Enlarged Oil Return Passages

The 96-98 Cobra heads (not sure about 99+ heads) have oil return passages with a 15mm OD cast-in portion adjacent to the head gasket surface and a 13mm drilled portion extending between the cast-in portion and the top of the head. Ford did provide a nicely contoured surface profile (e.g., funnel like) leading down the head surface to each return passage. Unfortunately, it terminates at a flat surface where the 13mm drilled portion of the oil passages are formed. As a result, each OE oil return passage has a flat ledge and a sharp 90-degree edges at their entrance. (i.e., formed by drilling/boring into a flat surface). The flat ledge, 90-degree edge and relatively small passage diameters (i.e., the 13mm drilled portion) adversly affect flow into the oil return passages. It seems as though Ford intentionally uses a relatively small drill (i.e., smaller than 15mm) to accommodate manufacturing tolerances for ensuring that the drilled portion of the passage falls within the perimeter of the cast-in portion of the passage. As the position of the revised holes can be located to a much higher degree of precision in a custom forming operation, the exterior wall of the revised hole can be formed to be completely concentric with the 15mm OD cast-in portion of the oil return passage. The thinnest wall section surrounding the passages is nominally .300", which is measured at the 15mm cast-in portion of the return passage adjacent to the head gasket surface. So, properly opening up the drilled portion of the passage to 15mm will only result in the same wall thickness at the drilled portion as already exists at the cast-in portion of the oil return passage. There are no cavities adjacent to the drilled portion of the oil return passages, only adjacent to the cast-in portion of the oil return passages. Below are a few pictures of the enlarged passages and some illustrations showing the before/after oil passage configurations.

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