When specifying aluminum alloys, there are usually two important numbers used. As an example, C355-T61 is the alloy Dart uses for all of our cast aluminum blocks and cylinder heads. The C355 number refers to the specific elements contained in the alloy, and the T61 refers to the heat treatment used to condition the metal after it is cast to shape.
Heat treating the aluminum improves its tensile strength and fatigue resistence; important factors in high performance engine components. Our research has shown that the T61 condition is considerably superior to the more commonly used T6, so Dart has adopted the T61 heat treat exclusively for our blocks and heads.
The T-61 treatment invloves three steps, the first of which is a solution heat treating process. During this step the metal is heated to a precise temperature and held at that temperature for a very specific length of time. This allows the individual elements present in the alloy to form a solid solution. The alloy is then cooled rapidly by immersion in a quench tank in order to preserve this quality in the final metal. The final step is referred to as “aging” the metal. During this phase the temperature of the metal is controlled carefully in order to stabilize the solution formed during the first step of the procedure. The end result is an aluminum alloy that has excellent strength and fatigue resistance.