Measurement of oil film thickness between piston rings and cylinder liner was conducted on a single cylinder version of a Cummins L-10 diesel engine using a laser induced fluorescence technique. The oil was illuminated with blue laser light (λ=442 nm) that causes the oil to fluoresce at a longer characteristic wavelength (λ=500 nm). This fluorescent light intensity is proportional to oil film thickness. A single fiber (50 μm core) was used to carry the laser light to the oil and to return the fluorescent light back to a photomultiplier tube.The paper presents results of oil film thickness measured under motored engine conditions for varying engine speeds, intake boost pressures and cylinder liner temperatures. The following conclusions were drawn from the experimental data. Oil film thickness increases with engine speed showing hydrodynamic lubrication. An increase in liner temperature decreases oil film thickness. The variation in intake boost pressure did not produce any regular pattern, but at 1900 rpm the oil films remained constant for the top compression ring. Statistical analysis showed cycle-to-cycle variations in the oil film under the top ring of +/- 2 to 3 microns.The paper also presents results of oil film thickness calculated from theory. Trends predicted by theory for speed, temperature, and pressure correlated well with measurements. The most consistent differences were observed in comparing after TDC predictions and measurements of thickness.