As the egg heats up, the air inside the bubble expands. As the hot air pushes outwards, it puts pressure onto the shell, making it crack. You can prevent this by making a pin-pick in the bottom rounded end of the egg.
How do you keep hard boiled eggs from cracking? Simple Tip! Adding a teaspoon of vinegar to the water may help keep egg whites from running out if an egg does crack while cooking. Also some people find adding 1/2 teaspoon of salt to the water helps prevent cracking as well as making the eggs easier to peel.
Why did my eggs crack when I boil them? Most of the time it’s the result of extreme temperature changes ie fridge to boiling water or worse boiling water + salt (higher temperature). Take eggs from fridge and warm them under hot tap. Submerge eggs at least 1 inch below in non salted boiling water.
Are eggs still good if they crack while boiling? Bacteria can enter eggs through cracks in the shell. Never purchase cracked eggs. However, if eggs crack on the way home from the store, break them into a clean container, cover tightly, keep refrigerated, and use within two days. If eggs crack during hard cooking, they are safe.