An implantable medical pulse generator is disclosed herein. The pulse generator is for administering electrotherapy via an implantable medical lead having a lead connector end on a proximal end of the lead. The pulse generator includes a can and a header coupled to the can. The header includes a first lead connector end receiving receptacle and a retainer configured to secure the lead connector end within the first receptacle. The retainer includes a member and a first collar, which is coaxially aligned with the first receptacle. The first collar includes an inner circumferential surface and a gap in the inner circumferential surface. The inner circumferential surface extends generally continuous and unbroken between a first face of the gap and a second face of the gap. The member is configured such that acting on the member causes a gap distance between the first face of the gap and second face of the gap to decrease, thereby reducing an inner circumferential diameter of the first collar.