A balancing device comprises three rotating balancers mounted within a housing 24 detachably mounted on a reciprocating piston, internal combustion engine E. The balancers are driven by the crankshaft 4 of the engine and the device generates a balancing force F b chosen to cancel a second harmonic component of a vibrating force Fr produced by the engine. The vibrating force Fr lies on a vibration line S, and the balancing force F b is spaced a distance l therefrom, whereby a oscillatory moment M is generated. The balancing device also generates a balancing moment -M, chosen to cancel the oscillatory moment M.