An improved replaceable lamp unit for use in an automobile headlight wherein the exterior surface of the unit's metallic insert is provided with a roughened surface (e.g., 125 RMS) such that an improved bond between the insert and the unit's plastic holder is possible. Alignment between the unit's lamp (e.g., tungsten halogen bulb) and holder, and thus the reflecting surfaces of the reflector in which the unit eventually will operate, is significantly improved, as is the time for attaining same. The roughened texture may be sandblasted, knurled, acid-etched or embossed.