An investigation into factors influencing top-ring oil film thickness at TDC, in a diesel engine, was carried out using capacitance probes and surface thermocouples installed in the liner. Short term and long term trends in the data were observed, and many unexpected features were found. Significant, consistent differences in the film thickness around the cylinder were detected, and the thermocouples showed that for this engine, the top ring unexpectedly cools the wall for a short time near TDC.Due to irreproducibility of the data, two different data acquisition techniques were used. Acquiring consecutive cycles, for a short period of time, provided a “high resolution snapshot” of the process. This method however, was not sufficient to characterize the data, and it was found that taking non-consecutive cycles, over a longer period of time, provided much more knowledge about the long term trends in the data.Normal changes in oil and operational parameters, such as viscosity and peak cylinder pressure (respectively), seemed to have little effect on oil film thickness. However, changes in oil and operating parameters of large enough magnitude were detectable. For example, the compression stroke always showed a thinner film than the exhaust stroke, and when the cylinder walls were wetted by fuel, the film thickness dropped dramatically.The effect of the mechanical condition of the engine was also found to be very significant; an under-sized piston was found to cause much metal-to-metal contact between the ring and liner, whereas a properly fitted piston showed practically none.