Some differ in size on legnth more than anything, I've noticed from previous posts youre running a Hyper 7 so, For example Losi ones are longer then the Hyper 7 range as the engine on the Hyper 7 is further forward, The Losi has the engine situated a slight bit further back so the bell stretches further.
If youre in need of a Clutch Bell for youre Hyper 7 I've got one in my pitbox you can have if you want it, It's free as i've no use for it.
Most will fit onto the clutch with out any real issue but its more to do with legnth more than anything
(Not in a rude way)