In this video I fully explain how to rebuild the top end on a KTM or Husqvarna 2 stroke dirt bike. The principles covered in the video can be applied to all 2 stroke motorcycles! Video Notes and Updates are posted below.
Cylinder Base Gasket: In the video I explain that the cylinder base gasket thickness is determined by ones region which is partially true when looking at gas quality and combination performance; However, it should be noted that in most cases to best gauge the proper cylinder base gasket thickness, one should either use a decking tool to determine proper decking (gasket) hight (thickness), or one could simply measure the cylinder base gasket(s) that where removed during the rebuild. Note that new or aftermarket parts may have different tolerances than when the bike was assembled at the factory so cylinder decking hight may need to be adjusted to account for the different tolerances in new or aftermarket parts.
Piston Size I and II: In the video I state that there are two sizes of OEM pistons available for the 125cc KTM and Husqvarna Motorcycles and that each size piston corresponds with a specific stamp on the outside of the cylinder. While this stamp might be useful for assembly of the engine in the factory, it does not help one to determine which size piston needs to be installed during the rebuild process of the top end. As a result, one should instead reference The Official KTM Husqvarna 125cc Service Manual which specifies that there are two cylinder bore diameters for their 125cc motorcycles. KTM Husqvarna specifies that "Size I" corresponds to a cylinder with a bore diameter of "54.000...54.012mm" while "Size II" corresponds to a cylinder with a bore diameter of "54.012...54.025mm". The cylinder bore diameter can be determined using the following instructions provided by the KTM Husqvarna Service Manual. Using a cylinder bore gauge (micrometer) "Measure the cylinder diameter at several locations on the X and Y axes using a micrometer to identify oval wear". Then, based on the measurement, one can determine what bore size their cylinder is ("Size I" or "Size II"). Once the bore size has been determined, one can then purchase the proper size piston which corresponds to the bore diameter size. A bore diameter that is "Size I" corresponds to Piston I kpl. 125 (Piston size I) while a bore diameter that is "Size II" corresponds to Piston II kpl. 125 (Piston size II). If you do not have access to a bore micrometer, you could reference the piston size which should be stamped on the head of the OEM piston or you could measure the diameter of the piston itself. Based on this measurement you could then determine which size piston you have as well as the cylinders approximate bore diameter.
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