Wing bolt (脱落防止蝶ボルト)
This is a type of wing bolt whose threaded shank is processed with a “reduced shaft diameter” (ヌスミ加工), meaning the shank diameter is smaller than the thread diameter.
Thanks to this structure, when loosened, the threaded portion is still retained by the mating component, so the bolt does not fall out.
This type is commonly used in locations that require frequent disassembly, such as covers or inspection doors, to prevent bolt loss and ensure safe use.
