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Cylinder Head Crack Repairs
Ask any engine builder who has pulled a cylinder head off a late model engine and he or she will agree: cracks are a serious problem today.
By Larry Carley
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It seems as if many cylinder heads are doomed to crack from the very start because of their lightweight construction and design. Aluminum overhead cam (OHC) heads, in particular, are often found to be warped as well as cracked. But so too are many pushrod cast iron heads.
One of the causes of cracking in cast iron heads is stress created when the valve seats are induction hardened. The concentrated heating process that hardens the valve seats also creates residual stresses in the head that may cause it to crack later - even if the engine has never overheated. The cracks typically form where stresses are highest, which is often between the valve seats.
Some heads, such as Ford 2.9L V6 and Escort 1.6L heads, are notorious for cracking. Others with a known history of cracking problems include Ford 2.3L and 2.5L "HSC" (high swirl combustion) cast iron heads, General Motor's 2.5L "Iron Duke" heads, the GM 250 six-cylinder head with an integral exhaust manifold and 1987 and later Chevy small block V8 "Vortex" heads.
Dodge has also been having cracking problems with the cast iron heads on its late model 318 Magnum engines. Almost all of these heads are found to have cracks between the valve seats when the engines are rebuilt.
Regardless of the engine, cracks are often found between valve seats, in exhaust ports, between the spark plug hole and valve seats, around valve guides, between combustion chambers, even on top of the head.
They're often blamed as the cause of a head failure. But in many instances, the cracks are not the cause of the failure but a symptom of another underlying problem such as overheating, detonation or incorrect installation (wrong torque on head bolts, dirty bolt threads, etc.).
Why Heads Crack
Cracks typically form when a cylinder head undergoes too much thermal stress. Loss of coolant, severe overheating as well as sudden changes in operating temperature from hot to cold can all create the conditions that can cause cracks to form.
Simply put, when metal is heated it expands. Aluminum expands at nearly twice the rate of cast iron, which creates a mismatch in expansion rates on bimetal engines with aluminum heads and cast iron blocks. While the heads are designed to handle a certain amount of normal expansion, elevated operating temperatures can push a head beyond those design limits, causing the metal to deform. This, in turn, may cause cracks to form as the metal cools and contracts.
When overhead cam heads get hot, they often swell and bow up in the middle. This may cause the OHC camshaft to seize or break as well as cracks to form in the underside of the head. Pushrod heads are not as thick as OHC heads so are less vulnerable to this kind of stress and warpage. But even pushrod heads have their limit, and when pushed too far will also warp and crack.
If a crack forms between the cooling jacket and combustion chamber, port or any other external surface on the head, it may leak coolant. If the leak is in a combustion chamber, it may go undetected until the engine overheats from a loss of coolant. Coolant in the combustion chamber can accelerate ring and cylinder wear and is murder on the bearings. If the leak is large enough, it may even hydrolock the cylinder.
Combustion gases entering the cooling system through the crack can also accelerate coolant breakdown and corrosion. Pressure testing the block and heads to see if they hold pressure is a good technique for finding these kind of "hidden" cracks as well as porosity leaks in aluminum castings.
Coolant leaking into the exhaust system through a crack in an exhaust port can expose the catalytic converter and oxygen sensor to silicone contamination. Silicone has the same coating effect on these components as lead, and will eventually ruin the O2 sensor and converter.
External coolant leaks due to cracks are not as common because the outside of the head runs considerably cooler than the combustion chamber and exhaust port surfaces. Even so, cracks sometimes form in these areas that leak coolant.
"Dry" cracks which do not leak coolant may or may not cause problems depending on where they form. Cracks between or around valve seats in an aluminum head may allow the seats to work loose and fall out. Cracks around valve guides may lead to loosening of the valve guides which can damage the valves.
Even if a crack is causing no apparent problems, it should not be ignored because most cracks tend to propagate and spread over time. In other words, little harmless cracks can turn into big nasty cracks, and there's no way to predict when that may happen or how far a crack will grow.
Many cracks are obvious to the naked eye, but others have to be ferreted out by various means. Magnetic particle inspection has long been a popular technique for identifying cracks in cast iron heads. But this technique does not work on aluminum because aluminum is nonmagnetic. Aluminum heads must either be pressure tested and/or sprayed with penetrating dye to find cracks and porosity leaks. Cast iron heads should also be pressure tested to check for hidden cracks in cooling jackets and ports that are not directly observable from the outside.
The important point here is that all heads should be thoroughly inspected for cracks before any machine work is done. Better to find out the head is cracked before you rework the valve guides and seats than afterward. Think of crack detection as your first line of defense against comebacks.
Cracks do not necessarily mean a cylinder head has to be replaced. In fact, many cracked heads that were once thought to be "unrepairable" are now being fixed. Repairing a cracked cylinder head always involves a certain amount of risk, but when done properly is usually much less expensive than replacing a cracked head with a new or used casting. Most small cracks in cast iron as well as aluminum heads can be repaired by pinning. Larger cracks in aluminum heads typically require TIG (Tungsten Inert Gas) welding. Larger cracks in cast iron heads can often be repaired by furnace welding or flame spray welding.
Furnace welding a large cast iron diesel head is often well worth the effort because of the high value of the casting, but it also makes economic sense on many late model cast iron passenger car heads if the head can be repaired for less than the cost of a new or used casting. Many late model heads are fairly expensive and hard to find, so some heads that would have been scrapped because of cracks are now being repaired and returned to service. It all depends on the market value of the head and the cost to replace it with a new or used casting. The higher the value of the head and the higher its replacement cost, the more sense it makes to repair the head rather than replace it.
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