"We're delighted," said Conlon's attorney, Plato Cacheris, who said his client was with him around 5 p.m. yesterday when he was notified of Oberdorfer's ruling.

Conlon was accused in the indictment of "unlawfully and knowingly" supporting a federal contract with the American Bank Note Co. for an anti-counterfeiting system to be used for U.S. currency. At the same time, the indictment asserted, he had an arrangement concerning a future job with the private firm.

Conlon resigned as director of the bureau in mid 1977, and was made president of an American Bank Note subsidiary one day later, according to information compiled by the Senate permanent subcommittee on investigations.

The perjury charges concerned Conlon's denials to the grand jury that he supported the firm in its contract dealings with the bureau, and his further assertion that his decision to resign to was "spontaneous."

The grand jury alleged that Conlon knew well in advance that he was leaving the bureau when he did and that he had long planned to go to the firm.

Conlon, who began working at the bureau in 1942 as an apprentice plate printer, heatedly denied before a Senate panel that he was involved in any conflict of interest and said he was the victim of overzealous investigators.

Oberdorfer's brief order yesterday dismissing the charges was made only because of the looming trial date, he said, adding that a more complete explanation of his ruling would be made later.

He noted, however, that the government's indictment of Conlon alleged a seven-month period within which Conlon "was negotiating and had an arrangement concerning prospective employment."

"It fails, however, to allege that any agreement existed, when such an arrangement developed, or when, if ever, any negoiations took place," Oberdorfer said. " . . . . The indictment fails to charge a crime."

Indicted in a separate case involving similar allegations of conflict of interest with the same firm was Conlin's former assistant, Richard C. Sennett. Sennett's case is pending before another federal judge here, and the ruling in Conlon's case has no direct effect on it.

The bureau, at 14th and C streets SW, prints money, postage stamps, food stamps and other official government documents.