Package of 4 hinges. Offset pin allows more realistic door movement. With flaps allows gap for air to pass over the flap from underneath which in many applications is more scale and more efficient. Large hinge pad (presumably the one you'd attach to flap/gear door is 16x18mm. Small pad (at hinge pin) is 15x14.6mm. May be screwed or sewn (through factory pre-drilled holes) to surface.