A heavy, rhythmic knocking from inside the crankcase that gets louder under load. This is the most serious noise you can hear from a reciprocating compressor -- it means the crankshaft or connecting rod bearings are worn and the compressor needs a major overhaul or replacement.

What you'll see

A deep, heavy knocking or thumping sound coming from the bottom of the pump (crankcase area). It's rhythmic -- one knock per revolution or per stroke. The noise gets louder when the compressor is working hard (at higher tank pressures). You may also feel excessive vibration. In the early stages it might just be a light tapping that you almost can't hear over the normal compressor noise. As it progresses, the knocking becomes unmistakable. If you see metal particles in the oil or the oil turns gray/metallic, the bearings are shedding material.
Before you assume this is the problem

A rhythmic clicking from the head area is a valve problem, not a bearing. A squeal is a belt problem. A rattle from the outside is loose components. Bearing knock is always from deep inside the crankcase and has a heavy, dull quality. See: Worn Valves, Loose Mounting, Belt Squeal.

See all causes of unusual noise / knocking →

How to diagnose

  1. Listen with a stethoscope or screwdriver

    Press a long screwdriver against the crankcase (not the head) while the compressor runs. If the knocking is loudest here, it's a crankcase bearing issue. Compare to the sound at the head -- valve noise is louder at the head, bearing noise is louder at the crankcase.

    Result: Knock loudest at crankcase = bearing wear. Loudest at head = valve issue.

  2. Check the oil

    Drain a small sample of oil and look at it. Metallic particles or a gray/silver color indicate bearing material shedding. Healthy oil is clear (amber to brown). If you see glitter in the oil, the bearings are wearing rapidly.

    Result: Clean oil = bearings probably OK. Metallic particles or gray oil = bearing wear.

  3. Check for play in the connecting rod

    With the compressor off and the head removed, try to rock the piston/connecting rod assembly sideways by hand. There should be minimal play. If you can feel or hear a clunk as the rod moves on the crankshaft journal, the bearing clearance is excessive.

    Result: Minimal play = OK. Noticeable clunk or rocking = bearing worn beyond limit.

How to fix it

  1. Replace the bearings (if economically justified)

    Bearing replacement on a reciprocating compressor means splitting the crankcase, removing the crankshaft, and replacing the shells or bushings. On larger industrial compressors, this is standard maintenance. On small shop compressors, the labor cost may exceed the replacement cost of the entire pump or compressor. Get a quote for a rebuilt pump versus a new unit before committing to a rebuild.

  2. Replace the pump or entire compressor

    For smaller compressors (under 5 HP), it's often more economical to replace the entire pump assembly or the whole compressor than to rebuild the crankcase. Replacement pumps are available for most popular models. Make sure the new pump matches the motor speed, pulley size, and mounting pattern.

Common mistakes

Don't keep running a compressor with bearing knock hoping it will 'go away.' It won't -- it only gets worse. Continued operation with worn bearings leads to a seized crankshaft, which can break the connecting rod through the crankcase (catastrophic failure). Also: if you rebuild the bearings, always flush the oil system thoroughly and use fresh oil. Old oil with metal particles in it will destroy the new bearings in no time.

Parts & tools

Bearing shells or bushings (if rebuilding). Complete pump assembly (if replacing). Oil sample for inspection. Mechanical stethoscope or long screwdriver. Standard mechanic's tools for disassembly.

Review safety precautions before starting →

Safety

A compressor with badly worn bearings can seize suddenly, which is violent -- the motor stalls instantly and can trip breakers or blow fuses. If the knocking is severe, stop using the compressor. A seized connecting rod can crack the crankcase.

This issue can also cause

  • Overload Relay Trips Thermal overload or motor breaker keeps tripping: either an electrical issue or the motor is working too hard.