Abstract: An internal combustion engine designed to use gas as fuel. Exhaust valve (2) operation is effected via: (i) a push rod of two parts, the respective ends of which are pivoted to an intermediate connector sliding in a bore in the cylinder head and (ii) two rocker arms (11 and 16). Fulcrum point (18) of rocker (16) may be adjusted to reduce exhaust valve lift during part throttle operation or alternatively exhaust braking during overrun conditions may be achieved by cracking open valve (2) on the compression stroke: this is effected by inward movement of wedge (21) or by a cam raising the end of rocker (16). Modifications of exhaust manifold, rocker shaft mounting, etc are also described.