A fluid delivery device comprises a fluid reservoir for containing medicament. The fluid reservoir is sealed proximate one end with a sliding seal piston. A delivery path is configured to fluidly couple the fluid reservoir and a patient wearing the fluid delivery device. A basal engine mechanism is configured to directly or indirectly move the sliding seal piston in the fluid reservoir at a controlled basal rate. A bolus mechanism configured to move the basal engine relative to the fluid reservoir and directly or indirectly move the sliding seal piston in the fluid reservoir a discrete bolus amount at a time.