Piston Ring

What is an Air Compressor Piston Ring?

An Air Compressor Piston Ring is a ring-type part in the air brake system compressor that sits in the grooves of the piston and is pressed against the cylinder wall to stop air leaking to the lower side. Its role is to close the gap between the piston and the cylinder in the compressor, which draws in and compresses air through the motion from the engine. The parts that wear most are the outer face of the piston ring touching the cylinder wall and the side faces of the groove; when they wear, pressure drops and oil can mix into the air line. In the field, this part is also known as a piston ring, compression ring or oil scraper ring. Bushes and seals belong to other categories and are not covered by this page.

How does an Air Compressor Piston Ring work and what parts does it cover?

As the piston moves up and down, the rings touch the cylinder wall and keep the compression chamber sealed. The contents of the kit depend on the piston ring structure of the compressor; typically it includes:

• Top compression piston ring: the first piston ring that stops compressed air leaking to the lower side.
• Second compression piston ring: a second sealing piston ring that supports the top piston ring in multi-ring designs.
• Oil scraper piston ring: piston ring that scrapes oil off the cylinder wall so it does not mix into the air.
• Piston Ring spring: spring that presses the oil scraper piston ring against the cylinder wall, found on some models.
• Piston groove: the slots in the piston where the rings sit; they must be clean and sound for the kit to work properly.

Air Compressor Piston Ring types

• Compression piston ring set: preferred when there is pressure leakage and reduced compression efficiency.
• Oil piston ring set: preferred when oil residue and oil carry-over are seen in the air line.
• Complete piston ring set: preferred when both groups are worn together, after checking the piston groove.

How to choose the right Air Compressor Piston Ring?

Selection should be made by reading the type and direction marks on the removed piston ring, the condition of the piston groove and the system type the compressor is fitted to. Looking only at appearance leads to a wrong choice; the side faces of the piston ring and the points where it contacts the groove must be checked. Even within the same model range of Mercedes-Benz, MAN, Scania, Volvo and DAF, different layouts can exist, so choosing by model name alone is a mistake.

Installation and maintenance tips

• Groove cleaning: no carbon deposits should remain in the piston grooves; the groove must be cleaned so the piston ring sits in place fully.
• Fitting direction: piston rings must be fitted with attention to their direction; a piston ring fitted the wrong way causes oil and air leakage.
• Gap alignment: the gaps of the rings should be placed so they do not line up at the same point.
• Lubrication: a light coat of compressor oil should be applied to the cylinder wall and piston ring surface before fitting; running dry scores the surface.
• Start-up check: the compressor should first be run at low load, and leakage and oil carry-over should be watched as the pressure rises.

Common faults and solutions

• Delayed pressure build-up: the compression piston ring may be worn and leaking from the compression chamber; solution is to replace the compression piston rings.
• Residual oil in the air line: the oil scraper piston ring may be worn; solution is to replace the oil piston ring and clean the air line.
• Compressor running longer than normal: loss of sealing causes the compressor to run continuously; solution is to check the piston ring and piston groove.
• Increased oil consumption: oil control on the cylinder wall may have been lost; solution is to inspect the oil piston ring and cylinder wall together.

Frequently Asked Questions

Does oil consumption decrease when the rings are replaced?

If the oil piston ring is worn, oil consumption and oil carry-over into the air line usually increase. When a new oil scraper piston ring is fitted this problem is largely reduced, but the cylinder wall must also be checked.

Can a piston ring be fitted without cleaning the piston groove?

No. A piston ring fitted to a groove with residue does not sit properly and leaks. The piston ring should be fitted only after the groove has been cleaned.

Should the compression and oil rings be replaced at the same time?

If only one group is worn, only that group can be replaced. If both groups are worn, a complete set should be chosen, otherwise the new part may not work well with the old one.

Is a piston ring enough if the cylinder wall is scratched?

No. A scratch on the cylinder wall wears the new piston ring quickly too. For this reason the condition of the wall must always be checked before fitting the piston ring.

How is piston ring leakage identified?

Delayed pressure build-up, oil residue in the air line and the compressor running continuously are the first signs. A soapy water test shows bubbles at the connection points, which reveals where the leak is.