A number of direct vibration measurements in a single cylinder diesel engine have been made and are presented in the form of films and spectra. The effect of cooling water temperature on vibration was investigated under controlled engine conditions by means of spectrum analysis. It was shown that a reduction in the water temperature increases the mean vibration in the frequency range 500 to 2000 c/s while the reverse is true in the 5500 c/s region. This effect was considered sufficient to require a control of temperature when conducting investigations into the effect of other engine variables on vibration. Piston slap was investigated by motoring the engine and removing certain sources of the vibration from the engine. As a result piston slap was identified and the instants of slap were determined. These were compared with the instants of slap calculated from a theoretical analysis of the dynamics of the moving parts of the engine. The effect of piston slap was shown to be important in the 2000 to 4000 c/s range and to be the predominant source of vibration in the cylinder block.