How to Tell If the Piston Rings Are Worn

Written by john stevens j.d. | 13/05/2017

In every internal combustion engine, one or more pistons travel the length of a cylinder. A small gap exists between the circumference of the piston and the wall of the cylinder to allow the piston to slide through the cylinder. To keep the oil underneath the piston and the air and fuel above the piston separate, pistons generally have three rings. If those rings become damaged, the engine’s power and fuel efficiently both decline. Fortunately, ring wear can be detected without disassembling the engine.

Remove all of the engine’s spark plugs with a sparkplug wrench.

Direct an assistant to depress the vehicle’s throttle pedal to the floor.

Insert a compression gauge into any one of the sparkplug openings. Note that compression gauges are available in one of two designs. The more common design features a rubber hose with a threaded metal fitting, which is threaded into the sparkplug hole until relatively tight. The second design instead features a rubber plug at the base of the gauge which is pushed into the sparkplug opening and held in place.

Direct the assistant to crank the engine with the ignition key.

Cease cranking the engine when the needle on the compression gauge reaches a maximum reading, then record that reading.

Repeat steps 2 through 5 for the remaining cylinders and compare the compression results. If the reading is lower on one or two cylinders than for the remaining cylinders, spray a small amount of water into the cylinder with the low reading using a spray bottle and repeat the test. If the compression reading is momentarily higher, that piston’s rings are likely damaged.

Things you need

  • Spark-plug wrench
  • Compression gauge
  • Water
  • Spray bottle

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