When you top up the water level, only use BMW or MINI coolant. This client diligently topped up with an after-market coolant, whenever it was required. However, because the ingredients in the coolant had a chemical reaction with the metals in the engine, the block corroded. Even replacing the cylinder head gasket cannot prevent the water from seeping through onto the pistons, when it has reached this stage. It is best to check the coolant levels first thing in the morning, when the engine is off and cold.