Here's an easy way to measure brake pad wear, without
having to do any disassembly:
1) Using a Dial Caliper, measure the brake disc thickness... on my bike this is 0.175" (4.45mm)
Note: The minimum service limit for the front disc is 0.140" (3.5mm)
2) After squeezing the brake lever to seat the pads against the disc, measure across the backing plates of both brake pads... here I have a measurement of 0.775" (19.68mm)
3) This last photo shows the measurement that was taken from a set of new
OEM Honda/Nissin front brake pads, which is 0.710" (18.0mm)
For aftermarket brake pads, just measure the pad set when new and make a note of that measurement for future reference.
Next, I subtract the brake disc thickness of 0.175" from the measurement taken in step 2...
0.775" - 0.175" = 0.600"
Then, subtracting this result of 0.600" from the thickness of the new brake set...
0.710" - 0.600" = 0.110"
This result of 0.110" represents a 15% difference between the worn brake pad set, and that of a new OEM Honda pad set... so, I have 85% of the usable*
friction material remaining on my front brake pad set.
The CBR250R/RA Service Manual lists the brake pad service limit as being "to the grooves" that are cut into the friction material.