In most high-speed engines of the OHC (over head camshaft) type, a number of gears are engaged with the crankshaft gear to drive the valve gear mechanism, the fuel injection pump, and other accessories such as the oil pump or power steering system.Each of the gears usually has a significant mass and moment of inertia. We investigated the influence of the masses and moments of inertia of the gears on three-dimensional vibrations of the crankshaft system. A four-cylinder in-line diesel engine (4 - ϕ115 × 110, 140ps / N = 3200pm) was used for a series of experiments and analyses. The three-dimensional vibrations of the crankshaft system were measured by the hammering tests and the shaker tests. We calculated the vibration behavior by applying an idealized simple modeling for the crankshaft system and the gear train.From the series of experiments and analyses, we found that the gear train should be engaged to the crankshaft system at the crankshaft rear end (flywheel side), as close as possible to the nodal point of the crankshaft torsional vibrations, so that the influence of the moments of inertia of the gears on the crankshaft vibrations could be minimized.