Crossfire front calipers have two pistons on one side only and rears have one piston on one side only. You count the pistons on each side when defining what you have.

SLK = 4 & 2
SRT = 2 & 1


I would love to have the SLK55 8 & 4 piston calipers and rotors, but I dont know if they would fit in my SRT wheels. Or how much the mounting difference is.