A 2003 Yamaha YZ125 was brought to us in need of engine work and new fork seals. It arrived at our shop partially disassembled after an attempted (and unsuccessful) DIY repair. We believe the DIY repair was unsuccessful because the piston wristpin clip was installed incorrectly. This caused severe scoring in the cylinder, and consequently more damages as well as repair expenses.
In this particular engine, the cylinder has a nickel coating as opposed to other cylinders that have a cast iron sleeve. Due to its coating type, repairing it is a process that we have to outsource and cannot perform in-house. The company we exclusively use for this service is called Millennium Technologies (link) based out of Plymouth, WI. These guys are the mega-nerds in our industry in the most complimentary of ways. Whenever we head out to the dealer shows we always make a point to stop by their booth and pick their brains.
They will repair the scars in the cylinder wall by welding them up, as well as re-bore/re-plate/re-fit a new piston. Millennium Technologies uses their own proprietary plating process called “Nickel Silicon Carbide”, or NSC for short. Rather than restoring to factory specifications, we prefer to send a new piston with the cylinder so that it can be properly fitted for a precise fit, as each piston can vary by marginal amounts.
After further inspecting the rest of the motor and speaking over the options with the customer, we decided to rebuild the bottom half of the motor as well. This will include replacing the main bearings and seals as well as installing a new crankshaft and rod assembly.
In the past several years, some of the larger aftermarket companies in our industry have begun producing complete engine rebuild kits for common models. Although this may seem insignificant to some, it greatly reduces the amount of time required to look up dozens of individual part numbers as well as eliminates the margin of error in ordering incorrectly, which tends to increase steadily as the number of part numbers do. This way we simply order one part number that includes every part needed for a complete engine rebuild. This is also typically more cost-effective for the consumer than ordering each part individually. We were happy to find that such kits were readily available for this YZ125.
At the time of this writing, we are awaiting the cylinder’s return from Millennium Technologies and are eager to reassemble and get this dirt-bike on the track once more!
