What do Social Security workers have in common with fire marshals who don't have fire insurance and pilots who won't let their families fly?

They don't practice what they preach.

Nobody who works for the Social Security Administration or any other federal agency, is under the compulsory pension system the government runs for 100 million nonfederal workers. Government employees who want Social Secrity coverage have to get it before they work for Uncle Sam, after they retire, or by moonlighting.

Within the next couple of years, however, odds are that all government workers will be brought into Social Security. Although the House voted yesterday to continue the exclusion for civil servants and postal workers, it did authorize a study into the feasibility of dovetailing the Social Security system with the generous federal staff retirement program. That will probably happen within the next five years.

When and if mandatory coverage comes, Congress is expected to use it to plug a loophole that has bothered many people in and out of government. It permits federal workers who can in some cases, retire at age 50 to benefit from their Civil Service pension while working at second jobs in the private sector to qualify for Social Security coverage, too.

The Civil Service retirement is a staff plan.It gives workers benefits that are about triple the payments availble under regular Social Security. For their higher pensions, and earlier retirement, govenment workers pay significantly more. And they also are taxed on their pensions after they have recovered all the money they put into the CS fund. That usually takes about 18 months.

Social security payments are tax-free. Obviously the Social Security system and the Civil Service retirement system are two very different things. Blending them won't be easy - at least if it is to be done right.

Although the House agreed yesterday to keep federal workers apart from Social Security for a while, the mood in Congress clearly seems to be toward universal converage. That would mean that civil servants - and state and local government employees, too - probably will be brought into a dual-rack pension system that would give them retirement benefits from both Social Security and from the federal pension program.

The questions that HEW and the Civil Servic Commission must answer in their newly mandated study of the pension linkups are tough. Among them:

How to protect existing and promised benefits for federal workers who joined the government, at least in part, because of its better retirement system.

How to guarantee that government workers won't pay through the nose when they are brought undera two-track retirement system that pays different levels of benefits at different ages.

What sort of benefits would a federal worker - also covered by Social Security - get? What would he or she have to pay?

Do employees get one check from the government when they retire and another from Social Security later on when they come eligible for benefits?

The House Ways and Means Committee clearly wanted to bring government workers under Social Security beginning in 1982. But committee arguments were not enough to withstand the lobbying onslaught from federal and postal union leaders who felt the action was ill conveived and at least partly motivated by politics.

By bringing government workers under Social Security Congress could have avoided the even greater increased in Social Security taxes it must now vote to finance the system.

The amendments to "strike" federal workers from universal coverage, and instead turn the matter over to study group, came from Reps. Joseph Fisher (D-Va.) and Gladys Spellman (D-Md.). It will give the experts the time they need to decide whether combining the systems would to feasibel and if so, how to do it.

Universal coverage may be a good thing. But Congress was wise not to order mandatory coverage until it has a few more facts. It also must provide some guarantees to government workers that their pension system - which they pay for - won't be hurt.

American Foreign Service Association has held exploratory talks with major federal employee unions about some kind of coalition status. AFSA leaders have talked with the National Treasury Employees Union, and the American Federation of State, County and Municipal Employees. Idea is that there is strength in union - or affiliation - and the AFSA members need all the help they can get in these days of real, and rumored reorganizations of State, AID and USIA.