What temperature condition should be used when measuring piston ring end gaps?

What temperature condition should be used when measuring piston ring end gaps?

• A. Measure at ambient room temperature.
• B. Measure after engine has run to operating temperature.
• C. Measure with the engine fully cooled.
• D. Measure with feeler gauge at operating temperature per manufacturer spec.

Answer: (D)

The space between the ends of a piston ring must be checked at the temperature the engine will reach in service because the rings expand with heat and the end gap grows when hot. If you measure at ambient or fully cooled conditions, you’ll get a smaller gap than the ring will have when the engine is running, which can lead to a tight or binding fit and possible damage. The manufacturer’s procedure specifies the correct temperature and how to measure (often requiring a hot engine or a specific hot-condition setup) and using a feeler gauge to verify the gap. Following that spec ensures the end gap is correct for normal operation.

The space between the ends of a piston ring must be checked at the temperature the engine will reach in service because the rings expand with heat and the end gap grows when hot. If you measure at ambient or fully cooled conditions, you’ll get a smaller gap than the ring will have when the engine is running, which can lead to a tight or binding fit and possible damage. The manufacturer’s procedure specifies the correct temperature and how to measure (often requiring a hot engine or a specific hot-condition setup) and using a feeler gauge to verify the gap. Following that spec ensures the end gap is correct for normal operation.