562For thermal strength

While, ideally, a full thermal stress analysis would be carried out, it is found that, in practice, relative freedom from thermal problems will be experienced if certain empirical rules are followed. In particular, with conventional cast iron cylinder heads manufactured from good quality unalloyed flake graphite grey cast iron, over a wide range of engine sizes, the gas side temperature of the valve bridge material should be kept below 400°C and preferably below 380°C. With temperatures much above 400°C cracking in service under cycled load conditions will be severe and at even higher temperatures will be catastrophic.

It should be emphasized that these temperature limits may not apply to engines fitted with integral cylinder heads, which will have less restraint due to the absence of the cold outer areas of the cylinder head deck. Here, as with cast iron pistons, somewhat higher temperatures may be acceptable. For freedom from thermal cracking, a peak temperature for the crowns of aluminium alloy pistons of 380°C has been quoted by Law18 but the author would prefer to see a value of 30°C lower than this.

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