Bristol produced a straight-six until 1961, based on a BMW design, that was also used by many small automakers such as AC Cars and Frazer Nash. The Bristol engine had a distinctive valvegear arrangement. The hemispherical combustion chambers meant that the inlet and exhaust valves had to be mounted on separate rocker shafts, giving the engine the appearance of having double overhead camshafts. In fact the engine used a variation on the overhead valve layout, with a camshaft mounted in the engine block driving the inlet rocker arms via traditional vertical pushrods. The inlet rocker shaft had additional cams which drove horizontal pushrods acting on the exhaust rockers on the opposite shaft. The result was an engine with very efficient gas flow due to the placement of the valves and the shape of the combustion chamber but the numerous interfaces between cams, pushrods, rockers and valves required dedicated maintenance to maintain and the engine made a lot of noise when the clearances were allowed to be become excessive.