Joined
·
1,184 Posts
The number one mistake to avoid when replacing a blown head gasket is to simply install a new gasket without checking or repairing anything else. In many instances, a blown head gasket is not the real problem but a symptom of some other underlying condition such as a hot spot, overheating or detonation. If the underlying problem is not identified and corrected, the new gasket will likely suffer the same fate as its predecessor.
Always inspect the cylinder head for cracks or other problems when it is removed, especially if the engine overheated. Aluminum overhead cam heads are much more likely to warp and crack than cast iron heads when an engine gets too hot. If an OHC will not turn once the followers have been removed, the head is probably warped and will have to be straightened and/or align bored.
Cracks are not always visible to the naked eye. Porosity leaks in aluminum heads may not show up unless the cooling system is under pressure. To minimize the risk of a repeat gasket failure, cast iron heads should be Magnafluxed (magnetic crack detection) to check for cracks. Penetrating dye will reveal cracks in aluminum. Pressure testing is also an excellent method of detecting internal cracks and porosity leaks in both cast iron and aluminum.
The cylinder head and block should also be checked for flatness before the new head gasket is installed. Flatness specs vary depending on the application, but on most pushrod engines with cast iron heads, up to .003 inch (0.076 mm) out-of-flat lengthwise in V6 heads, .004 inch (0.102 mm) in four cylinder or V8 heads, and .006 inch (0.152 mm) in straight six cylinder heads is considered acceptable. Most aluminum heads, on the other hand, should have no more than .002 inch (.05 mm) out-of-flat in any direction. Aluminum OHC heads should be checked for flatness in two places: across the face of the head with a straight edge, and down the OHC cam bores with a straightedge or bar.
If an OHC aluminum head requires resurfacing, the amount of metal that can be safely removed is usually quite limited. If a head has been resurfaced and the installed height is too short, cam timing can be adversely affected. Too much compression may also create detonation problems. To compensate, a copper or steel shim may be used with the head gasket to raise the head and restore proper head height (if available). Otherwise, the head may have to be replaced.
Surface finish is also very important. As a rule, most push rod engines with cast iron heads can handle a surface finish of anything between 54 to 113 microinches RA (60 to 125 RMS). But many aluminum OHC heads require a smoother finish to seal properly. Many late model Japanese engines have "multi-layered steel" (MLS) head gaskets that require a very smooth finish of 7 to 15 RA! Such heads should not be resurfaced unless the head is warped or the surface is damaged.
Finally, if the engine has torque-to-yield (TTY) head bolts, replace them. Reusing TTY bolts is risky because you have no way of knowing how far they are been stretched. Also, make sure you have the latest head bolt torque specs. Vehicle manufacturers often revise their original head bolt torque specs to correct problems that have arisen in the field. The new specs can be found in technical service bulletins (TSBs) from the manufacturers.
Engine Rebuild Tutorial Links:
Always inspect the cylinder head for cracks or other problems when it is removed, especially if the engine overheated. Aluminum overhead cam heads are much more likely to warp and crack than cast iron heads when an engine gets too hot. If an OHC will not turn once the followers have been removed, the head is probably warped and will have to be straightened and/or align bored.
Cracks are not always visible to the naked eye. Porosity leaks in aluminum heads may not show up unless the cooling system is under pressure. To minimize the risk of a repeat gasket failure, cast iron heads should be Magnafluxed (magnetic crack detection) to check for cracks. Penetrating dye will reveal cracks in aluminum. Pressure testing is also an excellent method of detecting internal cracks and porosity leaks in both cast iron and aluminum.
The cylinder head and block should also be checked for flatness before the new head gasket is installed. Flatness specs vary depending on the application, but on most pushrod engines with cast iron heads, up to .003 inch (0.076 mm) out-of-flat lengthwise in V6 heads, .004 inch (0.102 mm) in four cylinder or V8 heads, and .006 inch (0.152 mm) in straight six cylinder heads is considered acceptable. Most aluminum heads, on the other hand, should have no more than .002 inch (.05 mm) out-of-flat in any direction. Aluminum OHC heads should be checked for flatness in two places: across the face of the head with a straight edge, and down the OHC cam bores with a straightedge or bar.
If an OHC aluminum head requires resurfacing, the amount of metal that can be safely removed is usually quite limited. If a head has been resurfaced and the installed height is too short, cam timing can be adversely affected. Too much compression may also create detonation problems. To compensate, a copper or steel shim may be used with the head gasket to raise the head and restore proper head height (if available). Otherwise, the head may have to be replaced.
Surface finish is also very important. As a rule, most push rod engines with cast iron heads can handle a surface finish of anything between 54 to 113 microinches RA (60 to 125 RMS). But many aluminum OHC heads require a smoother finish to seal properly. Many late model Japanese engines have "multi-layered steel" (MLS) head gaskets that require a very smooth finish of 7 to 15 RA! Such heads should not be resurfaced unless the head is warped or the surface is damaged.
Finally, if the engine has torque-to-yield (TTY) head bolts, replace them. Reusing TTY bolts is risky because you have no way of knowing how far they are been stretched. Also, make sure you have the latest head bolt torque specs. Vehicle manufacturers often revise their original head bolt torque specs to correct problems that have arisen in the field. The new specs can be found in technical service bulletins (TSBs) from the manufacturers.
Engine Rebuild Tutorial Links:
