HRC recommends replacing the piston every 300 miles or 2.5 hours of running timing. We replace the piston ring, piston, wrist pin, wrist pin bearings, and the clips that hold the wrist pin in the piston. But that's only one area where wear occurs. In a Honda RS125 engine, the ring groove and the wrist pin bosses are also primary wear points and these areas are not easy to check. Because it's difficult to check these areas, it's best to just change the piston at the recommended mileage to avoid a piston or related part failure causing major damage to the cylinder, head, and possibly crankcases.
Some people try to save money by only changing the ring and then checking piston clearance.
We don't recommend this unless the piston has less than 100 miles on it and is still in excellent condition. This is because the piston and ring wear together and the new ring may not properly seal in the used piston. If the ring sticks or leaks while the engine is running at high speed it may lead to engine seizure, because the hot combustion gases will blow past the ring and heat the sides of the piston, causing the piston to expand excessively. Excessive piston expansion can also lead to an excessively painful experience as you hit the pavement, or it could just cause a huge hole in your pocket. Other wear points that people frequently do not think of include the wrist pin bosses on the piston, the wrist pin and the bearing.