cam follower 41 for sliding vertical movement and the housing 53 in connecting rod 57 contains cam follower 43 for sliding vertical movement the pins 51 each receive resiliently flexible stops in the form of o rings 58 over the exposed ends of the pins 51 connecting rods 55 and 57 the o rings 58 locating between the cam followers 41 43 and the connecting rods 55 57 respectively to prevent metal to metal contact at the ends of the travel of the connecting rods 55 57 the pins 51 are received in bushes 59 press fit into the connecting rods 55 and 57 at the top and bottom of the housing 53 cam follower 41 is biased to a central position within the housing 53 in connecting rod 55 by an upper spring 60 and a lower spring 61 received in recesses 63 in the connecting rod 55 and circular recesses 65 in the cam follower 41 similarly cam follower 43 is biased to a central position within the housing 53 in connecting rod 57 by an upper spring 60 and a lower spring 61 received in recesses 63 in the connecting rod 55 and circular recesses 65 in the cam follower 41 the assemblies of cam follower 41 and connecting rod 55 on the one hand and cam follower 43 and connecting rod 57 on the other hand are mirror images of each other but are otherwise identical the.