Honda Unicam

Honda's Unicam single-overhead-camshaft (SOHC) four-valve cylinder head first appeared on the 2002 CRF450R. The benefits of the Unicam design are a powerful, yet shorter engine. The liquid-cooled head incorporates a carburized single camshaft that directly actuates two intake valves (the CRF250/450X series utilize titanium intake valves). In the CRF250R/CRF450X models, the camshaft's single exhaust lobe actuates two steel exhaust valves via a forked, low-friction roller rocker arm. The Unicam system in the CRF450R/150R is slightly different, with the two exhaust valves operating through two separate low-friction roller rocker arms. The SOHC configuration contributes to a compact design that saves weight over a comparable dual-overhead-camshaft (DOHC) motor and also permits a narrow included valve angle. This, in turn, flattens the combustion chamber to facilitate free flame propagation, allowing a high compression ratio. A roller bearing on the rocker arms reduces friction and therefore wear, allowing the cam lobes to be narrower-and lighter-than conventional designs. Since less space is taken up in the cylinder head, the camshaft sits lower in the head for a more compact engine and a lower center of gravity.