Unstopping a clogged sink drain is an annoying home repair problem that usually occurs on a holiday weekend when a plumber is impossible to reach. Fortunately, in most cases it is a fairly simple repair the homeowner or apartment dweller can handle - assuming the individual has two or three inexpensive tools needed for the job and is familiar with the basic techniques involved.

Widely advertised chemical drain cleaners work on most simple grease and hair stoppages but often fail to penetrate heavy grease, soap, and foreign matter accumulations completely blocking the drain pipe.

When using a prepared chemical drain cleaner, it is important to follow the package directions carefully, because some are caustic and harmful to skin and eyes. Generally, they work best when there is only a small amount of cold water in the sink. If the bowl is full, bail most of it before using the cleaner.

If the chemical cleaner doesn't work try a "plumber's friend" - a rubber suction cup attached to a wood handle, sometimes called a plunger. This tool is used by placing the bell-shaped rubber cup flat over the drain and by pumping up and down to create pressure and suction. In order for it to work effectively, however, there must be enough water in the bowl to cover the bottom half of the rubber cup when it is in position over the drain opening.

If the sink has a stopper, remove it first. A bathroom sink will probably have a built-in metal stopper that is raised by lifting a knob or moving a lever near the spout. To remove the stopper, raise it as high as possible, then twist a half-or quarter-turn.If this doesn't work you may have to unscrew a nut from the linkage under the sink and get it out.

Stuff some rags into the overflow opening at the top of the sink (located under the rim) and place the rubber force cup directly over the drain opening. Press down with a quick, firm shove, then lift with a sharp yank. Push down again, then immediately yank upward. Usually, four or five such up-and-down motions will be adequate to do the job.

It is important to place extra emphasis on the upward yank, because in most cases this stroke is more effective in breaking a clog than the downward push. It creates a suction drawing up the clog so it can be removed by the subsequent rushes of water.

If repeated use of the plumber's force cup does not free the clog, more drastic action will be required. All sinks have U-shaped traps directly under the drain which is where many clogs develop.Most traps have a cleanout plug, but those that do not are designed with removable slip-nuts at each end of the trap.

To clean the trap, remove the cleanout plug (if it has one) by unscrewing it carefully with a wrench or a large pair of offset pliers, making sure there is a bucket beneath the plug to catch water and material that will fall. Now use a flexible plumber's "snake" (a springy, steel wire or ribbon) to probe the inside of the trap, and the pipe beyond.

Keep twisting the wire or snake as it is pushed into the pipe (some of these snakes have a handle fashioned so the user can keep turning the snake while it is being pushed into the pipe). First, probe into the short vertical length that leads to the sink drain. If nothing is found, probe into the wall or floor where the drain pipe continues.

If the trap has no drain plug, the trap itself will have to be removed. Loosen the large nuts at each end and slide them up the pipe. The trap can then be slid down and off, but be careful not to lose the rubber washers under each nut.

When probing the drain lines with the snake, remember that with steady pushing and twisting, the flexible wire can be maneuvered around bends and through elbows. When an obstruction is encountered, break it by twisting the end and by pulling with a series of short pushes and pulls.

Toilet bowls have built-in traps, so there are no separate U-shaped traps underneath. As the drawing shows, a snake can be used to clear obstructions or to removed foreign objects lodged in the reverse-shaped trap.

A plumber's force cup will work on some models and is always worth a try, but on many modern units they are hard to use effectively, because it is difficult to get a tight seal between the rubber cup and the toilet bowl.

A snake called an auger works best on toilet bowls (and is equally effective on sink drains). It is more a long, tightly-wound spring than a flexible piece of wire, and it has a twisted or hooked end used to dissolve obstructions. If the object blocking the drain is fairly solid, it is possible to hook the end of the auger into it, and remove the object.

In extreme cases, where neither a force up nor an auger will do the trick, the only solution may be complete removal of the toilet - but this is a job best left to the services of a professional plumber.