Two Stroke Top End Rebuild

This video by Rocky Mountain ATV shows how to rebuild the top end of a two stroke dirt bike. In this demonstration, work is performed on a 2005 Yamaha YZ250.

Specifications with regard to clearances and dimensions are always model specific so the rebuilder must have the service information for the specific machine available.

Engine work such as this should never be taken lightly. While most people are likely capable of performing this service, attention to detail is critical in every aspect. For an engine to perform reliably as intended, the fitment of parts must be exact and cleanliness is paramount. Further, manufacturer's recommendations must be heeded.

Be aware that, though all two stroke dirt bike engines are very similar, there are differences. Many very early models have no power valve at all (1980's and earlier, some later). Power valve designs can vary significantly and it is wise to service them when the top end is rebuilt. Reed valve locations can vary as well, some are located at the base of the cylinder (case reed) and some are mounted as shown here (intake reed). Some case reeds can be damaged if proper attention is not given to installation of the cylinder.


Two stroke top end rebuild. 9:46 minutes.

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