Stihl only recommend their 2 point, 270mm shredder blades for their Clearing Saws of at least 1.9bhp or more, and their 320mm shredder blade only for the FS5xx series. The SGS GPS520 is 1.6hp. The FS120 is 1.8bhp and not intended/engineered to take the Stihl shredder blades.
The Oregon 3 point blade is 300mm and over diameter for all Stihl clearing saws except the FS5xx series machines. Stihl don't do a 3 point blade with good reason, they are heavier and have more inertia that has the potential to lead to overloaded drive train components as unfortunately demonstrated by Grassyass - "you can see the strimmer shaft flex if you hit a solid object with the Oregon blade" – enough said.
The fact that Oregon make those 3 point blades to fit everything doesn't make it a good idea to use them without considering the potential overload issues. They don't have to pick up the bill for worn/broken drive train components on brushcutters/clearing saws.
There is also an issue with the 20mm alloy spacer ring disengaging from the Stihl Clearing Saw blade drive flange arbor while in use allowing the blade to be knocked off centre.
Cheers.
