Workshop: Kawasaki Z1 rebuild: 1

Kawasaki Z1

It's not often I get the opportunity to delve deep into the heart of the King of Motorcycles, but when this scarce 1972 Z1 turned up at our workshops in a cloud of oil smoke, I relished the prospect of giving it a thorough mechanical overhaul, particularly as it was such a lightly used example. While the bike looked basically sound, the original black finish on the engine castings had deteriorated, and there were signs of oil weeping from the top end. On to the bench with it, then, and out with the spanners.

Step-by-step images and captions are available in the image gallery.

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Image gallery captions
Picture one: This bike is a US import Z1 and looks to have led a fairly easy life. Most of the engine fasteners look to be in good condition, which should make a pleasant change from the usual wrangling with rusted and sheared studs. There's some signs of an oil weep from the cambox area, no doubt due to the age of the gaskets and rubber half moons on the cylinder head. This is a very early Z1; note the unusual cylinder head nuts that were later replaced with cast versions.

Picture two: The first Z1 models had an engine finished in satin black, with polished outer engine covers. The black paint used at the factory proved to be unequal to the task, and used to flake off the engine as you can see here. In the face of endless warranty claims, Kawasaki caved in and changed the finish to plain aluminium for the Z1B and later models. As this engine is being completely stripped, we've decided to take the pricey option of having all the major engine castings powder coated in satin black. This will be much more durable than the original finish, and should look superb.

Picture three: Before commencing the strip down I've warmed up the engine by running it for a few minutes, then popped it up on the bench and drained the engine oil while it’s still warm. Next, I'm removing the oil filter, which lives above this large circular housing retained with a single hollow bolt (which also contains the pressure by-pass valve). This one comes away easily, I've encountered some in the past that have been so over-tightened I've had to weld a socket to the central bolt to get it off.

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Picture four: While the last of the oil is draining I can remove the exhausts. The Z1 follows what we now recognise as conventional Japanese practice, each header pipe being located into the head with two large steel collars. All the header clamp nuts unscrew easily; I've had many problems with some of these in the past, especially on middleweight Kawasakis that use M6 studs and nuts. The nuts are directly in line with the front wheel and will rust onto the studs given half a chance. A liberal application of copper grease on re-assembly is the best way of preventing problems like this in future.

Picture five: At the inlet side of the cylinder head the carburettor assembly pulls clear once I've slackened off the hose clips. With the throttle cables disconnected I can then drain and store the carb assembly out of harm’s way until I need it for the rebuild. Note the basic air cleaner hose assembly – much easier to work with than later, more sophisticated designs that featured short lengths of moulded rubber hose. There's plenty of room here to manoeuvre everything out of the way.

Picture six: On the right hand side of the engine now, I've removed the front engine cover to reveal the contact breaker back plate. This bike still has its original points ignition set-up, and to make life easier during the rebuild I've scratched tiny reference marks on the back plate to help relocate it in its original position. Note the dummy nut on the end of the crank; this is useful for turning the engine over for servicing tasks; here I'm using it to position the engine at 1 and 4 TDC, the default position for commencing any engine work.

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Picture seven: Round the back of the cylinder barrels, I'm next removing the camchain tensioner. This early engine has a manual tensioner, so I leave the pinch bolt nipped up and simply remove the two mounting bolts to release it. Once the tensioner is on the bench I can measure how far the plunger was protruding from the housing, then release the pinch bolt and measure the plunger at full extension. This will give a rough measure as to how much life is left in the camchain. Academic in this case, as I'll be fitting a new camchain as a matter of course, but it’s interesting to see how badly worn the old one was.

Picture eight: With the cambox cover removed I can now access the cams and top idler wheel for the camchain. Back in the late 60s when the Z1 design was conceived, designers assumed the camchain would have to run on rubber mounted idler sprockets, and there are three in the Z1 engine. This is the top one, which has to be unbolted and removed before the cams will come out. Note the rubber blocks on the mounting flange, and be careful they don't come loose and drop into the engine as you lift it clear. As the design developed over the years, the rubber-faced slipper blade took over as the preferred camchain guide, and these idler sprockets look quaint and over-engineered to modern eyes.

Picture nine: Two caps, numbered to match their correct positions on the cylinder head, now retain each camshaft. Working on one cam at a time I can now slacken all the cap bolts evenly and lift them away as the camshaft sits up on its followers. Each cam cap has two dowels to locate it, watch these don't fall out as the caps come away from the head. The first generation dohc Kawasakis all used shell bearings for the cams to run in – this practice gave way to the cheaper option of machining the bearing faces direct into the head on later models. That does mean that the cam bearings can be replaced, and I'll be fitting a new set of shells when it goes back together.

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Picture 10: With both camshafts removed I can now slacken and remove all the cylinder head bolts, working to the pattern in the workshop manual to ensure the head doesn't distort as I do so. A gentle tap with a rubber mallet frees the head from the barrels, and I can lift it clear. Note I've replaced the cam caps in their original positions to make sure nothing goes astray. I've also allowed the camchain to fall down the tunnel into the sump; not a problem as this engine is being completely stripped. It's not a major job to fish it out with a piece of bent coat-hanger wire if necessary. Note the two idler sprockets now visible forward and aft of the camchain tunnel.

Picture 11: The rear idler sprocket is a complex looking affair, and forms an assembly with the rubber wheel that bears against the camchain to tension it. Note the two rubber damping blocks mounted at each end of the idler spindle – these will be replaced with new ones later to keep the tensioner mechanism running smoothly. Again, be careful one of these doesn't drop out and go down the camchain tunnel if you're only planning a top-end strip. If you're lucky it will land in the sump and be easy to find. If you're not…

Picture 12: The front idler sprocket is similar, but simpler in design to the rear one as it is not connected to a tensioner. Each of these idler sprockets runs on a needle roller bearing and has rubber tracks alongside the sprocket teeth to quieten the camchain. In theory these rubber tracks harden with age and go brittle, but the idler sprockets are hideously expensive to replace and have no discernable effect on the healthy running of the engine, so I generally reuse them. Unless there's obvious signs of trouble, that is.

Picture 13: Down at the bottom of the engine, now, and here's a Z1 peculiarity. The biggest problem the factory faced when unleashing the Z1s mammoth 58bhp on the word was the rear chain; and paranoia ran deep as to its capability to handle the stresses of hard acceleration. Apart from plastering the bike, and the owner’s handbook, with warnings that chain breakages were at the owner’s risk, Kawasaki’s only other plan was to build in this chain oiler, set to deliver a drip feed to the rear chain from a tank under the left side panel. Chain technology has advanced in leaps and bounds in the intervening years, and modern O-ring chains have rendered this system entirely redundant, but the pump is an original period piece and adds a touch of authenticity.

Picture 14: The outer engine cover lifts away from the crankcase, and you can now see the whole chain oiler set-up. Inboard of the outer cover is this steel chain guard, mounted onto a raised lip on the mission cover, and clearly intended to prevent major crankcase damage if a drive chain did let go under pressure. Our bike has been fitted with a thoroughly modern endless O-ring chain so no longer needs any of this lot, but it will all be staying in the name of originality.

Picture 15: Round to the engine’s left side, now, and I've removed the generator cover to reveal the rotor, starter clutch and starter reduction gears within. Note the alternator stator fixed inside the cover with three Allen screws. There's no need to remove the stator from the cover unless it needs attention, or unless you're sending the cover out for polishing. I am, so out it comes. The starter reduction gear spins on a hollow spindle that pulls away from the crankcase to release the gear. Note the thrust washer on each side.

Picture 16: Round the other side again, I've now removed the points back plate and the auto advance unit from the end of the crank. This may look like a random sequence to this part of the strip down, but what I'm actually doing is disconnecting, wrapping and sorting the electrical parts from each side of the motor prior to engine removal. The auto advance unit, along with its central bolt and dummy nut, goes into storage with the points back plate.

Picture 17: Aft of the contact breakers is the large oval cover that hides the clutch. With this removed I can strip out the pressure plate and clutch plates, and then use this special tool to hold the clutch drum while I slacken off the centre nut. At this stage the rear drive chain is still connected, so I have the option of holding on the back brake to get a purchase on the clutch centre nut.

Picture 18: Moving to the drive side, I've used a similar technique to slacken and remove the engine sprocket, and have then removed the alloy cover (known mysteriously in Kawasaki circles as a ‘mission cover’) that covers the gear change mechanism. The shift shaft (don't you just love these pidgin Japanese translations) has two spring-loaded claws that hook over the end of the selector drum. Holding the claws apart, I can pull the shaft away from the crankcase. Note the clutch pushrod above the selector drum, this can also be removed and stored at this stage.

Picture 19: With both sides of the engine stripped and the cylinder head removed, the motor is now small and light enough for me to unbolt it and remove it from the frame single handed. I would normally remove the barrels and pistons first, but these are proving reluctant to shift off the crankcase mouth so I've opted to struggle with it on the bench where I can get at it. To begin with, I've inverted the engine on the bench and removed the sump, revealing the oil pump and strainer screen. There are some signs of sludge in the sump, but otherwise everything looks fine at this stage.

Picture 20: The oil pump is located onto the lower crankcase with three bolts. With those removed it lifts away as an assembly, complete with the drive gear that drives it from the crankshaft. Note the O-rings which seal the pump against the oilways on the crankcase, and the dowels on the bolts. With the pump removed, I can turn the engine right way up and it will now sit stably on the flat machined surface of the lower crankcase half. With a bit of cardboard below to protect the mating surface, I have now reduced the engine to a size I can struggle with to get the barrels off without it wobbling about on the bench.

Picture 21: However, the barrels still don't want to come free, so after a contemplative cup of coffee I've opted to strip the engine further, to see if that presents any more options. The crankcases are easy to split, once all the bolts are removed, leaving all the engine internals visible in the upper crankcase half. Note the crankshaft at the top, driving the clutch basket on the right. The clutch sits on the end of the gearbox input shaft, below it is the output shaft and finally the kick-starter assembly. On the Z1 the clutch cover is too narrow to permit the removal of the clutch basket, so getting it off requires a crankcase split.

Picture 22: I've removed the gearbox shafts and clutch now. This central cap on the crankshaft locates the centre main bearings and must be removed before the crank can be lifted out. At this stage I'm nurturing a rather forlorn hope that the pistons might be capable of exiting the barrels through the crankcase mouth, which would leave me with just an upper crankcase and barrel stuck together, but of course the piston skirts are wider than the holes in the crankcase moth, so that won't work either. Time for another brew and a bit of head scratching.

Picture 23: I hate to do this, but when all other avenues are exhausted… Working as gently as possible, and being extremely careful to position the drift where it will do the least possible damage, I can bash the barrels off the crankcase with a big hammer. This engine is coming apart, remember, for attention to the bores and piston rings, so leaving the barrels on is not an option. Careful and considered application of force in the right place is the only thing that will now work.

Picture 24: I'm immensely relieved when the barrels finally separate off the crankcase. To ensure the final stage of removal goes smoothly I've temporarily rejoined the crankcases so the engine is a stable assembly as I lift the barrels clear. This kind of problem is a frequent hassle for any restorer working on older Japanese classics, and I've come across instances where the barrels have had to be terminally damaged to save the crankcases. I wasn't expecting trouble of this kind on a bike as clean and well cared for as this one, which goes to prove something.

Picture 25: Usually, stuck fast barrels are the result of heavily corroded front cylinder studs, rusted into their holes by effects of years of winter salt and pressure cleaning. In this case it appears that the base gasket itself, which appears to have been stuck on at the factory with a gasket cement with similar qualities to Araldite, caused the problem. Horrible. I've finally been able to remove the pistons from the con rods, and last job before separating the crank from the upper casing is to remove this final part of the camchain idler system, a rubber wheel which sits in the crankcase mouth to locate the front and rear chain runs. 

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