Head gasket failure

HGF is a Acronym

HGF = Head Gasket Failue

Signs of head gasket failure tend to include oil in the coolant expansion bottle, or oil becoming emulsified where the coolant from the coolant ways seeps through the gasket into oil. This can normally be seen as a "mayonnaise" type substance inside the oil filler cap or the engine oil dipstick.

Upgraded Head Gasket and steel dowels

Original designs of the head gasket were held in place by dowels that allowed minute movement of the gasket. Later gaskets are held in place by steel dowels, which do not allow movement of the gasket.
If you suspect HGF, do not drive the car and have it checked by a reputable garage. This can include checking the coolant for hydrocarbons to see if oil/gases have escaped past the gasket into the coolant ways. If the engine has overheated and the cylinder head has become warped, then the head will need to be skimmed so that it can be reseated onto a new gasket at a later date.

To avoid head gasket failure, always check the coolant level, hoses for damage and jubilee clips for tightness. Warming the engine up gradually from cold may also be beneficial.