(8) When you hear about an engine tuner “grinding” a cam, it’s not a guy in overalls on a bench grinder. A valve that slaps hard against its seat won’t last. The opening and closing ramp is the first and last part of the duration spec and is very important in making the extremely fast (35 feet per second closing speed on a 14,000 rpm engine) valve reciprocation smooth. Duration is described in number of crank shaft degrees, counted starting at 1mm and ending at 1mm of lift. (7) The shape of the lobe determines how far the valves open (lift) and how long they stay open (duration). The rest of the KTM lineup utilizes finger follower rocker arm designs that come into contact with the cam lobe early, and ultimately allows for a long valve stroke and more lift.

The KTM 450SXF is also a single cam design with a forked rocker arm. Honda’s Unicam design utilizes one cam to actuate a combination of a cam-over-bucket and a forked rocker arm. All Japanese motocross bikes, except Hondas, use a dual-overhead-cam (DOHC) configuration that assigns a separate cam for the exhaust valves and intakes valves. In the era of pushrod engines, the camshaft could be located below the cylinder head. (4) Modern motocross bikes have overhead cams. The KTM 450SXF’s single overhead cam (SOHC) forked rocker arms. The Honda CRF250/450 uses a forked rocker arm to actuate two valves and a cam-over-bucket design for the other two. KTM still uses rocker arm engines (sometimes with very short rocker arms called finger followers and sometimes with forked rocker arms).

(3) If the camshaft isn’t positioned directly over the valves in the head, the motion can be transferred through a rocker arm.

This transfer of rotational motion to linear motion would be far too violent for the valve stem at actual engine speeds, so a bucket (or tappet, in automotive terms) is used to transfer the motion. As the cam spins, the eccentric lobes push the valves open, while valve springs push them closed.

(2) The cam has lobes which are like eccentric bulges that the valves ride against.

How a dual overhead cam works