One should take many safety precautions when using a STIHL TS400 saw. For example, one should wear gloves, a helmet, and safety goggles. In addition, one should keep hands away from the blade as much as possible.
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If I was you I would head to your nearest tool hire shop and ask them if they can print you a parts breakdown. Its all relatively simple just take whatever parts need off to access it then take it out and change it. Then parts back on i order they came off there will be a tensioning nut usually the same as the one that fits your stihl tool simply loosen off to adjust the tension. Alternativly send me it and I will fix it 4 you. I used to work in a tool hire shop and fixed them all day long. But if in doubt ask a local hire shop to help you they should be more than willing.