Quote:
Originally posted by Revracing
It is not as common nowadays but in the "old days" iron castings such as engine blocks were deliberately "weathered" for several years prior to machining for use. This was to allow the material to "age harden".
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You're confusing two terms here. Cast iron was left to age, not to age harden - age hardening is a characteristic of aluminium alloys. The rate at which a casting cools is related to cross-sectional area, so thinner parts of the casting cool more quickly than thicker parts; this differential cooling tends to set up streses in the casting, which can lead to distortion when it is machined. 'Weathering' a casting subjects it to numerous thermal cycles over a long period, which relieves the casting stresses. BMW found that by using old blocks for their racing engines they were getting the same effect.