bmx freewheels that fit on regular flip flop hubs go down to 13 tooth (the odyssey ones are the best), take your wheel to a decent shop and they should let you try to thread it on before you buy, the freewheels come in an unsealed cardbord box so i cant see why they should have a problem with that. Bmx cassette hubs can go down to an 8 tooth and i guess you might be able to thread one onto a 28" (36 or 48 hole) rim but i dunno if that would work. Try the odyssey 13th freewheel, they're solid and last long as hell
bmx freewheels that fit on regular flip flop hubs go down to 13 tooth (the odyssey ones are the best), take your wheel to a decent shop and they should let you try to thread it on before you buy, the freewheels come in an unsealed cardbord box so i cant see why they should have a problem with that. Bmx cassette hubs can go down to an 8 tooth and i guess you might be able to thread one onto a 28" (36 or 48 hole) rim but i dunno if that would work. Try the odyssey 13th freewheel, they're solid and last long as hell