Automotive Engine Crack Detection

Once engine parts have been cleaned, everything should be carefully inspected. This inspection should include a check for cracks, especially in the engine block and cylinder head. If cracks in the metal casting are discovered during the inspection, they should be repaired or the part replaced.

Cracks in metal castings are the result of stress or strain in a section of the casting. This stress or strain finds a weak point in that section of the casting and causes it to distort or separate at that point ( +++0-35). Such stresses or strains in castings can develop from the following:

¦ Pressure or temperature changes during the casting procedure may cause internal material structure defects, inclusion, or voids.

¦ Fatigue may result from fluctuating or repeated stress cycles. It might begin as small cracks and progress to larger ones under the action of the stress.

¦ Flexing of the metal may result due to its lack of rigidity.

¦ Impact damage may occur by a solid, hard object hitting a component.

¦ Constant impacting of a valve against a hardened seat may produce vibrations that could possibly lead to fracturing a thin-walled casting.

¦ Chilling of a hot engine by a sudden rush of cold water or air over the surface may happen.

¦ Excessive overheating is possible due to improper operation of an engine system.

Methods:

Cracks can be found by visual inspection; however, many are not easily seen. Therefore, engine rebuilders use special equipment to detect cracks, especially if there is reason to suspect a crack.

Pressure Checks--Pressure checking a cylinder block or head is done in the same way a tire is checked for leaks. All of the coolant passages are plugged with rubber stoppers or gaskets. Compressed air is injected into a water jacket and the point of air entry is sealed.

The block or head is then submerged into water.

Bubbles will form in the water if there is a leak. The spot where the bubbles are forming is the location of the leak.

Magnetic Checks--Magnetic particle inspection (MPI) uses a permanent or electromagnet to create a magnetic field in a cast iron unit. When the legs of the detector tool are placed on the metal, the magnetic field travels through the metal. Iron filings are sprinkled in the surface to detect a secondary magnetic field resulting from a crack. Because the secondary magnetic field won’t form if the crack is in the same direction as the magnet, the magnet must be rotated and the metal checked in both directions.

+++0-39 Cracks appear as red lines when a dye penetrant is used. LOCK-N-STITCH Inc.

Dye Penetrant--Another common way to detect cracks is by using three separate chemicals: penetrant, cleaner, and developer. The part to be checked must be clean and dry. This check must be done according to the following sequence:

1. Spray or brush the penetrant onto the surface.

2. Wait 5 minutes.

3. Spray the cleaner onto a clean cloth.

4. Wipe off all visible penetrant.

5. Spray the developer on the tested area.

6. Wait until the developer is totally dry.

7. Inspect the area. Cracks will appear as a red line.

Crack Repair--If a crack is found, a decision must be made as to whether the part should be replaced or repaired. This decision should be based on the cost of repair as well as any other repair that the part may need. Further inspection of the cylinder block and head and the service and repair procedures for each are covered.

Next: Example Study / Terms / Summary / QUIZ / ASE-Style Review Questions

Prev.: Cleaning Engine Parts