The piston rings fit into horizontal grooves in the pistons. The rings expand against the cylinder wall and seal the combustion chamber. This ensures that the engine maintains a consistent level of compression. Stuck piston rings are uncommon, but they do occur. There are two positions in which the rings can get stuck. They can become stuck inside the piston grooves, or they can stick against the cylinder wall.

Seized Engine
If the rings are stuck against the cylinder wall, the crankshaft will not rotate. This condition usually happens when an engine has been sitting for a long time. The rings rust and bond to the cylinder wall. If an engine is seized, it is important to eliminate other causes, such as failed crankshaft bearings, or broken valve-train components, since these can also seize an engine. The typical solution to this condition is to pour penetrating oil into the cylinders. Over a period of days, the oil will usually free the stuck rings.

Low Compression
If the rings are stuck inside the ring grooves, they cannot seal the combustion chamber. A compression tester must be used to determine if each individual cylinder is maintaining compression equal to the others. Rings can become stuck in the ring grooves from a buildup of carbon on the piston. They can also break and remain in the ring grooves for many miles.