Federal workers weighing the impact of the old and new pension plans on their future Social Security benefits need to move fast to get an official statement of their Social Security coverage and work history. Employes who make the requests quickly will be okay.

Those who delay may be caught up in the deluge the agency is expecting later this year when any of the 2 million federal workers who didn't get the word or didn't act panic and write for the data.

At stake is the amount of the Social Security benefit workers will eventually get based on Social Security-covered military service or private employment. More than half of all federal retirees get some kind of Social Security benefit in addition to their civil service pension.

Under legislation passed in 1983, the Social Security benefit of many federal workers will be reduced -- particularly if they remain in the current pension plan -- based on the amount of their civil service annuity. The new Federal Employees Retirement System could minimize the impact of the so-called windfall offset law, meaning workers would get a bigger Social Security check. Although the House Ways and Means Committee is working on a bill to close that loophole in the new pension plan, the FERS program as now written protects employes from much of the Social Security offset.

Because FERS provides the Social Security benefit protection, many employes hired before 1984 plan to move out of the old civil service retirement system and into FERS. FERS covered most workers hired since the end of 1983.

The open season for making the switch to FERS starts next month and ends Dec. 31. Those who change plans cannot switch back to the old pension program.

During the open season, workers must decide whether to stick with the old plan -- in which benefits are based on salary and seniority -- or to move into FERS. It is a private-sector-style pension program, combining Social Security, a less generous federal pension and an optional employer-employe matching tax-deferred savings plan that is considered one of the nation's best.

But before workers can figure out which retirement plan best protects any Social Security benefit they have earned, they need an official year-by-year breakdown of their work history when they paid into, and were covered by, Social Security. The Social Security Administration says it will supply the data within three to five weeks of getting an official request, provided it gets the applications from federal workers by mid-August.

To apply, a Form SSA-7004 FERS, available at agencies' personnel offices, should be filled out and mailed as soon as possible. Arrangements have been made by postal employes unions to have the work history information sent directly to postal workers, so only those who don't get the information this month need send in applications. For other government workers, the forms are (supposed to be) in personnel, and the time to act is now. Job Mart

General Services Administration has an opening for a Grade 12 contract specialist. Call Terri Green at 566-1805.

International Trade Administration needs a GS 9 computer programmer analyst. Call 377-3276.

Army's Military History Center wants a GS 11 visual information specialist. Call David Armstrong at 272-1907.

National Institutes of Mental Health is looking for GS 5 through 9 chemists/biologists for full-time and part-time work. Call Jean Barr at 496-5337.