BMW Cylinder Heads

Cracked Head Checks

Overheating is one of the most frequent causes of cracked cylinder heads. This technical sheet provides a checklist of problems that can cause overheating, and it is in your own long-term interest to ensure that you make these checks carefully to avoid the possibility of the replacement head failing.

  1. Thermostat

    The thermostat is designed to open when the water reaches a certain temperature. You can check that it opens correctly by placing it in a jug of boiling water. Also check that it has no visible damage.

  2. Radiator

    The core should be 100 percent intact without corrosion or damage. Check cooling capacity with a flow test; any competent radiator reconditioner will do this for you. If you live in a hard-water area beware, the inside of your radiator could be furred-up leading to drastic reduction in efficiency from reduced flow.

  3. Radiator Cap

    Check for correct pressure release and that the rubber seal is in good condition.

  4. Header Tank

    Check for splits due to pressurising or old age on nylon tanks, on metal tanks check all soldered joints and mountings.

  5. Fan and Fan Clutch

    The fan should run in balance and have all its blades. The fan clutch, whether friction or viscous type, should cut in at the correct temperature to ensure appropriate cooling. It should not freewheel when the engine is hot.

  6. Auxiliary Electric Fan

    If fitted, this is usually thermostatically controlled. Check for correct cut-in cut-out.

  7. Hoses

    Check for age hardening, split ends, cracks, and damage from the jubilee clips. If in doubt, replace.

  8. Hose Clips

    These should always be replaced with new clips.

  9. Heater Valve And Core

    These often leak unbeknown to the driver. Check for wet carpets around the centre console.

  10. Trapped Air

    When any cooling system is filled with water or antifreeze it needs bleeding to allow trapped air to escape. The bleed screw is usually on the thermostat housing. If air remains trapped in the cylinder head area then the coolant will not be in contact with the hot metal and the cylinder head will crack again.

And After Fitting A New Cylinder Head ...
  • Run the engine for a few minutes and check that the cooling system is pressurising correctly.
  • Remember to bleed trapped air from the system.
  • If you have to add water to the cooling system, check why.
  • Do not add cold water to the cooling system when the engine is hot.
  • Do not try to drive home with an overheated engine.