Crank pulleys OEm has rubber to help stop some harmfull vibration, when you go to a light crank pulley like those Ultra light from UR they dont have a Rubber balancer in them, so all that harmfull vibration is going threw the engine causing serious issues, Oil pump is critical when going to the light crank pulley, if you want a light pulley if you have a B series...Use CTR or ITR they still have the Rubber but yet is light!