got the assembly from my Magnum FT-160 (copy of the OS) and it fits perfectly. if you're looking to use the FEMA on an OS FS-120, get this assembly. the only drawback is that you will not be able to get an alumnimum spinner on unless you do some custom work. but a plastic spinner would go on just fine. the assembly requires that you use a spinner as the bolt will run out of threads before you get it all on. with a prop only, the FT assembly can not be used. unless of course, it's a huge arse prop. :-)