The exhaust passage (21) of the cylinder head (12) of the engine (10) is provided with an inner duct (24) which surrounds the stem (16) of the exhaust valve (18). The outer end of the duct (24) is clamped against the cylinder head (12) by the exhaust manifold (32) so that there is an annular gap between duct (24) and exhaust passage (21). This gap is open to the exhaust gases passing through the valve so that a jacket of hot stagnant gas surrounds the duct (24). This jacket of hot gas ensures complete combustion of any unburnt hydro-carbons before they are discharged to the atmosphere. In an alternative design the duct (24) is provided with an additional jacket of thermally insulating material.
