How to Check Piston Rings

Piston rings seal off the pistons in the combustion chamber so that no energy escapes during the compression stroke. Normally, three rings come with each piston: an expansion ring, a compression ring and a thicker oil ring. They work in concert with the piston by riding up and down the cylinder wall with it.

Piston rings must maintain a near-perfect seal while subjected to extreme heat, moisture, and sometimes dirty and thinly-polluted oil. For such small and delicate parts, the task they perform harnesses the engine's power output and transfers it to the driving wheels. Anyone can diagnose piston ring problems if they know what to look for.

1Pistons rings are subjected to extreme heat and flex.Listen for any noises originating from the sides of the engine or near the oil pan underneath the vehicle. Although connecting rods and bearings can make knocking noises, piston slap has a hollow, bell-like sound, which means the piston rides with too much sideplay inside the cylinder wall. Worn rings can be the cause of this condition.

2Look for bluish white smoke pouring or intermittently puffing from the exhaust tail pipe. Oil burns with this color, which may be due to worn rings. Valve guides and seals would be the only other parts that would cause excessive oil burning.

3Unhook the coil high-tension wire from the engine. Remove all of the spark plugs with a socket and wrench. Have an assistant crank the engine six or seven times while you take readings on all the cylinders with a compression tester. Write down the pounds per cylinder for each one. Add a small cap of oil to each cylinder and test for compression again. Compare the numbers. If the second test has made the cylinder numbers climb higher than 30 pounds in any cylinder, the rings are worn and should be replaced.

4Inspect the rings with a magnifying glass once the pistons have been removed from the engine. Look for any dark discoloration on the ring end's outer surface and bottom side. Good rings look shiny and clean in these areas. Bad rings will have soot, oil or carbon buildup.

5Examine the ends of the rings for lines, scratches or grooves. Such distortions point to metal shavings and/or contaminated oil. Any small pit marks on the ring's outer surface indicates water contamination that has caused rust. The rings must be replaced if this condition exists.

6Use a feeler gauge to measure the ring gaps when fully open. Consult the manufacturer's specifications in your owner's manual for the correct gap thickness.  Generally, too large of a gap allows escaping combustion gases to pass through the ring contact-to-cylinder wall area. A narrow gap indicates the ring has lost flex strength.

7Look for small amounts of metal slag on the ring's outer surface. Piston rings that show melted edges have been subjected to too much heat. The rings might possibly crack when handled or bent to test their spring-back flexibility. Rings that do not bend easily cannot exert enough tensile strength to make a proper seal.

8Inspect the largest ring on the bottom of the piston. This is the oil ring, which has an internal structure or weave that allows it to retain oil. The small honeycomb structure should not be caked with carbon or gunk. Replace the ring sets if this condition exists.

9Examine the rings for uniform thickness. They should not have any thickness variations from one end to the other. A very thin section denotes a cylinder bore that has worn out-of-round or taper, which has distorted the ring's surface. Replace as needed.