The sleeve is the liner the piston slides in. Whether the cylinder is sleeved from the factory or Nikasil-plated, a scored, oval or over-worn bore can be saved with a new sleeve: the engine gets its compression back, for far less than a complete cylinder.
Here you will find sleeves for KTM 4-strokes: EXC 520, EXC 525 RACING, EXC-G 525 RACING, MXC 520… Fitting is done by an engine machinist or an equipped workshop (cylinder machined to take the sleeve, heat fitting, boring and honing to the piston size). Order the piston and the top-end gasket kit with the sleeve.
