Cylinder heads have an extremely complex shape withlarge areas of concave casting surfaces. The concavecasting surfaces are often associated with metalexpansion penetration problems or other surfacedefects, e. g. surface shrinkage. The defects cause highproduction costs due to component rejection andincreased fettling time. This report presents aninvestigation of the microstructure in grey cast ironclose to the sand-metal interface affected bymetal penetration in a complex shaped casting.The dominant penetration defect observedin the cylinder heads was expansion penetration.Even pre-solidification penetration and sandcrack defects were observed. The microstructurefound in the non penetrated areas is typicalfor solidification of grey iron cast in sand moulds.