Rod's now getting within touching distance of finishing our overhaul. Still a lot to be done, but the engin'es now in place…
Step-by-step images and captions can be viewed in the image gallery.
Enjoy more Classic Motorcycle Mechanics reading in the monthly magazine.
Click here to subscribe & save.
Picture one: Now the engine’s lower end is bolted in I can start to fit the transmission parts. The kick-starter shaft has been engaged with its return spring (top left), followed by the alloy spacer collar that keeps it centralised on the shaft. Don’t forget the shouldered collar behind it that supports the spring. The easiest way to tension the spring is to clamp it in a pair of long nosed mole-grips, then wind it around to engage the spring in the shaft, finally sliding the collar home. Next, I'm fitting the gear-change shaft.
Picture two: At this stage it’s possible to run through the gear selector to make sure all the gears engage as they should. Without oil the gearbox will feel stiff, but rotating the respective shafts while operating the lever should, with persistence, allow selection of all five gears. Next, the clutch basket fits over the input shaft and engages with the primary drive gear on the crank, then is located with the central boss and roller bearing, which I've lubricated with fresh gearbox oil. Don't forget the thrust washer and collar behind the basket.
Picture three: The clutch follows conventional Japanese design, alternating plain and fibre plates until the outer pressure plate can be bolted up against its six springs. The spring bolts are M8 on the Kettle, and don't be tempted to over tighten them. Unusually the clutch lifter uses a pull-rod assembly, which must be fitted through the centre of the pressure plate before fitting it. Don't miss out the needle roller thrust race between the pull-rod and the pressure plate.
Picture four: Before the outer clutch cover goes on I'm assembling the clutch lifter mechanism. This is reminiscent of old British designs, with three balls riding up on egg-shaped ramps, when the mechanism is rotated by the cable. The pull-rod passes through the centre of this assembly as the casing goes on, and the clutch lifter arm is then secured to the pull-rod with two nuts locked together on the shaft. Set the free play on the lifter before connecting up the cable and fitting the inspection cover.
Picture five: Problem time again. The barrel studs we removed during the strip down were badly corroded, and I've now discovered that replacements are almost impossible to find. Between us Roy, the bike’s owner, and I have managed to track down enough second-hand studs to do the job, but the small stud at the rear of the cylinder base proves impossible to find until Roy tracks down this scrap set of crankcases. To avoid any damage to this rare and valuable stud I spend some time carefully slicing up the scrap casing with an angle grinder and hacksaw until it finally drops free.
Picture six: With all the studs finally present and correct I can begin to turn my attention to re-assembling the top end. Like all two-strokes, the GT750 uses needle roller small end bearings, and I've lubricated each one with fresh clean engine oil before fitting it into the con rod. These two stepped thrust washers go on either side of the bearing before offering up the piston.
Picture seven: The pistons on the GT750 are handed – the centre and left pistons are the same, but the right-hand piston has its porting arranged differently to suit the position of the carbs and exhaust pipes. The pistons were changed several times during the production run, we're using the most recent version. I always fit one circlip to each piston before offering it up to the small end, then the pin slides in and I can clip in the outer circlip.
Picture eight: Also common to other two-strokes, the Kettle pistons use pegs in the ring grooves to locate the rings. Without these pegs the rings could rotate in the grooves as the engine runs, with catastrophic results if the end of a ring popped out into one of the ports. Each ring has a couple of notches at each end to engage with the peg, and it's important to make sure they're fitted correctly before proceeding.
Picture nine: Now the barrels can go on. This requires some care, as it’s a heavy lump, and there are no cylinder sleeves protruding below the base flange to help engage the pistons. However, with care, it’s quite possible to engage each piston into its respective bore and lower the barrels to the crankcase mouth, though the job is made easier with the help of a willing assistant. Note the new cylinder base gasket and the O-rings around the coolant ports at the cylinder base.
Picture 10: The cylinder head is a one-piece casting, which simply lowers into place over the new gasket and bolts down. These composite washers were added to later models, apparently to stop coolant travelling up the cylinder studs and leaking at the head. The parts list also shows plain and spring washers on the sleeve nuts, and it's not clear which combination of washers should be fitted to which model. Careful examination of a couple of show Kettles at Lotherton, and some friendly advice from their owners, results in my decision to refit the sleeve nuts with a single spring washer above each composite. Inox Fasteners supplied some excellent stainless washers for the purpose, and advised they have small quantities of stainless sleeve nuts to match if required. Roy had already bought a set of original chromed ones for this bike, alas…
Picture 11: The M8 chrome head bolts for the cylinder head were in a pretty shabby state, but are still available as new parts. Final torqueing down of the cylinder head will wait until they arrive in the post, for now I'm adding these classic emblems to the sides of the cylinder barrel. Although these are adhesive backed, they prove reluctant to follow the curved contours of the cylinder barrel and keep springing up at the ends. Eventually a couple of drops of Superglue makes them behave.
Picture 12: Though it’s probably not strictly necessary, I've dropped a brand-new thermostat into the recess on the cylinder head before bolting down this cover, which is also the take off point for the top radiator hose. There's no gasket included in the kit for this joint, so I'm using a good quality Silicone RTV sealant before tightening down the three bolts. The flat casting at the rear of the housing was originally for a temperature sender, part of the fan assisted cooling system fitted on the J models. The fan was found to be unnecessary for most road use, so was offered as an option from the L model onwards, though all the mountings for the fan on the frame (and all the related wiring in the main harness) was present until production ended.
Picture 13: The SRIS pipes are designed to recycle unburned lubricant from the crankcase up to the cylinders, and must be connected to the correct inlets to work correctly. The centre and right pipes cross over at the front of the crankcase, while the left pipe routes under the engine to connect up at the back of the left-hand cylinder, above the starter motor. The inlet stubs themselves are quite fragile, and the rear one was damaged during cylinder removal so I've had to replace it. These two at the front are quite serviceable but stuck fast in the block, so, although I have new replacements to hand, I'm leaving well alone.
Picture 14: It seems to have taken ages, but we finally have a complete exhaust system ready to fit to the bike. New exhaust collars have been a nightmare to find after we had to destroy two of them during the strip down. Most of the original pipes and silencers looked reclaimable, but Bob at CRS chromers has had a mammoth task replating them, and we still have the odd dent in the silencers. Roy discovered the centre left silencer is still available new so one of those has found its way to the bike. Note the chromed steel cover in front of the SRIS pipes at the engine’s right front, just visible behind the main frame tube.
Picture 15: The ‘three pipes, four silencers’ layout is achieved by using two (one-piece) pipe and silencer assemblies on each side, and this one into two arrangement in the centre. The centre header pipe branches into two beneath the engine, and the two inner silencers fit onto it with these clamps and stepped rubber seals. Before tightening everything up I can waggle the centre silencer to make sure it clears the centre-stand by the same amount on each side.
Picture 16: The outer silencers are welded to their respective header pipes and are quite heavy to hold up while fitting the bolts. I've slotted a screwdriver through the mounting holes to take the weight while I get everything into position. A single bolt at each side here, passes through the bolt-on support bracket and screws into the welded-on mounting point on each centre silencer.
Picture 17: The final step on the exhaust system is to add a set of four new baffles to the pipes. Each baffle simply slides in, though I've taken the precaution of smearing copper slip around the inside of the pipes first so they don't rust in place later on. Each baffle has a single M5 retaining screw, which I've replaced with specially supplied stainless Allen screws from Inox. The baffles are service items, and need to be removed at regular intervals to clean off excess carbon from the exhaust.
Picture 18: Back to the topside now, and this water bypass hose fits to a stub in the top crankcase, passes through a grommet in the starter motor cover plate, and connects to the stub at the bottom of the thermostat housing. I'll be adding a new clip to seal it before putting any coolant into the system. Note the left-hand SRIS pipe rising through the starter motor cover and connecting to the back of the right-hand cylinder.
Picture 19: The carburettors were in reasonably good condition, though one of the float bowl threads was stripped on the centre carb. Roy has been shopping for a replacement while I've had some of the smaller fittings off for zinc plating. Now I can rebuild the carbs with these Keyster rebuild kits from Bike Barn, which include jets, needles and gaskets. Some of the jets supplied aren't the same as the originals, so for now I'm keeping the original settings until we have the bike running and can do a carburation check.
Picture 20: The carb assembly simply pushes onto the three rubber inlet stubs and clamps up with the clips. I've fitted both throttle cables first, as they are frequently tricky to get at later on. Inox have provided smart stainless screws for all the covers and float bowls, but a couple of the smaller fittings including one choke lifter have gone missing somewhere between my workshop and the zinc platers. I'm grateful to Mick Hopson for coming to my rescue with a spare set of carbs at Lotherton, which I can pirate for the bits we've lost.
Picture 21: Behind the carbs, the air cleaner box now slides in sideways and forward into place. I've fitted the three hoses to the housing beforehand, and to make sure the hoses are fully engaged I've pushed the assembly fully home on the carbs and tightened the retaining clips before fitting the two bolts at the top of the air cleaner, which clamp it to the frame. The housing is new, as Roy found one for a bargain price. Next, the air cleaner element goes in, followed by the two chrome trim covers to close it up, also new.
Picture 22: Roy has actually been on a bit of an international shopping spree, and keeps turning up at my place with his arms laden with more rare bits for the bike, sourced from Kettle specialists and enthusiasts worldwide. This is a brand-new radiator, found as NOS, and still in its original box. The fitting instructions with the rad give clear directions to use shim washers when bolting it up to its mountings, presumably so it won't suffer from vibration fractures. A selection of shims were provided, but were zinc finished, so I've used stainless body washers instead.
Picture 23: For some reason I have three brand-new radiator top hoses in the box of bits, and no bottom hoses. However it turns out the bottom hose is the same diameter but shorter, so I can trim one down to fit. And surprisingly, the trimmed bit is exactly right to connect up to the new filler neck assembly, which sits in the centre of the top frame tube. New clips complete the picture. It's hard to imagine a current chassis designer allowing a radiator filler neck to be routed right through the middle of a major structural frame tube at the headstock! How things have changed.
Picture 24: The radiator overflow tank sites between the frame rails, and will be secured with a brand-new rubber strap. This small diameter hose connects through a banjo bolt to the filler cap neck. The overflow tank allows coolant in the system to expand and contract without affecting the level in the radiator. Many modern designs actually have their filler caps fitted to the header tank instead of the rad.
Picture 25: Now all the plumbing’s hooked up securely I've fitted the re-chromed crash bar around the radiator and bolted everything up. The final touch on the cooling system is this brand-new plastic trim panel, mounted to the radiator sides with four small shouldered chrome screws and rubber O-rings. The original plastic cover was scrubbed at one corner due to accident damage, but the scrapes on the crash bar have been expertly polished out by the chromers.
In the final part, we enter the finishing straight. Will our Kettle fire up and run without drama, or will it put up a fight before gong back on the road?Enjoy more Classic Motorcycle Mechanics reading in the monthly magazine. Click here to subscribe.