All About Cylinder Head Hardness

With the advancement in engine technology more components than ever before are being manufactured out of aluminium.  The 70’s and 80’s seen the introduction of aluminium cylinder heads into mainstream family cars and now almost all new engines use aluminium heads and in many cases an aluminium engine block as well.

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Southside Cylinder Heads
Digital Hardness Tester

Although both strong and light, aluminium is also a relatively soft metal that needs to be alloyed with another metal in order to have enough hardness to be used in an engine.  Typical alloying compounds are silicon manganese, copper manganese and iron.

When we talk about cylinder head hardness it is basically the ability of the component to resist being permanently deformed when a load is applied. Testing the hardness of your cylinder head is a vital part of the reconditioning process and the most widely used hardness test in the automotive field is Brinell Scale.

Depending on the manufacturing process and alloying metals used, cylinder heads will have different hardness levels from new, so for instance a new Mazda 6 cylinder head may be 125hb but a Toyota Corolla head may be only 110hb.  You would expect a new cylinder head to be in the vicinity of 100-130hb.

The only accurate way to test your cylinder head hardness is with a calibrated digital tester.  Our testing is conducted in 3 spots on the gasket face of each cylinder head and the average result is recorded.  Industry standard minimum hardness is 65hb and any heads with a reading below this level are unsuitable for reconditioning and must be replaced.  Unfortunately many workshops still use the outdated and unreliable manual “ball tester” that relies on a ball bearing in a tube to bounce above a line.  This type of tester is relatively cheap but has proven to be inaccurate in determining a cylinder heads suitability for reconditioning.

What causes the hardness level to deteriorate?

The single biggest factor that effects your cylinder head hardness is excessive heat.  Most modern engines operate at a temperature of approximately 88°c – 105°c.  When your engine overheats and exceeds its normal operating range, the increased heat puts major stresses on the aluminium cylinder head and starts to deteriorate the hardness value of the head.

Once the temperature of the aluminium reaches about 200°c – 250°c permanent softening of the alloy begins and the hotter and longer your engine is exposed to these conditions the more damage is done.  To minimise the damaging effects of overheating be sure to switch off your engine at the first signs of overheating and have your vehicle checked.

What will happen if I use a soft head?

Using a cylinder head that is soft (tested at 65hb or less) greatly increases the likelihood of premature head gasket failure as it has lost its structural integrity and ability to withstand load and stresses that occur during engine operation.  As well as gasket failure a soft head may also be more susceptible to warping and cracking as the weakened component continues to be subjected to the high loads and stresses of the combustion engine.

A key sign to look out for is head bolt recess or indentation caused when the load applied by the head bolts is too great for the soft aluminium and the bolts will start to sink into the head. These bolts pulling through the aluminium will prevent the correct load being applied to the head gasket and failure will be imminent.

If you have overheated your engine and suspect the cylinder head hardness may be affected, or you just want to be on the safe side, bring your head in to us and we can do a hardness test while you wait (in most cases), and best of all its free!

For more information on cylinder head hardness check out: http://www.aagaskets.com.au/PDF/CylinderHeadResearchInvestigation.pdf