The largest of the cap and neck sets at 58mm, intended for substantial header and expansion tanks. A wide neck earns its place on race and heavy-duty cooling systems for practical reasons: filling a large-capacity system through a small neck is slow and prone to airlocks, whereas a 58mm opening lets coolant in quickly and gives you a clear view of level and condition. It also makes flushing and refilling between events much less of a chore. Weld the neck square to a flat, unstressed area of the tank top and make it the highest point in the circuit so trapped air can be purged on filling.