McDonnell & Miller 63-M-HD Head Mechanism

The McDonnell & Miller 63-M-HD Head Mechanism (Part Number 143200) is a heavy-duty replacement head mechanism designed for the Series 63 Low Water Cut-Off. Engineered for both hot water and steam boilers, this manual reset mechanical control ensures critical safety by interrupting the electrical circuit to the burner in 15 psi steam or 50 psi water applications. It features a robust float-operated design and includes the No. 2-M switch, providing reliable SPDT action for low water protection and alarm signaling. As a head mechanism only, it allows contractors to perform a complete rebuild of the internal components—including the bellows assembly and float mechanism—without the need to cut into or disrupt the existing 1" NPT equalizing pipework, significantly reducing labor time on-site.

Key Benefits

• Includes the No. 2-M switch with an integrated manual reset to prevent automatic burner restarts after a low water event.
• Rated for a maximum boiler pressure of 50 PSI (3.5 kg/cm2), making it compatible with residential and commercial hot water and steam boilers.
• Simplifies maintenance by providing a drop-in replacement head mechanism that utilizes the existing float chamber and piping connections.
• Rugged construction featuring a packless bellows assembly to ensure long-term leak prevention and mechanical sensitivity.
• Specifically compatible with Series 63 and 63-M models for seamless field integration.

Common Questions

Can I use the 63-M-HD on a steam boiler with higher than 15 PSI?
While the McDonnell & Miller 63-M-HD is rated for up to 50 PSI in water systems, for steam service, it is primarily utilized in low-pressure applications; always verify your specific boiler's operating range against the 50 PSI maximum threshold before installation.

Does this unit include the gasket and mounting bolts?
Yes, this head mechanism typically includes the necessary CO-12 head gasket and hardware required to seal the mechanism to the existing float chamber once the old assembly is removed.

Why choose a manual reset over an automatic reset for low water protection?
A manual reset mechanical low water cut-off requires a technician or building operator to physically inspect the boiler for the cause of the water loss before restarting, providing an essential layer of safety in commercial and industrial environments.

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Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.