This is an outer centrifugal clutch housing (bell housing) for a 47cc and/or 49cc Chinese 2-stroke engine. These small engines are used in miniature motorcycles (pocket bikes) and and number of other motorized things like skateboards and Razor shooters. Yours is a high performance one that's cut open for extra air cooling. They come in different gear ratios (tooth count) for application. The maker of this one is unknown but the bolt pattern holes are the same as is the mechanical dimensions inside because all the Chinese engines use the same standard bolt pattern and internal dimensions regardless of the manufacturer. The one in the first URL is the most common part made in China. The second URL shows custom ones with a chain guard. The third URL shows a racing clutch housing used on miniature motorscycles machined out of solid aluminum billet like the one you have.

This is an outer centrifugal clutch housing (bell housing) for a 47cc and/or 49cc Chinese 2-stroke engine. These small engines are used in miniature motorcycles (pocket bikes) and and number of other motorized things like skateboards and Razor shooters. Yours is a high performance one that's cut open for extra air cooling. They come in different gear ratios (tooth count) for application. The maker of this one is unknown but the bolt pattern holes are the same as is the mechanical dimensions inside because all the Chinese engines use the same standard bolt pattern and internal dimensions regardless of the manufacturer. The one in the first URL is the most common part made in China. The second URL shows custom ones with a chain guard. The third URL shows a racing clutch housing used on miniature motorscycles machined out of solid aluminum billet like the one you have.

References:

http://www.motopartsdealer.com/7-teeth-gear-box-clutch-drum-2-stroke-47cc-49c...