LLorenzo's XS650 has proven to be in rather better condition than I'd expected after being stored in a damp cellar for over five years. A full strip of the engine revealed no major problems, only the front camchain slipper blade being urgently in need of replacement. I've had the head and barrels bead blasted and we've opted to replace the camchain, so armed with a gasket set and a full set of replacement oil seals I'm now ready to rebuild the engine.

1. The roller bearings fitted to the main and big ends are more or less bulletproof unless a major lubrication failure has occurred. The crank has shown no signs of distress, so I'm happy about re-fitting it. The starter drive gear, indicated, shows some signs of wear but this is not excessive.

2. Before assembling the crankcases I start by re-fitting the starter crossover shaft and it's drive gears. This can be really difficult to engage later, as I discovered during the engine strip.

3. The gear selectors also need to be assembled into the top crankcase half at this stage. I've been careful to keep the three selector forks in order, and can simply slip them over the selector drum as I slide it into the crankcase. Each selector fork then needs its respective pin sliding into place until it engages with the track on the selector drum, then I slide a cotter pin into place and bend the ears over to secure the pin.

4. This cross shaft also serves to locate the three selector forks in the correct position. There's a fair amount of movement in the selector forks with everything correctly engaged which undoubtedly helps gear selection, but is also probably responsible for the traditional Yamaha "clunky" gearbox.

5. Now I can position the crankshaft and the two gearbox shafts into position in the upper crankcase half. I've placed new oil seals on each end of the crank and on the gearbox output shaft at this stage, and made sure the gearbox bearings are engaged with their respective locating rings in the crankcase. I'm fitting a new camchain too, so I've hooked it around the crankshaft before proceeding.

6. After applying a thin smear of Blue Hylomar to the jointing faces I can now fit the crankcase halves together and refit and tighten the bolts. The workshop manual gives a tightening sequence, but if in doubt I always tighten the bolts in a diagonal pattern working from the centre of the engine outwards. Make sure the shafts rotate easily in their bearings before proceeding. The starter motor fits underneath the XS650 engine, so I re-fit it now to save problems later. The starter reduction gears can then be fitted as an assembly to the left side of the crankcase.

7. Turning the engine right way up, I can now lower the barrels into place after greasing and fitting a new base gasket and "O" rings. Many people like to use ring compressors for this job, but I prefer to gently ease each ring into the freshly oiled bore with the aid of a small screwdriver. Once all the rings are engaged the barrels should tap gently home with a rubber mallet.

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