How can one tell visually if a cylinder head is cracked?

[kola]

Hi:

I've read that the 4.0 OHV cylinder heads are prone to cracking. I can confirm that having two Explorers--91 and 98--that I've had to replace the heads. I've also read that most cracks are hairline cracks not visible to the naked eye.

If that is the case, how does a mechanic know a head is cracked and it's not some other issue; a bad head gasket for example?

Just curious.

Thanks

 

[JDraper]

On aluminum heads they can use either dye penetrant or eddy current inspection. Most shops go for dye penetrant because it's cheap and easy to use. If the head is cast iron, they either use dye penetrant or mag particle inspection.

 

[mrboyle]

As for telling if the vehicle has a blown head gasket, there is a special tester that uses a chemical that reacts (by changing color) when exposed to CO2. 90% of the time, If an engine has a blown head gasket, CO2 from the exhaust will be forced into the cooling system. By performing a "Block Test" to the cooling system one can determine if there is any trace of CO2 in the coolant. Presence of CO2 indicates a blown head gasket/or possible cracked head.