Before assuming internal wear, check for the obvious: oil leaking out of the compressor. Common leak points are gaskets, seals, oil fill caps, drain plugs, and the crankshaft seal. Usually easy to spot -- look for wet, oily areas on and around the pump.

What you'll see

The oil level keeps dropping but you don't see oil in the compressed air or at the tank drain. There's oil on the floor under the compressor, or wet oily patches on the pump housing, crankcase, or fittings. The leak may be slow enough that it only drips when the compressor is running (oil pressure builds up) and stops when it's off. Or it can be a steady drip from a worn gasket or loose plug.
Before you assume this is the problem

If there's no visible oil anywhere on or under the compressor but the level is still dropping, the oil is being consumed internally -- carried past the rings into the compression chamber. See: Worn Piston Rings, Worn Cylinder Bore.

See all causes of oil carry-over & high oil consumption →

How to diagnose

  1. Clean the compressor and watch for fresh oil

    Wipe down the entire pump and crankcase with a clean rag so everything is dry. Then run the compressor for 15-30 minutes and look carefully for fresh oil appearing anywhere. Check the bottom of the crankcase, around gasket joints, the oil fill cap, the drain plug, the crankshaft seal area (where the shaft exits the crankcase), and any sight glass or dipstick tube.
    Result: Fresh oil appears at a specific location = you've found the leak. Everything stays dry = internal consumption.

  2. Check the obvious spots

    The most common leak points on reciprocating compressors: (1) Oil fill cap -- the gasket dries out or the cap wasn't tightened. (2) Drain plug -- loose or missing washer. (3) Crankcase gasket -- where the two halves of the crankcase meet. (4) Crankshaft seal -- where the shaft passes through the crankcase to the flywheel. (5) Oil sight glass -- the rubber gasket around the glass can shrink and leak.
    Result: Identify the specific leak point.

How to fix it

  1. Tighten or replace gaskets and seals

    For the oil fill cap: replace the gasket or o-ring. For the drain plug: tighten it or replace the crush washer. For the crankcase gasket: you'll need to partially disassemble the pump. For the crankshaft seal: this is the most involved repair -- you need to remove the flywheel to access the seal. Use the correct replacement parts for your compressor model.

  2. Check for excessive crankcase pressure

    If the crankcase gaskets and seals keep leaking even after replacement, the crankcase pressure may be too high. This happens when piston rings are worn and blow-by pressurizes the crankcase. The excess pressure pushes oil out through every gasket and seal. In this case, fixing the seals is only treating the symptom -- you need to fix the rings. See: Worn Piston Rings.

Common mistakes

Don't just keep topping up the oil without finding the leak. Oil on hot compressor surfaces is a fire risk, and a slow leak on the floor eventually becomes someone's slip-and-fall injury. Also: if you find high crankcase pressure causing the leaks, don't just drill a bigger breather hole. Fix the rings. The breather is sized for normal blow-by, not for worn-out rings.

Parts & tools

Replacement gaskets, o-rings, and seals for your compressor model. Drain plug washer. Clean rags and degreaser. Torque wrench for crankcase bolts. Flywheel puller (if replacing crankshaft seal).

Review safety precautions before starting →

Safety

Oil on the floor is a slip hazard. Clean it up promptly. If the compressor sits on a shelf or stand, oil dripping onto a hot motor or exhaust can create a fire risk.