Piston Rings – When and How to Replace Them

Piston Ring
Piston Ring

The first sign of worn piston rings is blowby.

Puffs of blue smoke out the exhaust means the engine is burning oil.

Worn or damaged piston rings, You may notice that the engine oil gets low faster than before. Oil smoking first appears during engine cold starts. As the engine warms up, the pistons and rings expand, sealing the walls, reducing how much oil is blowing past the rings.

Eventually, the piston rings wear to the point that there is constant blow-by, and the car smokes all the time. This can also be due to worn valve seals as well and finding the right size rings can be hard if the cylindrical grinding techniques used to make these pistons is slightly off sizing.

Another issue with blow-by is combustion gases entering the crankcase. This means fuel and combustion byproducts in the oil. As these chemicals permeate the oil, the oil loses its viscosity and the ability to cool and lubricate the engine.

You must change the oil more often to keep the engine clean. This is also a big factor in bearing damage as well.

Eventually, the wear gets so bad that there is a loss of power,

  • Too much of the combustion gasses are being lost to the crankcase
  • Too much oil is getting into the combustion chamber.

This leads to fouled spark plugs and a poorly running engine.

Typical Ring Configuration

Compression Rings or Pressure Rings

The compression rings provide sealing above the piston and prevents the gas leakage from the combustion side. Located in the top grooves are the compression rings. However, this may differ according the design of the engine. The main function of these rings is to seal the combustion gases and transfer heat from the piston to piston walls.

Second or Intermediate Rings

The oil in controlled by shearing the layer of the oil left by oil ring, thus providing the top compression rings enough lubrication. Moreover, it also provides help to the top compression ring in sealing and heat transfer.

Oil Control / Scrapper Rings

The oil control rings controls the amount of lubricating oil passing up or down the cylinder walls. These rings are also used to spread the oil evenly around the circumference of the liner. The oil is splashed onto the cylinder walls. These rings are also called scraper rings as they scrap the oil off the cylinder walls and send it back to the crankcase. These rings do not allow oil to pass from between the face of the ring and the cylinder.

Most engines today have very tight piston-to-wall clearances (.001 in. or less) to minimize blowby and reduce piston rock. The more stable the piston, the better the rings can maintain a tight seal. Close tolerances also make for a quieter running engine, especially after a cold start when clearances are greatest).

Anti-scuff moly-based coatings are used on the sides of many stock and performance pistons. Skirt coatings not only protect the pistons, but also allow tighter clearances between the piston and cylinder. As a result this will reduce piston rocking and blowby.

Checking Ring End Gap

Check Ring End Gap
Check Ring End Gap

When measuring piston ring end gap, check the gap with the piston rings at the top and the bottom of the bore. If the bore has taper the end gap will be larger at the top and smaller at the bottom.

Use the bottom position to set the end gap. If you use the top of a worn bore to set the end gap, the end gap will be too small when the piston reaches the bottom of the bore.

The ends of the ring may hit each other causing the ring to bind and scuff. Cylinders that have more than .003 to .005 inches of taper wear should probably be bored. Refer to the engine manufacturer’s specifications for the maximum allowable taper wear.

Installing Piston Rings

Installation is just as important as the type of pistons or rings that are used in an engine. One of the most common installation errors is installing one or more rings upside down.

  • If only one second oil ring is accidentally installed upside down in a V8 engine, it can double the engine’s oil consumption!
  • If every second ring on all the pistons are reversed, the engine will have an unquenchable thirst for oil – which may be mistakenly blamed on improper ring break-in, seating or cylinder wall finish.

Piston rings are usually marked with a dot, which must always face up. If no mark is provided, rings with a bevel on the inside diameter must be installed with the bevel facing up. On rings with no mark and a groove on the outside diameter, install the rings with the groove toward the bottom of the piston.

Another common error is spiraling piston rings onto a piston.

Piston Ring Pliers
Piston Ring Pliers

This will usually deform the ring which can affect ring rotation and seating. Always use a ring expander. Position the ring on the expander and expand it just enough so it can be slipped onto the piston. Do not over extend the ring.

Position the rings so the end gaps are staggered 180 degrees apart. This will reduce blowby.

Ring and groove depth should be compared prior to installing the rings to make sure the rings are the correct ones for the piston.

  • If shallow groove rings are installed on a deep groove piston, the rings will tend to pop off the piston before the piston is installed in the engine.
  • If rings designed for a deep groove piston are installed on a shallow groove piston, the rings will bottom out and jam against the cylinder.

Piston ring end gaps should always be measured to make sure the rings fit the grooves and cylinders correctly. Piston rings can be damaged during installation if the lower lip of the ring compressor is nicked.

For this reason, the underside of the ring compressor should be checked frequently for damage.

Finally, adequate ring and cylinder lubrication is essential for proper ring seating and protection when the engine is first started.

Scrubbing out the cylinders with hot soapy water after the block has been honed is an absolute must. This will help to remove all the abrasives and other contaminants that can damage new piston rings.

Remember, wear will always be greatest at the top of the cylinder where cylinder pressures and temperatures are highest.

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