 In situations where a complete "C20XE" is still fitted to a vehicle, the presence of Welch plugs (or lack of) has proven to be the sole means of differentiating between "GM" and "Coscast" heads.A reinforced version of the "GM" head became available in the later years of the "C20XE"; however, these reinforcements meant that it had smaller inlet/exhaust channels than the other two.Since an engine's oil circulates at much higher pressures than its coolant, oil in a porous head has a tendency to gradually seep into the coolant galleries.