Z650 ENGINE STRIP
It may look like a smaller version of earlier Z fours, but look inside and it’s very different
Kawasaki’s rather excellent inline four spread over the workshop floor in lots of easy-to-lose parts
Kawasaki took the double-overhead-cam inline fourcylinder engine from the realm of Grand Prix racing and put it within reach of the street rider. The breakthrough 900cc Z1 four of 1973 was a 130mph-plus sensation, but there was a drawback – the King of Motorcycles and its Z900 and Z1000 variants were bulky machines with imprecise handling.
Gyoichi ‘Ben’ Inamura, who had overseen the design of the Z1 engine, refined the dohc four concept to create a more compact unit that was cheaper to make. The result was the 652cc Z650 (or KZ650 in the US and other markets) launched in late 1976 to offer riders an agile and versatile four with sharp acceleration and a near-120mph maximum. Kawasaki’s claim that its 650 possessed 750 performance was backed up when factory-prepared KZ650S set 750cc-class endurance records at Daytona in 1977, including 100 miles at 130mph. More proof of reliability came from American Rich Willet’s 9550-mile journey to break the record for riding around the coasts of Australia.
Although the 652cc engine looks like a smaller version of the earlier Z fours externally, it differs considerably inside. Instead of a built-up crankshaft with roller bearings, the smaller engine has a one-piece shaft with plain shell main and big-end bearings. Primary drive is also different with an inverted-tooth Hy-vo chain to an intermediate shaft, rather than direct gears. Few changes to the design were made during a production run that lasted until 1983. The most significant were a self-adjusting cam chain tensioner in 1979, the introduction of a quieter Hy-vo cam chain in 1980, CDI ignition in 1981 and four-ring pistons in 1983.
The engine here dates from 1979. The crankcase has five cast-in housings for the main bearing shells and four more for gearbox
shaft bearings, all on the axis of the horizontal split. A cast-alloy oil collection sump is screwed to the underside of the crankcase.
The crankshaft’s crankpins are disposed at 180° intervals; the outer two are at top dead centre when the inner two are at bottom dead centre. The integral sprocket for the cam chain is located centrally, with teeth for the drive output chain cut in the crank metal to the left of it.
A keyed taper at the left-side end of the shaft carries the alternator’s magnetic rotor, which surrounds a 12-coil stator. The other end of the shaft drives the ignition points unit, which has four contact breakers and an auto-advancing mechanism.
The conrods’ split big-ends are held together by bolts with heads shaped to lock against the rods as the nuts are tightened. The gudgeon pins bear directly on the rod metal in the small-ends, supporting conventional three-ring pistons that have their skirts extended at the front and rear where rubbing is heaviest. The cylinder block is in one piece, with bore liners that extend down into the upper crankcase and a central cavity for the cam chain. It mounts onto the 12 studs with nuts on their tops that hold down the one-piece cylinder head, with a gasket at the head-to-barrel joint. There are also two small bolts holding the head to the barrel at the front and rear of the cam chain tunnel.
The pistons have gently-domed crowns with indents to clear the valves; spark plugs are off-centre of the evenly-concave combustion chambers, tilted for accessibility. The cylinder head casting incorporates the lower half-journals for the camshafts, four for each shaft. Each half-bearing is completed by a top cap secured by two bolts, with numbers and directional arrows on the caps to ensure correct assembly; an oil-tight cover screws to the top of the head. The eight cam lobes press on bucket tappets that slide in the head metal and envelop pairs of valve springs under collars fixed to the stems by split collets. Spring seats fit over the tops of the valve guides, which carry oil seals retained by ring-clips. A flange at the centre of each camshaft carries a sprocket for the cam chain.
A carrier bolted to the head holds a free-running jockey sprocket to bear against the chain’s top run from above. Two more guide sprockets that bear against the outside of the front and rear chain runs turn on needle roller bearings on short shafts slotted into the top face of the barrel and held in place by rubbers. The rear shaft additionally supports a carrier for a wheel made of synthetic material held against the rear chain-run to maintain tension. A block bolted to the rear of the barrel contains the selftensioning device, a spring-loaded pusher with a rectangular pad that bears against the chain. Another plastic guide wheel between the chain’s vertical runs turns on a shaft held in notches by rubbers at the crankcase joint face.
The Hy-vo primary chain takes drive to a jackshaft supported by ball bearings in the upper crankcase behind the crankshaft. The receiving sprocket transmits drive to the shaft via a shock absorber inside it with four vanes and eight synthetic rubber inserts. An output gear on splines at the right-side end of the shaft engages with a larger gear attached to the back of the clutch via a cush drive. Also on the shaft are a gear to drive the oil pump and a starter clutch mechanism with three spring-loaded rollers. When the starter motor (buried in the top of the crankcase) operates, it drives an idler gear turning on its own short shaft, and that in turn drives the clutch’s gear, causing it to lock to the jackshaft.
The five-spring clutch has six plain driving plates, plus one integral with the splined hub, and seven driven friction plates. A worm mechanism in the output sprocket cover on the right of the unit activates a rod sliding inside the gearbox primary shaft, applying force to the pressure plate via a ball bearing and mushroom pusher.
Ball races located by half-rings in grooves support the gearbox shafts at their most loaded ends, behind the clutch on the primary shaft and adjacent to the output sprocket on the left side for the secondary shaft, while the other ends run in needle rollers. The
‘THE OUTER CRANKPINS ARE AT TOP DEAD CENTRE WHEN THE INNER TWO ARE AT BOTTOM DEAD CENTRE’
method of gear selection is similar to that in the bigger fours – a cam drum carries one selector fork, while two others engaged with it are carried on their own shaft. The drum is supported in a roller bearing on its left side, where it is activated from the gearlever’s shaft by spring-loaded claws engaging with six pins slotted into the end of the drum. The kickstarter shaft, with a ratcheted driving gear and return spring on a plastic collar, is at the rear of the lower crankcase half. When CDI ignition was introduced in 1981, the kickstarter was dispensed with, as was a spiral gear drive for a mechanical rev-counter off the exhaust camshaft.
The oil pump is a rotary type, unlike the gear pumps in earlier Z fours. Drawing from the wet sump through a gauze strainer, it pumps oil through a replaceable filter housed near the back of the sump, which has a relief valve in case of blockage. A passage in the sump casting then carries oil forward into crankcase drillings for distribution to the crankshaft bearings, from where oil splashes up to the bores and pistons. From the two outer main bearings, oil ascends vertical passages in the cylinder barrel to the head, where its is led into the hollow camshafts via the bearing journals. After lubricating the cam lobes and buckets, it falls down the central tunnel, lubricating the cam chain as it does. Oil is also pumped to both gearbox shafts. On top of the gearbox, a crankcase breather chamber with a removable cover settles out oil mist before venting to the airbox via a hose. To cut emissions, reed valves in the cam cover also vented to the airbox on Us-market models from 1979.
‘BALL RACES LOCATED BY HALFRINGS IN GROOVES SUPPORT THE GEARBOX SHAFTS AT THEIR MOST LOADED ENDS’
Primary drive jackshaft with starter clutch Ignition autoadvance unit Stator of post-1977 single-phase alternator Standard three-ring pistons with extended skirts
Crankshaft differs from earlier Z fours’ built-up items – it’s a one-piece job with plain shell main and big-end bearings
Like the rest of the engine, the gearbox components are rugged and reliable