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Suzuki's first ever litre bike: The GS1000 engine stripped & analysed

Published: 20 September 2015

Updated: 18 September 2015

Launched to compete with Kawasaki’s Z1, the GS1000 was the first of Suzuki’s fearsome litre bikes. Prepare to be a very little explorer inside a very big engine as we take a microscopic view of one of the great powerplants...

uzuki’s first ever one-litre machine was launched in 1978, following on from the GT750 two-stroke triple, the short-lived RE5 Rotary and the 1976 GS750 dohc four cylinder. Offering 130mph performance, comfort and exceptional road-holding for its class, plus solid reliability and a moderate price tag, the conventional-looking big Suzuki was lauded as the best all-round buy in its class. Variants included the GS1000S with a cockpit fairing, the custom-style GS1000L for the US and the shaft-drive GS1000G. The ‘GS-thou’ was a common sight on UK roads during the bike boom at the turn of the 1980s and the final GS1000ET version stayed in the range until the end of 1983. 

The same bottom-end design was still being used by Suzuki in the 124bhp 16-valve GSX1150 of 1986 and twenty years of dominance in drag racing served to emphasise the unit’s robustness. Its reputation was also boosted by racing success. The factory’s GS1000R XR69 track version successfully challenged Honda, especially in the contentious TT Formula races of 1980 and 1981, while in America Wes Cooley won the national Superbike series in 1979 and 1980 for Yoshimura Suzuki. Cooley also won the 1980 Suzuka 8-Hour race, partnered on an XR69 by Graeme Crosby, winner of the 1981 Formula 1 TT. 

Kawasaki influence

The GS1000 engine derives from that of the GS750, Suzuki’s first four-stroke four of 1976 and the layout of both is obviously influenced by Kawasaki’s dohc Z fours, originating with the Z1 of 1972. But the GS1000 does differ from the earlier 750 four in several respects. Commendably, the bigger engine is lighter and shorter. A crankshaft with non-circular webs rather than full flywheels reduces weight, as does the absence of the GS750’s kickstarter and its shaft, which also accounts for the unit’s reduced length. Nearly 18in (480mm) long, the built-up crankshaft, which has its outer crankpins disposed at 180 degrees to the inner pair, is supported on six main bearings clamped in housings integral with the crankcase castings. Five are caged roller units, but the bearing in the right-side crankcase wall is a ballrace, located by C-rings in slots to govern end-float. Wide (20mm) big-ends eyes on the steel connecting rods carry needle roller bearings, while at the small-ends the gudgeon pins run direct in the rod metal.

The first Japanese four, Honda’s CB750 of 1969, had a car-type one-piece crankshaft with plain main and big-end bearings, but as Kawasaki did with its biggest Z fours, Suzuki stuck with the built-up type they knew from two-stroke multi production for several years, before switching to plain bearings. A helical gear bolted up to the right-side crank web of cylinder number three transmits primary drive to a larger gear at the back of the clutch. A slim gear at the leftward end of the crankshaft, outside the crankcase, receives drive from the starter motor pocketed in the upper case, via reduction gears and a clutch with centrifugal rollers. 

Outboard of the gear, the three-phase alternator’s magnetic rotor bolts onto the tapered end of the shaft. Eighteen stator coils are fixed in an outer cover that also encloses the starter drive. The ignition contact-breaker unit, with two sets of points and condensers, is at the other end of the crankshaft. A sprocket cut into the mid-section of the crankshaft drives the camshaft chain, which runs in a slot cut in the centre of the cylinder barrel and head. The inclined aluminium cylinder block with iron bore liners is held on 12 long studs. Flexible O-rings are used to seal the bores where the block seats onto the crankcase.

The cylinder head casting incorporates the camshaft housings and is held down to the block on a one-piece head gasket by nuts threaded onto the tops of the 12 studs, plus two side bolts threaded into the barrel. The two-valve combustion chambers are hemispherical, with the shrunk-in inlet and exhaust valve seats almost meeting in the middle and the spark plugs to one side, angled outwards for accessibility. The pistons have moderately domed crowns, three rings and full skirts that are recessed on each side around the gudgeon pin bosses. 

Set at an included angle of 61 degrees, the valves slide in cast-iron guides, with oil seals on their stems. Double springs seat on steel bases and have caps retained by collets in the normal manner. Lifter buckets fitted over the springs are recessed on their tops for shims, available in thicknesses from 2.15mm to 3.10mm in increments of 0.5mm, which are used to set clearances. While the top end is similar in appearance to Kawasaki’s early Z type, in this case the camshafts run directly in the head metal rather than in shells. Two journals for each shaft are completed by bolt-down holding caps, marked from A to D for correct assembly. Skew gears take rev counter drive off the exhaust camshaft near the lobe for cylinder three and a single top cover bolts to the head. 

Shims over buckets

Bucket-type cam followers have opened valves in high-performance dohc engines for almost 100 years. Clearance gaps, vital to ensure that valves close tightly regardless of heat expansion and for precise timings, were usually set by pre-sized shims on the valve stem tops under the buckets. To make adjustment simpler without disturbing the camshafts, shims on top of the buckets were introduced by several car makers in the 1960s and subsequently by Kawasaki, Suzuki and other bike manufacturers. Because they are rubbed by the cams, they have to be of a durable material with a large surface area, so must be bigger and heavier than under-bucket shims. If engines are over-revved, shims can pop out. 


The cam chain runs over sprockets bolted and keyed to the shafts with a fixed guide under the chain run between them. A straight guide for the downward-running front portion of the chain is held in a support bolted to the base of the crankcase at its lower end and located in a slot in the head under the cam cover at the top. The bowed tensioner blade bearing on the rearmost chain run shares the same lower support and has another mounting bolt that threads into the top face of the crankcase. A spring at the back of the rear blade automatically adjusts chain tension as it stretches in used.

The oil pump in the lower crankcase is driven by a gear behind the clutch. Running on a narrow needle roller bearing on the mainshaft, it has two dogs that engage with notches in at the back of the clutch gear. Oil is drawn from the wet sump via a mesh screen, and pumped through a cartridge filter located inside the front of the lower crankcase. From there it proceeds up a large-diameter vertical oilway in the upper crankcase to a T-junction feeding a gallery running across the upper case behind the cylinders. There are six feeds to the crankshaft from the transverse gallery, five directly into the roller mains and one pushing oil into space between the ballrace and its adjacent oil seal. 

Near each end of the gallery, oil travels across recesses in the joint face with the barrel to cavities surrounding the rear outer cylinder studs, up which oil is sent to the cylinder head, where oilways distribute it to the camshafts. After lubricating the top end, it falls through the cam chain tunnel and returns to the sump. A plate bolted to the upper crankcase above the T-junction carries the oil pressure switch for the instrument panel’s warning light. A take-off in the recess under the plate sends oil into galleries supplying the gearbox internals.

High flow, low pressure

While Kawasaki used a gear-type oil pump in its roller big-end fours, Suzuki opted for a gerotor (generated rotor) pump in the GS750 and GS1000. Also known as a trochoidal pump, it has a peg-driven rotor with four lobes, turning eccentrically in a five-lobed chamber that also rotates. The spaces between the rotors continuously expand and contract, creating suction and pressure to transfer oil between intake and output ports on the side of the chamber. The simple and inexpensive gerotor does not deliver the high pressures of a gear pump, but a roller bearing crank needs relatively low pressure. This pump passes 1.85 litres per minute at 500rpm. 


A primary gearbox shaft carries the clutch on the right side and final drive output from the secondary shaft is on the left. Both shafts are supported in ball bearings with locating C-rings at each end, with larger-sized bearings behind the clutch and the output sprocket. Oil seals are used with the bearings, to keep lubricant inside the gearbox compartment, where it drains to the sump. The left-side gearchange pedal rotates a shaft extending to the other side of the crankcase, where it carries an arm with rack teeth on and a return spring. The rack meshes with teeth on a wheel that turns the selector drum in the box via a ratchet and pawl for positive-stop activation. Cam slots on the drum move three selector forks pivoted on two shafts.      

Clutch operation is unlike the original GS750’s, which had a pushrod. Here, the cable pulls on a lever fixed to the top of a vertical shaft housed in the clutch cover. At its lower end, pinion teeth engage with a rack on a pull-rod. A disc with a radial needle roller thrust bearing and washer on the end of the rod frees the plates by pulling the alloy pressure plate outwards against the force of six coil springs in cups. Eight friction driving plates engage with the outer basket and seven plain driven plates are splined to the alloy centre hub. As in a Kawasaki Z, springs housed in slots in the driven primary gear act as shock absorbers between it and the alloy outer basket. 

Specifications

1980 Suzuki GS1000

Type air-cooled dohc in-line four Capacity 997cc
Bore x stroke 70 x 64.8mm 
Compression ratio 9.2: 1 
Valve sizes inlet 38mm, exhaust 32mm 
Valve angle 61 degrees 
Valve timing Inlet opens 28° BTDC, closes 68° ABDC 
Exhaust opens 66° BBDC, closes 26° ATDC 
Valve clearances inlet 0.03mm – 0.08mm exhaust 
Carburation 4 x 26mm Mikuni VM 
Ignition battery and coil   
Firing order 1-3-2-4 Spark timing 37° before TDC (full advance) 
Points gap 0.35mm-0.40mm 
Lubrication system wet sump 
Oil capacity 4.2 litres 
Primary drive helical gears 
Gearbox five-speed 
Output sprocket 16 teeth 
Power output 87bhp @ 8,000rpm 
Weight 198lbs (90kg) 

Running one today

Copious power and a reputation for solid reliability make the GS1000 an attractive proposition. Those virtues also mean that some have been ridden into the ground and an engine strip may reveal horrors. But while sourcing exhausts, seats and tanks may be problematic, the situation with engine parts is fairly heathy. It is likely that the GS1000 will be covered by Suzuki’s Vintage Parts Programme, which provides newly-made spares. Leading GS1000 exponent John Sims, the specialist who provided the engine seen here. He restores and builds the fours in both stock and racing forms, the latter including a 1040cc-kitted engine with which Michael Dunlop lapped the TT mountain course at 119mph in 2010. “It takes a lot to kill one of these engines,” John says. “With regular oil changes, they should go for ever. But you do see some that have been damaged by ham-fisted owners, or in accidents. Electronic ignition is worth fitting and the shock absorber springs at the back of the clutch need to be checked, as they wear out.” 

Words Mick Duckworth Pics Simon Hipperson

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