Piston rings seal the gap between the piston and cylinder wall. When they wear, compressed air leaks past them into the crankcase (blow-by), causing hissing sounds, increased crankcase pressure, oil mist from the breather, and a gradual loss of pumping efficiency.
What you'll see
If you mainly hear a clicking or rattling from the head area (not the crankcase), the problem is more likely valve damage, not rings. If the only symptom is oil consumption without noise, see: High Oil Consumption > Worn Piston Rings. If performance is fine and the only issue is noise, check mounting and valves first.
How to diagnose
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Check the crankcase breather
Put your hand over the crankcase breather or oil fill cap while the compressor is running. You should feel little to no air pressure. If you feel strong, pulsating air coming out -- or see oil mist spraying from the breather -- that's blow-by from worn rings. The more pressure you feel, the worse the ring wear.
Result: Little or no air at breather = rings OK. Strong pulsating air or oil mist = ring blow-by. -
Check oil consumption
Track oil level over a few days of running. If you're adding oil more frequently than before, and there are no external leaks visible, the oil is likely being carried past the worn rings into the compression chamber and out with the compressed air.
Result: Oil level stable = rings likely OK. Rapid oil consumption without external leaks = worn rings. -
Measure cylinder compression (optional)
If you have a compression tester, you can check each cylinder. Remove the intake filter, seal the tester into the intake port, and crank the compressor by hand or briefly with the motor. Compare the reading to the specification. Low compression indicates worn rings, worn cylinder, or valve problems.
Result: Near-spec compression = rings OK. Low compression = worn rings or valves.
How to fix it
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Replace the piston rings
This is a more involved repair. You'll need to remove the head, valve plate, and cylinder (or pull the piston from below on some designs). Replace all rings on all pistons at the same time. Check the ring end gap with feeler gauges -- too much gap means the cylinder is also worn. New rings in a worn cylinder won't seal properly.
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Check and possibly replace the cylinder
While the piston is out, check the cylinder bore for scoring, taper wear, and out-of-round. Run your fingernail across the surface -- if you can feel ridges or scratches, the cylinder is worn. Some compressors have replaceable cylinder sleeves. If the cylinder is beyond spec, new rings alone won't fix the problem.
The most common mistake is putting new rings in a badly worn cylinder and expecting good results. If the cylinder is tapered or scored, new rings can't seal against the uneven surface. Also: install rings the right way up. Most compression rings have a dot or mark that goes toward the top (piston crown). And stagger the ring gaps -- never line up the end gaps of multiple rings, or blow-by goes straight through.
Piston ring set for your compressor model. Ring compressor tool. Feeler gauges for checking ring end gap. Torque wrench. Clean rags and light oil for assembly. Cylinder hone (if cylinder needs light re-honing).
Do not disassemble the pump while it's hot. Let it cool completely. Bleed all pressure. Support the pump properly if you need to remove the cylinder.
This issue can also cause
- Oil Carry-Over & High Oil Consumption Oil leaving the compressor faster than normal: either ending up in the compressed air, or disappearing from th...
- Not Building Pressure / Air Blowing Out Inlet Compressor runs but pressure builds slowly or not at all: often air blows back out through the inlet filter.