The owner of a downton bar and a man described by law enforcement officials as one of the city's top rackets figures were convicted yesterday of using the bar as a front for a cocaine distribution ring.

The men were convicted under a racketeering statute that will allow the government to seize Sylvester's the bar at 1129 21st St. NW, when they are sentenced.

Federal officials said the convictions mark the first time that the racketeering statute ahs been used in connection with a narcotics enterprise, possibly the first time any business in the District will have been seized under the statute.

Convicted by U.S. District Judge John H. Pratt after a nonjury trial were Joseph S. Swiderski, also known as Joe Winters, of 3710 Stewart Dr., Chevy Chase, owner of the bar, and David A. (Shaggs) McGowan, 42, of 5225 Pooks Hill Rd., Bethesda.

The government presented testimony during the two-day trial before Pratt that portayed Sylvester's as a "clearinghouse" for drug transactions, where "summit meetings" of druy dealers were held in which cocaine would be sold in quantities of up to two ounces.

Among the witnesses were four codefendants in the conspiracy, who earlier had entered pleas of guilty in the case. They said the cocaine was brough to Sylvester's from New York, and Swiderski would then arrange for various other persons to purchase and use the narcotic.

Pratt refused a request from Assistant U.S. Attorney Barry L. Leibowitzof the major crimes division that McGowan be jailed immediately after the verdict. Leibowitz pointed out that McGowan had convictions for forgery, gambling, shoplifting and firearms, and currently was on parole or probation for some of those offenses.

Terming it a "close case," Praff said he decided to allow McGowan to remain free on bond largely because he needs to help his attorney prepare for another cocaine trial involving McGowan scheduled next week before the judge.

Law enforcement sources describe McGowan as one of the city's top rackets figure in the loosely-knit world of alleged organized crime here. Leibowitz said in court yesterday that McGowan was involved in numerous "criminal activities," and is under two separate investigations in an alleged gambling operation.

McGowan and Swiderski were convicted of using Sylvester's which bills itself as the city's only "Polish pub" and features kielbasa sausage sandwiches and Kiakus beer, as part of a conspiracy to distribute cocaine.

The charges grew out of a long joint investigation by the Drug Enforcement Administration, the FBI and the Metropolitan Police Department into the alleged widespread use and sales of cocaine at bars in the 19th and M Streets NW area.

With the help of an informant, DEA Agent William J. Logay said that he was able to infiltrate the drug conspiracy by posing as Bill Lorenzo - a smooth-talking, high-spending import-export businessman who was involved in drug dealings in Chicago.

Logay testified during the trial that he purchased cocaine at Sylvester's twice, once for $1,650 for one ounce and then $3,500 for two ounces. The difference in price reflected the purity of the drugs being purchased, Logay said.

The purchases were made in a third-floor office at the restaurant, according to the trial testimony.

Logay, who told Swiderski and others that he could not "snort" the drug because of sinus problems, also testified that he had seen various persons use cocaine in the restaurant.

Howard J. Kolbenheyer and Donald R. Donato, both of Southhampton, N.Y., testified during the trial that they brought four ounces of cocaine from New York to Sylvester's on two different occasions in false-bottom briefcases.

Kolbenheyer who said he became acquainted with Swiderski several years ago when both worked as bartenders in Manhattan and Long Island, said Swiderski would arrange for other drug dealers to meet the men from New York and purchase the narcotic at Sylvester's Both Kolbenheyer and Donato pleaded guilty to narcotics conspiracy charges.

William C. St. John of New York and George T. Kenney of Fairfax City also said they had supplied cocaine to Swiderski for distribution at the restaurant. Kenndy said he had a particularly good source for the drug, which provided him with cocaine of up to 94 per cent purity.

Both Kenney and St. John pleaded guilty to narcotics conspiracy and racketeering charges.