Here's a breakdown:
* Minimal Damage: If the head gasket failure is localized and hasn't caused significant overheating or contamination of the coolant with oil or vice-versa, a head gasket replacement might be all that's required. This involves removing the cylinder heads, replacing the gasket, and then reassembling. You'll need to check the cylinder heads for warping (machining may be needed) and potentially resurface the block's mating surface.
* Moderate Damage: If overheating has occurred, you might need to check for warped cylinder heads and possibly cracked heads or block. You might also need to replace the head bolts. This increases the repair cost and complexity.
* Severe Damage: Extensive overheating can lead to cracked engine blocks or heads, warped cylinders, scored cylinder walls, or damage to pistons and other internal components. In such cases, a full rebuild or even engine replacement might be necessary. The cost and effort would be significantly higher.
In short: You need a proper diagnosis to determine the extent of the damage. A mechanic should inspect the engine to assess the damage before deciding on the necessary repairs. Simply replacing the head gasket is usually the cheaper option, but severe damage from overheating mandates much more extensive work.
