Piston stopper tool for engines with 10mm threads. Works on Zenoah G230RC / G260RC / G240RC / G270RC / G290RC / G320RC / G260PU / G260PUH / G231PUH / G231PUM / G260PUM / GZ25N14 / GZ25N23 engines. Also works on Chung Yang CY23RC / CY26RC / CY27RC / CY29RC / GP290 / GP420 / GP460 engines and all other engines that use a CMR7H or similar spark plug. Also fits some pocketbike engines if high-performance headkit with 10mm spark plug is installed. In general, fits engines with 10mm spark plug hole threads (for example, engines which use NGK "C" class spark plugs like the CMR7H, CR8HIX, etc). Threads into spark plug hole and stops piston from moving, allowing easy removal of flywheel and other rotating parts. This stopper is interchangeable with Zenoah part # 4810-96220 481096220, and with HPI part # Z966.