The reasons why in-situ repair is impractical and risky are:
* Accessibility: Accessing the crack properly for repair is nearly impossible without removing the head. You need a clear view and the ability to properly prepare the surface for repair.
* Proper Preparation: Successful repair requires meticulous cleaning, surface preparation (machining is often necessary), and proper application of the repair material. This cannot be done effectively with the head still installed.
* Heat Dissipation: Cylinder heads get extremely hot during operation. The repair material must be able to withstand these temperatures, and proper heat dissipation is crucial; this is hard to guarantee in-situ.
* Material Compatibility: The repair material must be compatible with the head material (aluminum or cast iron) and engine coolant. Improper selection can lead to further damage.
* Stress Concentrations: Cracks often introduce stress concentrations. The repair must alleviate these stresses, which requires proper clamping and curing, difficult to accomplish without removal.
* Leakage Testing: After repair, thorough pressure testing is essential to verify the repair's effectiveness. This is virtually impossible in-situ.
While some very small, superficial cracks *might* be temporarily sealed with a specialized sealant, this is a very risky approach, offering only a temporary solution at best. It's highly unlikely to hold up under the pressure and heat cycles of a running engine, and could lead to catastrophic engine failure.
In short: Removing the cylinder head is the only reliable and safe way to repair a crack. Any other attempt is highly discouraged and should only be considered as a last resort in an extreme emergency, fully understanding the risks involved. If you have a cracked cylinder head, the proper course of action is professional inspection and repair, or replacement, by a qualified mechanic.
