So I am mounting cbr600rr brakes onto 05 zx10 forks. Which bolts do I use? (caliper ones or fork ones). The cbr bolts are 10x57, the zx bolts are 10x60.

Which ones and why. Only concern is whether there is enuff depth in fork thead right?

 

That is correct, I'm not sure how it's setup with the ZX caliper mounts, but you also need to see that the bolts don't penetrate too far and touching the rotors or anything else in the way of the bolts, once you install them, spin the wheel in the air just to make sure all is clear. If you had some pictures it would be nice to see how it's setup.
Oh yea, there are no stupid questions when it comes to safety

 

The only stupid safety questions are the ones that are NOT asked.

 

As you`re dealing with radial brakes and the bolts are being screwed into aluminum (bottom of fork leg), I say use the longest ones possible that do not bottom out, thereby ensuring maximum grip and penetration....

 

But these are radial mount calpers so the mounting is parallel to the rotors (will never touch). Thats why its not so obvious the answer.

 

Same answer you already had above...

Use the longer bolts if they will thread though the caliper mount and into the fork leg fully.. if not, use the shorter

For steel, a basic rule of thumb for thread engagement length for a screw in a tapped hole should be 1.5 the Diameter, where Diameter is the major diameter of the screw.

I've also seen it suggested engagement length of 1.0D for steel, 1.5D for cast iron, and 2.0D for aluminum.

I try for 1.5D in any steel and use 2.0D in any non ferrous metal.

 

I am often left wondering why I even bother........