Most of the 120,000 U.S. workers in hard-to-fill jobs who got special raises in 1987 won't get the 2 percent pay increase that is effective this month for the vast majority of the government's 1 million-plus white-collar civil servants.

Uncle Sam now pays anywhere from 3 percent to 30 percent above normal federal scales to special-rate employes in hard-to-fill occupations. Most special raters are engineers, scientists and selected medical personnel.

Last April the government added 32,000 clerical employes in the Washington area to the special-rate category. Those local federal workers in such Grade 2 through 7 jobs as clerk-typist, clerk-steno and data transcriber got special raises ranging from 3 percent to 23 percent.

The government makes regular surveys to see which jobs agencies have trouble filling. The Office of Personnel Management can authorize special rates for workers by grade, occupation or geographic location.

Officials said yesterday that some special-rate engineers in GS 5 through 7 grades who didn't get the special raises last year will be included in this month's increase.

Some special-rate employes could get raises later this year if the government again determines it is having a hard time getting and keeping them.Savings Plan Rates

The January interest rate for money in the federal Thrift Savings Plan will be 8.875 percent. The tax-deferred plan is open to active duty federal and postal employes. Those covered by the Federal Employees Retirement System can put up to 10 percent of pay (or a maximum of $7,000) into it. Anyone under FERS who invests 5 percent or more gets a matching 5 percent tax-deferred contribution from the government. Persons covered by the old Civil Service Retirement System can put 5 percent into the plan but get no matching funds from the government.

Congress late last year exempted the federal Thrift Savings Plan from rules covering similar tax-deferred plans in the private sector. That means the 5 percent or 10 percent employe contribution rate is guaranteed.

Interest on the savings plan's investments in Treasury securities varies monthly. In November and December the rate was 9 percent.

Employes who want to enroll in the savings plan, withdraw from it or change the amount they are having withheld from their paychecks have until Jan. 31 to make changes. The next investment open season will run May 15 through July 31. Lump-Sum Lawsuit

Fifty recent federal retirees have retained the law firm of Neill, Mullenholz & Shaw to represent them in a lawsuit challenging the right of the government to tax lump-sum pension plan payments. They argue that they have paid taxes on their pension plan contributions while working.

When Congress first made the lump-sum option available, it said the payments were to be tax-free. But the Tax Reform Act changed that and requires retirees to pay taxes on a major portion of any lump-sum payment they get. For highly paid workers with long service those lump-sum payments could be worth $70,000 or more.

The Senior Executives Association has been coordinating the effort to challenge the tax ruling.