In the stock head gasket of the C20XE (2.0 16V) is a weak point.
This weak point is the fact that the reenforcement bridges are missing between the cylinders.
When tuning these engines this can cause some problems.
This weak point is easy to resolve by changing the the headgasket to one of a C20LET (2.0 16V turbo). This gasket has the reenforcement brigde.
The weak point is easy to see on the following pictures.
C20XE head gasket.
Here it is easy to see the weak point and the results of it.
C20LET head gasket.
Here the reenforcement bridge is in place.