Originally Posted by simon
OK chaps, thanks. IF it is the crank/gearset is it a major teardown jobbie to replace the bits?
If the rattle is indeed bottom end K-bike rattle, you don't need or want to fix it.
If I were considering purchasing a bike that had this characteristic I'd do the following:
Have a trusted, competent mechanic do a compression, and leak down test, and check everything functions normally.
Do the test I described in a post above in this thread, i.e. hold the brakes, slip the clutch and see if the rattle quiets or goes away. If it does, it is consistent with the K-bike rattle and does not need to be fixed.
Listen around the motor with a mechanic's stethoscope (or a long screwdriver held against the ear). If the sound is loudest from the bottom right side of the engine case then it is consistent with K-bike rattle.
If the bike checks out and you are willing to live with the rattle, buy the bike. If you don't like the rattle, pass on the bike. Finding a different bike will be lots easier then doing the engine teardown and checking the shims that limit axial movent of the crank and counter shafts. And even if you did all that, there's no guarantee that after you put it all back together the rattle will be gone.
Some of these K motors do rattle more than others and it is normal. They are noisy at tickover rpms, like rocks in a can.