Abstract: A rotary valve for blocking opposed apertures of a valve body includes a central assembly comprising laterally offset hemi-cylindrical first camming surfaces on an exterior of a rotatable spool and substantially hemi-cylindrical seal plates with hemi-cylindrical first cam following surfaces which engage the first camming surfaces. Stop pins fixed to the valve body run in rebates in the seal plates to limit rotation of the seal plates to 90°. Each of four clearance members is supported by a wedging block loosely located in a recess in the spool, slidingly extends through a radial slot in one of the seal plates and prevents the spool jamming the seal plate against an inner face of the valve body during closing of the valve until a projecting end of the clearance member enters a recess in the valve body.