The Norton Mercury was a 646 cc (39.4 cu in) air-cooled OHV parallel twin motorcycle made by Norton-Villiers from 1968 to 1970.

There was a large inventory of parts from previous models that would not be used on the Commando, and to use up this stock the Mercury was conceived.

[6] The engine in the Mercury was based on that of the 650SS,[2][7] which itself was a development of Bert Hopwood's 1948 500 cc twin first used on the Model 7 Dominator.

The pre-unit air-cooled OHV vertical twin engine used an alloy head that was fed by a single Amal Concentric carburettor.

[3] Primary drive was by chain to a multiplate wet clutch and was enclosed in a pressed steel chaincase.