The paper presents the results of the bench tests to measure mechanical vibrations of a new aircraft opposed piston engine with reciprocating pistons. The PLZ-100 engine is a three-cylinder, six-piston, two-shaft drive unit with a two-stroke diesel cycle. This type of engine is dedicated for powering light aircraft, e.g. autogyros. The tests were carried out on a test bench at the Lublin University of Technology. The engine was loaded with constant torque, for several fixed values of rotational speed of a crankshaft. The angle of the start of diesel injection was changed for each of the rotational speeds. The mechanical vibrations that accompanied the operation of this drive unit were recorded with three measurement transducers and a National Instruments conditioning system. Each of the transducers was mounted on a different axis of the engine. The signals were analyzed from their courses with the DIAdem software. The results were the courses of effective speed and vibration acceleration to conduct a vibration-acoustic evaluation of the PLZ-100, detect and prevent various types of defects or failures.