McDonnell & Miller 51SHD Replacement Head CapMcDonnell & Miller 51SHD Replacement Head High Cap for 51S 135800 is a high quality replacement head cap designed to restore proper operation of your boiler control system. This durable component provides a secure seal and reliable safety performance for everyday industrial use. Engineered for compatibility with the 51S family, the 51SHD offers rugged construction, corrosion resistance, and easy installation. Keep your boiler running safely and

Simple wiring and maintenance to streamline servicing

This model helps prevent accidental activation and tampering

hydronic and radiant heating

Sensor: Bulb-type coiled copper for precise readings

High-volume output suited for demanding systems

Designed to fit C/cl/cu/cp configurations with M10 threads

Shaft: ceramic

Ideal for hot and chilled water loops

with clear flow specifications of 6–10

Beckett igniter kit for reliable ignition and compatibility with Beckett systems

it carries UL and CSA listings

Interlock Lockout