A subassembly for a high pressure fuel injector control valve includes, a control valve member and a sleeve attached to a stem of the control valve member, the sleeve extending along at least a partial axial length of the stem of the control valve member, and wherein a clearance, provided between the sleeve and the control valve member, is in fluid communication with the control chamber such that during use, pressure compensation occurs, as the sleeve is caused to expand and seal against the control valve bore, thereby preventing leakage of fuel within the control valve assembly.