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Overheated Aluminium Heads

Aluminium Blocks and cylinder heads are a great advancement in automotive engineering. However they do have a serious shortfall and that is when overheated they can become unserviceable. This relates to a very expensive repair job or even a total renewal of the part. I want to just talk about cylinder heads here but the principals would be the same for an alloy block. Cylinder heads are made in a variety of aluminium alloys and the heat treatment applied to the casting varies according to the alloy used. The hardness of the alloy depends on how the head was cast but generally a hardness of oven aged heads might be approx 80-90 Brinell, where a cylinder head that was solution heat treated might have a hardness of 110-120 Brinell. I am not sure if todays manufacturing processes and alloy combinations are producing harder heads compared to the 80's.

When a cylinder head has been overheated such as the coolant has been lost from the engine and the vehicle has been run until it basically stops then I call this rissoled. Overheating is when there is still some coolant left in the radiator and it is boiling which can be heard bubbling away after you pull over. When you rissole an engine the radiator is not hot and can be touched. That is because there is no water in the engine to pump into the radiator, the lack of hot water allows the radiator to cool. Rissoling an engine is very bad because it can cause alloy components to anneal, basically meaning it looses its strength and becomes more ductile. Generally the result from overheating and rissoling will be a blown head gasket, warped cylinder head and/or cracked head. Rissoling can cause major damage to the block and pistons as well. To determine whether the cylinder head can be straightened, it must be hardness tested. If the test indicates a brinell value lower than 70 then you may find that no business will want to attemp to straighten the head. Sometimes it is possible to test the softness of the head yourself. When the head is removed take note of the area where the head bolt washer if fitted comes in connect with the cylinder head surface. If the washer has sunk into the head then this may well indicate the head has gone soft and I am afraid it is junk, but get it checked first before you dump it. A softened cylinder head cannot be re-hardened or repaired with any great success and don't expect that anyone will guarantee their workmanship under these condtions. If you re-use a softened head, the head bolt will eventually sink further into the head which will release the tension of the bolt and next thing you know, you have a blown head gasket and possibly an overheated engine- again. It is at your risk, ask a professional mechanic or engine builder for their advice before you go ahead and take a chance.

Now if the cylinder head proves to still be serviceable, then straightening can be performed. This was a very common practise back in the 70's and 80's. However it will never be perfectly straight again. The head gasket side can be machined to get the surface straight, however if the head is an OHC then the cam tunnels cannot be straightened for perfect alignment. The tradsman my get it close enough to be an acceptable risk to re-use and then machine the head surface to get that side straight. You must then understand that the camshaft will be under slight stress in the cam tunnel because of the small amount of misalignment. In some cases, not many, Re-sizing and line boring the cam tunnels and then fitting bearing shells may be an option, but the cost of such a procedure needs to be weighed againt buying a new or second hand head. In some case the camshaft journals can be built to a larger diameter and the cam tunnels line bored to a biger size. If the head has no camshaft fitted then most of the time the head surface is just ground and the head refitted.

When a head is straightened it is heated from 150deg C to approx 250 deg C depending on the alloy composition and as a matter of interest the head will soften at about 190 - 300C once again depending on the alloy and amount of time it is under this heat. So it is vital for the tradesman that is attempting to straighten your head that they do not exceed the recommended temperature.

It is also vital that the cooling system be checked regularly and maintained to ensure you never overheat and even worse rissole your engine. If you do, the news is all bad, I know, I have come across warped and cracked heads many a time. They are expensive to repair, (if a trademan even wants to attempt it for you) and trying to find second hand heads and blocks from older vehicles is getting considerably harder to the point that it becomes un-economical to repair. Buying another car is sometimes the only option. Check out my article on Cooling systems in the Misc Topics section, this may help you in checking your system.