The head gasket seals between the cylinder and the head. When it fails, compressed air leaks out -- either externally (you can hear it hissing) or between stages on a two-stage compressor. The pump loses compression efficiency and the affected cylinder runs hot.

What you'll see

The compressor runs but pressure builds slowly or not at all. You may hear a hissing sound from the cylinder head area -- air escaping past the failed gasket. On a two-stage compressor, a gasket failure between stages allows high-pressure second-stage air to blow back into the low-pressure first stage, causing the first stage to overheat dramatically. The discharge pipe may feel cooler than expected because less air is actually reaching the tank.
Before you assume this is the problem

If the compressor just had the head removed (for valve work or inspection) and now won't build pressure, the gasket is the prime suspect -- it may not have been installed correctly, or it was reused (never reuse gaskets). If the head hasn't been off recently, check valves first as they're a more common failure. See: Broken Inlet Valves, Broken Outlet Valves.

See all causes of not building pressure / air blowing out inlet →

How to diagnose

  1. Listen for air escaping from the head

    With the compressor running, listen carefully around the cylinder head joint. A blown external gasket will produce a hissing or whistling sound at the gasket surface. You may feel air blowing out between the head and cylinder. Spray soapy water around the joint -- bubbles confirm the leak location.

    Result: No hissing or bubbles = gasket sealing externally. Hissing and bubbles = blown gasket.

  2. Check for inter-stage leak (two-stage compressors)

    On a two-stage compressor, the gasket may be leaking between the first and second stage passages. This won't produce an external hiss, but the first stage will run very hot and the interstage pressure will be abnormal. If the interstage pressure gauge reads much higher than normal, second-stage air is leaking back into the first stage.

    Result: Normal interstage pressure = gasket OK between stages. Abnormal = possible inter-stage leak.

How to fix it

  1. Replace the head gasket

    Remove the head bolts, lift off the head, and remove the old gasket. Clean both surfaces thoroughly -- all old gasket material must come off. Check the head and cylinder mating surfaces for flatness with a straight edge. Install a new gasket (don't use sealant unless the manual specifies it). Torque the head bolts in a cross pattern to the specified value.

  2. Check head and cylinder flatness

    Place a straight edge across the head mating surface and look for light underneath. Any warping means air will leak past even a new gasket. If the head is warped, it can sometimes be lapped flat on a piece of emery paper placed on a known-flat surface (like a thick glass plate). If badly warped, the head needs replacing.

Common mistakes

Never reuse a head gasket. Once compressed, it won't seal properly a second time. Also: don't overtighten head bolts trying to stop a leak. If the gasket is blown, tightening won't fix it -- and overtightening can warp the head or crack the casting, making things worse. Replace the gasket properly. Another common issue: after doing a valve job, the new head gasket was installed upside down or with wrong orientation -- some gaskets have specific passage holes that must align. Double-check before tightening.

Parts & tools

New head gasket for your compressor model. Gasket scraper. Straight edge for flatness check. Torque wrench. Clean rags. Fine emery paper on flat surface (if lapping needed).

Review safety precautions before starting →

Safety

The head area gets very hot, especially with a leaking gasket. Compressed air escaping through a blown gasket can be very hot. Let everything cool before disassembly.

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