THE WORM TURNS
While Kawasaki’s Bsa-influenced ohv W1 twin of 1966 used a pivoting lever to transmit clutch cable motion to the pushrod in the gearbox mainshaft, their triples and early dohc fours including the Z650 use a ‘worm’ or ‘quick-thread’ mechanism. It consists of inner and outer components with an interlocked coarse thread. When the inner part is rotated by a lever pulled by the clutch cable, the fixed outer part forces it to move axially and push against the rod. It’s a compact and sturdy system, with smooth action as long as the thread is free from dirt and gunge.
Clutch pushrod is operated by this system, seen on motorcycles since the 1910s