Did you just get a 520 front sprocket, if thats the case than it will be thinner because your rear will be bigger than the chain links and the front sprocket.

I dont know if thats what is being said but thats just what i got when you only put front sprocket 520 conversion
You need both front and rear sprockets as a set if you are changing the 525 to a 520 set

 

I am running into the same thing on my F4. Did you end up putting the spacer on the inside or outside? I am using Vortex sprockets with the RK chain that came as a kit and it says 1999-2000 CBR F4 on the front, so theoretically it is the correct kit.

 

My Driven sprockets fit the way they should have w/o the use of any additional spacers.