fig1 shows a valve operating mechanism having two camshafts 10 and 12 mounted in an engine having two mutually inclined banks of cylinders , commonly referred to as a ‘ v ’ engine . the engine cylinder block and cylinder heads have all been omitted from the drawings in the interest of clarity , only two intake poppet valves 14 a , 14 b and two exhaust poppet valves 15 a , 15 b being shown in the drawing . the exhaust poppet valves 15 a and 15 b are each operated in a conventional manner by a single cam on the camshaft 12 and their operation need not be described further . the intake valves 14 a and 14 b , on the other hand , are each operated by combining the effect of two cams , one mounted on the camshaft 12 and the other on the camshaft 10 . at least one of the camshafts 10 and 12 is coupled for rotation with the crankshaft by way of a phaser ( not shown in the drawings ) to allow the phase of the camshafts 10 and 12 to be varied relative to one another . as is well known , a phaser is a coupling that rotates the camshaft in synchronism with the crankshaft ( at half the speed in the case of a four - stroke engine ) but allows some degree of rotation of the camshaft relative to the crankshaft to vary to the phase of the cams mounted on the camshaft in relation to the engine operating cycle . several phasers are disclosed in the prior art , suitable examples being hydraulically operated vane - type phasers that can be incorporated in the cogs or pulleys driving the camshafts . each of the intake valves 14 a , 14 b is operated by a rocker 20 mounted in one of the two cylinder heads on a stationary rocker shaft 22 . one end of each rocker 20 acts on the tip of the stem of the associated valve 14 to open and close the valve . the other end of each rocker 20 carries a double ended summation lever 24 which is pivotable relative to the rocker 20 about a pin 26 . in the case of the valve designated 14 a , its summation lever 24 is acted upon at one end by a pushrod 30 a whose other end is in contact with a cam follower 36 a riding on a cam 12 a of the camshaft 12 . the other end of the same summation lever is acted upon by the cam 10 a of the camshaft 10 by way of a cam follower 38 a and a pushrod 28 a . similarly for the valve designated 14 b , its summation lever 24 is acted upon at one end by a pushrod 30 b whose other end is in contact with a cam follower 36 b riding on a cam 12 b of the camshaft 12 and the other end of the same summation lever is acted upon by the cam 10 b of the camshaft 10 by way of a cam follower 38 b and a pushrod 28 b . thus , for each of the valves 14 , the associated summation lever 24 is acted upon at its opposite ends by two pushrods each associated with a cam on a respective one of the two camshafts 10 , 12 . a torsion spring 32 acts on each rocker 20 and one of the cam followers 36 , 38 is of the type shown in fig1 a which includes a hydraulic lash adjuster , the other being a fixed cam follower of the construction shown in fig1 b . an adjustable stop 34 limits the maximum clearance . the cam follower of fig1 a has a main body 50 carrying a roller 52 . a piston 54 reciprocable within the main body is biased by a spring 56 and forms the movable wall of a hydraulic working chamber 58 into which engine oil is admitted through a non - return valve 59 . the cam follower of fig1 b has a body 60 carrying a roller 62 but its piston 64 does not move and for this reason the cam follower is termed a fixed cam follower . the spring 32 and the hydraulic cam follower together ensure that the ends of the pushrods remain at all times in their sockets in the summation lever 24 and in the cam followers . the embodiment shown in fig2 operates on a similar principle to that of fig1 but relies on an alternative summation linkage for combining the two cam follower motions transmitted via the pushrods . the embodiment of fig2 replaces the rocker 20 and the summation lever 24 by a first rocker 20 ′ having a fixed pivot point 22 ′ and a second rocker 24 ′ pivotable relative to the first rocker 22 ′ about a pivot 26 ′ carried by the first rocker 20 ′. one of the pushrods 30 b acts on the free end of the rocker 20 ′, the other pushrod 28 b acts on one end of the rocker 24 ′ and the opposite end of the rocker 24 ′ acts on two intake valve 14 b 1 and 14 b 2 by way of a bridge 40 which overlies the tops of the stems of both valves . the operation of this summation linkage , which is believed to be clear from the foregoing description is further explained within the context of an ohc engine in ep 1426569 . the embodiment of the invention shown in fig3 and 4 uses concentric cams 410 , 412 with followers 436 , 438 and separate pushrods 428 , 430 to operate a summation linkage similar to that shown in fig2 . the use of similar reference numerals in the 400 series is used to avoid repeating the description of the summation linkage . in the embodiments of fig2 and of fig3 and 4 , a torsion spring and a hydraulic cam follower may once again be used to take up free play and to ensure that ends of the pushrods do not come away from their sockets at any time . fig5 and 7 show an embodiment operating in the same manner as that of fig2 and , to avoid repetition , like parts are designated by like reference numerals but in the 100 series . fig5 shows a perspective view of the valve operating mechanism while fig6 shows a section through the mechanism when both cams are on their base circles and the spring 132 has opened a clearance between the valve 114 and the rocker 124 . fig7 shows the same section when one of the cams is at maximum lift , bringing the rocker 124 back into contact with the valve 114 at the point of valve opening . the essential difference in this embodiment of the invention is that the torsion spring 32 has been replaced by a helical compression spring 132 which , as shown in fig6 , biases both the summation lever 124 and the rocker 120 counter - clockwise to open a gap between the summation lever 124 and the valve 114 while maintaining contact with the pushrods 128 and 130 at both ends . in this embodiment also , the rocker 120 has been fitted with a manual adjuster 121 for controlling the clearance in the system . the manual adjuster removes the need for either of the cam followers to be fitted with a hydraulic lash adjuster . in the case of the embodiment of fig8 and 9 , which use like reference numerals in the 200 series to designate like parts , the springs 32 and 132 of the previously described embodiments , which act on the summation linkage , are replaced by a spring 76 arranged in one of the cam followers 238 , which is constructed in the manner shown in fig1 c and 10 d . the other cam follower 236 is of the same fixed design as used in the previously described embodiments and shown in fig1 b . the sprung cam follower shown in fig1 c and 10 d comprises a body 70 carrying a follower roller 72 . a piston 74 slidable in the main body 70 is biased by a spring 76 so that the cam follower can be extended , as shown in fig1 c or contracted , as shown in fig1 d . in this case , the summation lever 224 remains in contact with the valve stem 214 at all times and the clearance “ c ” in the system appears within between the main body 70 and the piston 74 of the cam follower . the embodiment of fig8 and 9 may use an extendable pushrod in place of an extendable cam follower to achieve the same effect . such an extendable pushrod 528528 , which would replace the fixed length push rod 228 is shown in its collapsed state in fig1 a and 11 b . the pushrod is formed in two parts 528 a and 528 b which can slide relative to one another and are maintained in alignment by means of a sleeve 528 c which is permanently attached to the lower part 528 a of the pushrod . a spring 528 d acts in a direction to separate the two parts and extend the pushrod into the position shown in fig1 c and 11 d . it will be clear from the various embodiments described above that the invention does not reside in the design of the summation linkage employed to combine the actions of the two cams but in the fact that the combining of the action of two cams is carried out within the context of a pushrod engine . in operation , a phaser is attached to each of the two cams to allow the phase of the cams to be adjusted relative to the engine crankshaft . by altering the relative phase of two cams acting on the same intake valve it is possible to vary the valve event duration and the valve lift . furthermore , when both cams can be independently phased relative to the crankshaft , it is possible to modify the timing of the valve event with the engine cycle .