Blowout preventer (BOP) and method for sealing a well. The BOP includes a body having first and second conduits, the first conduit being substantially perpendicular on the second conduit; a piston extending through the first conduit and being configured to reciprocate inside the first conduit, the piston having a body portion, a neck portion and a head portion in this order; a ram block disposed on the piston and configured to move with the piston inside the first conduit for closing the second conduit; and a shim configured to fill a gap between a back region of the ram block and the body portion of the piston.