Correct Use of Piston Rings

Piston rings are consumable components used in the harshest environments and therefore must be manufactured to the most exacting standards.

Despite the triple threats of searing temperatures, limited lubrication and corrosive atmosphere, well engineered and properly fitted piston rings can function for in excess of 1 year. This approximates to about 300,000km sliding distance per year. Every PRUK piston ring is precisely manufactured and thoroughly inspected to guarantee conformity to specification. This is to ensure correct functioning.

Each ring requires careful handling both prior to and during fitting. Without clean grooves, adequate lubrication or a well maintained cylinder liner, the piston rings cannot operate properly. In fact, countless engine malfunctions, for example incomplete combustion, result in a change in an engine’s characteristics which in turn damage its piston rings. All too often piston rings are blamed for the problem, but as any good engineer knows, the solution lies beyond simple replacement of the rings.

Storage

PRUK’s piston rings are carefully packaged to ensure they reach the end user in premium condition. Once with the end user they should be stored carefully to ensure damage does not occur to them prior to fitting. Rings should be stacked neatly in a flat position; this prevents twisting, which reduces the sealing ability of the rings.

Rings should also be stored so that they are protected from contact with moisture, despite the protective coatings put on PRUK’s piston rings. Prolonged exposure to moisture will cause corrosion and undermine performance.

Fitting

When fitting piston rings, great care should be taken to ensure no damage is caused to the ring. Always use the proper tools provided by the engine manufacturer, these should prevent over extending the ring while passing it over the piston. During fitting, the ends of the ring should not be extended to more than 9 times the thickness of the rings. As the ring is expanded over the piston, stress is placed on the ring which in certain areas may exceed the ‘elastic’ limit of the material. This could mean permanent deformation, resulting in distortion to the shape of the ring, preventing it from sealing the cylinder bore completely.

PRUK’s piston rings are designed to minimise deformation. However, their efficient operation relies on careful fitting by the engineer. Recent improvements in ring design have increased the importance of installing a ring in the correct orientation, i.e. the right-way up. Although a ring may look similar which ever way it is held, there is often a special surface profile, which although not easily visible to the naked eye, improves the rings operational performance. If however a ring is installed upside down, the functional characteristics of the ring may be destroyed, leading to increased oil consumption or other problems.

Rings should always be installed with any identification markings shown on the upper-side face of the ring. Commonly ‘TOP’ will be marked onto the ring to help identify the correct orientation.

Running-In

The purpose of running-in new rings is to help them form a seal as quickly as possible without causing scuffing of the running surface. scuffing can develop during the first few hours of a rings life, and if severe, may disrupt the lubricating oil film which leads to further damage.

Engine manufacturer’s instructions for running-in should always be followed closely; any short cuts taken at this early stage will ultimately shorten the rings working life through higher operational wear rates. It may also increase oil consumption.

New rings are carefully manufactured to assist straight forward running-in. The slightly roughened surface aids oil retention, vital for reducing scuffing. Some are surface engineered to have soft protective coatings. Rings which appear black usually have a phosphate coating. Other commonly used running-in coatings on PRUK rings include soft metals such as copper. These conform particularly effectively to the sides of worn liners.

The newer taper faced rings for large 2stroke engines help reduce running-in times by offering improved oil film-forming characteristics.

Ring Replacement

It is always difficult to state the frequency with which piston rings should be renewed. Replacement depends not only on the engine model, but also the engine’s age, condition and its usage patterns. In general terms, when the thickness of the ring has been reduced by 10-15% from the original size, it is potentially hazardous to continue use. At this point the loss of tension generated by the reduction in thickness is some 30-40% of the original. Furthermore the ring gap will have substantially increased, undermining the ring’s sealing ability.

Using rings which have passed an acceptable rate of wear will again increase oil consumption and liner wear, impair engine performance, and can lead to ring failure and potential combustion chamber damage. Always be aware of the manufacturers’ ‘limits of acceptable use’ and remember these are maximum wear limits. Rings should ideally be replaced prior to reaching this stage in their life cycle.

Piston Ring Re-use

It is recommended that all piston rings are renewed every time a piston overhaul is undertaken. In the long term this is the most economical choice, yet many engine operators continue to re-use rings in the belief they are saving money. As a ring approaches the end of its life cycle, its performance deteriorates and the likelihood of failure increases. Re-installing used rings normally exaggerates this problem for several reasons.

When rings are removed they must go through the same stress cycle as that which occurs during fitting, i.e. without due care and attention and use of the correct tools it is very easy to deform the ring shape. The ring is always weaker in its worn condition and engineers tend to be less careful when removing rings than during fitting. Damage to rings is very easy to cause, but very difficult to see.

Upon removal the ring should be cleaned and measured. If the ring has worn out of tolerance or is likely to exceed the tolerance by the time of the next overhaul, it should never be re-used. If the rings dimensions have remained satisfactory then all the sharp burrs on the outside lower and upper edges should be removed and brought back to the condition of a new ring, i.e. normally a small radius. Without carrying out this operation, the sharp edges may disrupt the forming of the oil film and may cause scuffing from high point loading.

Upon re-installation the rings should be orientated the right way up, although sometimes the identification markings may have been worn away, preventing identification of the top face. As with new rings, replacing a used ring incorrectly is again likely to promote operational problems.

Finally once re-installed, an engine with reused rings should be very gently run-in. A used ring’s running surface will be shiny and smooth. This doesn’t aid running-in since its oil retention is poor and conformity to the liner takes longer. Running-in periods should be extended when re-using rings. As previously stated, it is very often more economical and less problematic to renew all piston rings each time a piston is overhauled.