Half a million clerks, letter carriers and mail handlers working for the government's largest agency, the U.S. Postal Service, will get the first installment of a new 2.7 percent raise Fed. 8.

The increase is retroactive to July; a cost-of-living adjustment (COLA) that also will appear in the same paycheck is retroactive to November.

Employes will get another 2.7 percent adjustment this summer and a third 2.7 percent increase in July 1986.

The pay raise will average $640 a year and the cost-of-living adjustment $291.

The $4.2 billion, three-year contract has been billed as the largest arbitrators' award in the nation's history.

In addition to the three flat pay raises and the COLA, employes will get COLAs again later this year and in the fall of 1986.

Checks for the retroactive increases -- for both pay and COLA -- will be issued later, the postal service said yesterday.

Arbitrators wrote the new contract after bargaining talks broke down last summer between the postal service and unions representing most of its 650,000 nonmanagement workers.

The first employes to get the new raise will be those represented by three craft unions: the American Postal Workers Union, National Association of Letter Carriers and the Mail Handlers Union.

Increases for about 30,000 rural letter carriers will show up in Feb. 22 paychecks.

White-collar and blue-collar federal workers, who depend on the White House and Congress for pay adjustments, got a 3.5 percent pay raise this month.

Government and military retirees, whose pensions are indexed to inflation, also got a 3.5 percent raise in their checks this month, as did persons receiving social security benefits.