my c20let was burning some coolant at first it was nearly unnoticeable but by time it became like a smoke machine!. When I removed the spark plug there was a little water on piston 3 and therefore had to remove the head. Also found water in the inlet manifold port 3 and exhaust manifold. the gasket had a small visible leak near piston 3 and also found out that between all 4 combustion chambers the head had sunk in by about 1mm. the engine never overheated or anything, the head is skimmed to a point where the inlet valves are level with the head's face, had to skim it due to pitting before installing. also the block has steel rings around the bores so a standard gasket is used.
The only thing I can think of is that the head was to thin and therefore it wasn't able to take the stress of the tightening torque
Any ideas, the picture is of the head before fitting just marked from where it sunk in, seems that in those points there are the water ways.