What model of Stihl are you trying to fit? I believe they will only fit models with a LH thread arbor. The speed fees heada fit both my km-130 and fs240. I think Echo makes a 375, 400, and 450. The 400 should do for most trimmers. I have the 450 on the kombi because it fits larger line. My 450 is supposed to only hold 10' but my 400 holds 20'. I believe the 375 only holds 10'. And, one last note about the 400. It doesn't do lines much over 0.095. The line gets stuck in the pass through part of the head.
Oh and they do work just like they seem. Super well.