The oil film thickness for combustion engines describes the gap and thus the quantity of oil between piston and cylinder wall. Thus it is sometimes a determining factor for smooth operation and durability. As the oil film can only be integrated and really measured in the firing condition, it is extremely difficult to manufacture sensors for these environmental conditions and then also find space for them. Specially miniaturised eddy current sensors from Micro-Epsilon are capable for this. The smallest with only 2.4 mm external diameter is integrated directly into the cylinder wall and ground to its shape. There, it measures the distance from sensor to piston or the space available for the engine oil for lubrication in every stroke.