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Torquing Heads

The applied torque of the head bolts depends as much on the metallurgy of the head as anything else.

By Nigel Tait*

Click on pics to view larger images


This article was first published in 2007.

Most of the cylinder heads in vehicles today are made from some form of aluminium alloy. Different alloys and their heat treatments are designed to perform in different environments; consequently there are no uniform rules governing their characteristics. To understand the way aluminium and its alloys behave, we have to firstly understand and appreciate some fundamentals.

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In general terms, pure aluminium has little value as an engineering material. It is simply too soft, it has poor tensile and compressive strength and is subject to "creep" (if a plate of pure aluminium were to be stood on its edge, over time the bottom would be thicker than the top).

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Creep occurs due to the fact that there are no "keys" at the grain boundaries to stop them sliding (dislocating) over one another.

Typical alloying elements:

  • Silicon Si Magnesium Mg
  • Copper Cu Manganese Mn
  • Iron Fe

These alloying elements (and there are others) are used in varying proportions and mixes to produce aluminium alloys that display very specific properties, eg alloys can be designed to improve hardness, wear resistance, resistance to heat, dimensional stability, shear strength, castability, machineability and weldability, etc.

Generally, aluminium cylinder heads are cast (gravity or die cast). They are complex in section, and over the years as designers seek weight reduction and improved engine performance, there has been a gradual reduction in mass.

An alloy head that was produced, say, five years ago, is likely to display different mechanical characteristics today than those it displayed when first made, depending on alloy, age, working conditions, mass and section. As described above, alloying elements are used to impart specific characteristics to the material depending upon the application.

When copper is added to aluminium (4% max) and subsequently heated to a temperature of 480-590 degrees C (depending on the alloy) and allowed to soak for between one minute and five hours, copper will go into solution with the aluminium (a bit like sugar dissolving in water). This is called a "solid solution".

When the material is quenched, very small areas of a copper-rich precipitate (CuAl2) are formed. These are evenly distributed throughout the structure and are so small that they have no influence on the mechanical properties of the material. Yet if this material was left at room temperature for a period of time (years) these particles (precipitates) will grow and the material will naturally "age harden".

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These precipitates offer resistance to slip (dislocation), however, if they grow too large, the dislocation path will simply go around them and the material will soften. The age-hardening process can be accelerated by a process known as Precipitation Hardening. The alloy is heated to between 115 and 190 Degrees C for a period of between 5 and 48 hours after solution treatment (soak time and temperature will be dependant on mass and previous history, for example, if the alloy had been cold worked).

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Time and temperature will determine the precipitate size - too high or too long and they will become too large and the material will not be as hard as desired. This graph shows the hardness/time curve for a typical alloy. It is important to note that many aluminium alloys will not only age harden, but age soften also - the rate is dependant on alloy and use. The softening process, however, will accelerate dramatically if the material is subjected to heat.

So.....

  1. Always test the hardness of an alloy head prior to reconditioning;
  2. Always test head hardness if the head has been "cooked";
  3. Be careful when applying general or local heat to an alloy head - you may be softening it.

Using an alloy head with a Brinnell hardness number of less than 60 is asking for trouble. Why? Because to keep the integrity of the seal between head and block, it is essential that bolt torque recommendations are adhered to. This includes torque sequence, making sure the bolts are clean and have not been stretched or deformed, the block is clean and the threads are clean and free of debris. Also make sure the underside of the bolt heads are lubricated and also the threads, use engine oil.

Now if you do all of these things correctly and you have a soft head, guess what? You've got it - the seal will fail, not because of the gasket, but because of a loss of clamp load due to the reduction in yield strength in the alloy.

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Note in the second diagram the load exerted by the bolt has been absorbed in deforming (shearing) the soft alloy, thus reducing the clamp load available to clamp the head gasket to the block, leading to failure. It really does not matter how many times a head is re-torqued, all that happens is the bolts are pushed further into the head.

In general you can re-heat treat some aluminium alloys, but it is a specialist process. My advice is - if you don't know the metallurgy, leave it alone.

So some simple reminders:

  • Always check the hardness of the head
  • Always make sure surfaces are clean, straight and flat.
  • Dispose of your old gaskets sensibly - they may contain asbestos.
  • Always use correct bolt torques and torque sequence. It's always a good idea to have your torque wrench properly calibrated.
  • Always lubricate bolt threads and under bolt heads; avoid using stretched bolts.
  • When attempting to straighten or weld an alloy head, be careful when applying heat, you may be softening it.

*Nigel Tait is Chief Engineer of ACL, a major Australian manufacturer of engine parts.

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