When is a foundation crack something to repair immediately, and when is it something that may never be a problem?
Rule of thumb:
If the opening is more than a quarter inch, it’s a problem.
If the two sides of the crack are no longer level with one another, it is a problem.
(note the pen for size and also the show that the two sides of this crack are displaced.)
You can worry less about cracks that are smaller and cracks that are not displaced.
Thin cracks should be watched, but they are often no problem for home owners.
Cracks along the mortar in stone, brick, or concrete block foundations indicate that the mortar is drying out. This condition should be corrected by replacing the mortar (called pointing.) If you leave it long enough, the foundation will begin to move.
However, it is quite common in our old housing stock to find foundations with mortar so dry that it will flake off when you touch it. This is nothing to panic about. It’s normal.