What causes piston rings not to seal?

Piston rings lose their ability to create a tight seal against the cylinder wall when one or more of the following conditions exist:

This is the most common long-term cause of sealing loss.

• Cylinder Bore Wear (Taper and Out-of-Round): Over time, the cylinder walls wear unevenly, developing a taper (wider at the top than the bottom) or becoming out-of-round (oval instead of circular). Piston rings are designed to be circular; they can no longer conform perfectly to the non-uniform cylinder shape, allowing gases to leak past.
• Ring Face Wear: The contact face of the ring wears down, reducing the sealing tension and creating a gap between the ring and the cylinder wall.
• Ring Land Wear/Damage: The ring groove (or "ring land") on the piston widens or becomes damaged. If the groove is too loose, the ring cannot sit correctly or transfer heat efficiently, leading to poor seating and excessive ring flutter (vertical movement) at high RPMs.
• Ring Breakage: A broken ring obviously cannot seal and also causes rapid cylinder wall scoring.

Carbon deposits prevent the rings from moving and seating properly.

• Carbon Buildup in Grooves: As noted, carbon deposits and varnish accumulate in the piston grooves. This contamination prevents the rings from expanding outward to maintain contact with the cylinder wall. The ring becomes "stuck" or "frozen" in the groove, completely losing its ability to seal.
• Abrasive Contamination: Dirt, dust, or abrasive particles entering the combustion chamber (often via a faulty air filter) embed themselves in the cylinder wall and on the ring faces, accelerating wear and creating microscopic pathways for leaks.

The physical properties of the ring itself degrade.

• Loss of Tension: Piston rings are made to exert outward spring tension against the cylinder wall. Prolonged high temperatures and engine operation can cause the metal to lose its inherent elasticity, reducing the force that holds the ring against the bore.
• Excessive End Gap: Rings are manufactured with a small end gap (a break in the circle). This gap allows for thermal expansion when the engine is running. If this gap becomes too large due to wear, it creates a direct path for combustion gases to bypass the seal. Conversely, if the gap is too small (often after improper installation or excessive bore reduction), the ends can butt together when hot, preventing the ring from sealing properly and leading to breakage.

Sealing failure can start immediately if the rings weren't installed correctly.

• Incorrect Gapping: Failure to set the correct end gap before installation.
• Upside-Down Rings: Installing rings, particularly tapered or keystone rings, upside down prevents them from seating against the cylinder pressure correctly.
• Mismatched Components: Using rings that are the wrong size for the cylinder bore or that are incompatible with the piston groove style.

Understanding these causes is key to diagnosing whether a sealing issue requires a simple de-carbonizing or a complete engine overhaul and cylinder re-boring.