When F. H. Ainsworth and his wife bought their 18th century brick house in Princess Anne, Md., they did not know that it had significant structurtal problem and that tw that it had a significant structural problem and that they had the start of a new business.

Soon after moving in the Ainsworths found a jagged vertical crack in one of the brick walls that divides the interior of the house into four rooms. Someone had attempted to hide the crack with plaster but as the crack widened the plaster broke. Instead of applying more plaster, Ainsworth did the right thing. He looked for the cause of the crack.

Moisture was the culprit. At some point in the past 200 years, the drainage system on the roof had broken. Water was seeping down the walls to the foundation and the bond between the interior and exterior brick walls was broken. The walls were moving apart and if left alone, the house or part of it would have collapsed.

Ainsworth, a mechanical engineer, discovered and applied a very old solution to the problem. He ancored the moving wall to a stable wall, using tension rods, turnbuckles and large star-shaped castmetal washers.

It is those washers that are seen on the outside of many 18th and 19th century brick houses. The basic procedure is to drill holes in the holes in the two walls. A tension rod, with screw threads on both ends, is inserted in each hole.

A star-shaped washer is attached to the end protruding from the house and the two interior ends are joined with a turnbuckle. As the turnbuckle is rotated, the rod ends move toward each othe bringing the walls together. It is the hundreds of pounds of tension produced by the turnbuckle that stabilizes the moving wall.

The system is as complicated as it sounds. Ainsworth had to determine where to place the rods and then design a device to measure how much force to put on them. Since the rods were to be hidden in the space between the ceiling of the first floor and the floor of the second story, the flooring had to be removed and holes drilled in the supporting floor joists. And Ainsworth had to have the star-shaped washers forged at a local foundry.

When all of that had been done, the rods were inserted, the stars attached, and the turnbuckle tightened; the moving wall was pulled in by 3/8 the 7/16 of an inch. With that done and the drainage system repaired, the house was ready for the rest of the restoration.

Ainsworth now manufactures the turnbuckle stars. They are forged of iron or bronze and are for structural use. The two models are reproductions of stars made in 1845 and 1865 and used for bracing masonry walls. The originals were removed from buildings in New York City.

Not every crack in a brick wall will mean the same drastic treatment that Ainsworth had to apply. Some cracks occured soon after construction and were the result of uneven settlement. They open and close with the seasons and present no serious problem. Others, like the Ainsworth's, are the first signs of major problems expense.

Monitor all old cracks regularly and check new cracks for a full year before planning or doing any repair. Note and record the length and width of the opening each month. If at the end of the year the crack has grown vertically or horizontally, consult a restoration specialist. Use that time to look for the cause of the movement. Do not just patch it up and look the other way.

For mor information on the turnbuckle stars send a stamped, self-addressed envelope (with zipcode) to the Ainsworth Development Corp; Beckford, Princess Anne, Md. 21853.

Beverly Reece is associated with the Preservation Resource Group, a firm that conducts preservation workshops for homeowners here