Also available in brass, this is the part that seals the valve body to boiler fitting closed: there is no gasket at the seat end, it's a metal-on-metal seal. The associated gasket found on this shaft prevents steam from escaping out the knob side of the valve.
Attention CA Residents: Prop 65 Warning
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.