C20XE - 'COSCAST' (Early) and GM (Later) Head Differences

'COSCAST' (Early) and GM (Later) Head Differences

The C20XE's cylinder heads came in two notable forms. The original version, which first appeared in the Astra GTE, Calibra and Vauxhall Cavalier/Opel Vectra between 1988 and 1991, was cast by Cosworth and was marketed under the name 'Coscast'. In 1991, the Coscast items were replaced by GM manufactured heads, which first appeared in the Astra GSi. One of the most prominently recognised qualities of the 'COSCAST' head is its inherent lack of porosity; this was achieved by pumping the liquid metal into the mold rather than pouring it, hence, minimizing the presence of tiny air bubbles that usually form during the standard casting process. The Coscast head can be identified by a 'Coscast' logo which is stamped under the 3rd exhaust port and a ridge on the head under the distributor. The GM head was manufactured by Kolben-Schmidt and featured a slightly different oil/water gallery design. These design changes required that a pair of welsch plugs be pressed in at either end of the head. In situations where a complete C20XE is still fitted to a vehicle, the presence of welsch 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. A typical symptom of a porous head is usually a 'mayonnaise'-like substance forming somewhere inside the cooling system (usually, this can be found residing on the coolant reservoir cap). However, depending on the degree of porosity, symptoms of a porous head have a tendency to vary. Many C20XE operators have described the symptom as a curry-like residue or in more severe cases, a thick brown sludge which may overcome the entire cooling system. In such instances, engine oil will readily react with the sulfur in rubber components, hence quickly degrading coolant pipes and hoses to the point of failure. During the porous head debacle, GM faced bankruptcy - therefore dealers failed to recall affected models. Due in part, to the engine's immense prominence and demand, many businesses now specialise in the repair of porous GM C20XE/LET heads - by either sleeving the affected gallery or by injecting a polymer based substance into the porous region. Reportedly, a small number of total GM C20XE cylinder heads ever exhibited significant symptoms of porosity.

The early engines used round tooth cambelts while the later used square (with a plastic pre-tensioner). There are also subtle differences between the crankshaft, and visible difference in the pattern of the SFi airbox.