Piston rings seal combustion pressure, control oil on the cylinder wall, and transfer heat from the piston to the block. Each ring has a small gap between its ends, called the end gap. Getting that gap right is one of the most important checks during an engine rebuild.
Why End Gap Matters
Rings expand as they heat up. The end gap gives them room to grow at operating temperature.
- Too tight: as the engine warms up, the ring ends can butt together. That can scuff the cylinder wall, break the ring, or damage the piston.
- Too wide: more combustion gas escapes past the rings (blow-by), which can reduce power and increase oil consumption.
How End Gap Is Checked
- Clean the cylinder and the ring.
- Place the ring in the cylinder and push it down squarely with an inverted piston, so it sits level within the area of ring travel.
- Measure the gap between the ring ends with a feeler gauge.
- Compare the measurement to the ring manufacturer’s specification for that ring position and bore size.
Check each ring in the cylinder where it will be installed, because bore sizes can vary slightly from cylinder to cylinder.
Specifications Vary
The correct gap depends on bore diameter, ring position (top, second, or oil ring), ring design, and how the engine will be used. Some ring sets are pre-gapped for a specific bore size, while file-fit rings are intentionally tight so the builder can adjust them. Always follow the specification sheet supplied with the rings rather than a general rule of thumb.
Installation Tips
Install rings with any identification marks facing up, position the end gaps as directed in the ring instructions, and use a ring expander to avoid twisting or overstretching the rings.
Evergreen Pistons and Rings
Evergreen offers piston sets and engine rebuild kits that, depending on the part number, may include piston rings from established manufacturers such as NPR. Use the Evergreen eCatalog to find the right components for your engine and bore size.
