The clamp secures the quartz sleeve in its seat at the reactor end, holding the sleeve against the seal that keeps water out of the lamp cavity. It is a small part with a disproportionate consequence: a clamp that has relaxed or been refitted loose allows the sleeve to move under flow and lets water past the seal onto the lamp, which ends the lamp and can end the sleeve with it. Sized 46 mm — check the diameter of the sleeve in the reactor before ordering.