The Torbeck B661AT-1/2B Brass Tailed Bottom Entry Fill Valve (often sold under the Fluidmaster or Opella brands) is a highly regarded, durable toilet cistern valve that has long been a favorite among professional plumbers for its reliability and ease of installation. 
Key Features and Specifications
1/2" Brass Shank: The primary feature is the robust, high-quality 1/2-inch BSP (British Standard Pipe) brass threaded tail connection. This prevents the common issue of cross-threading associated with plastic shanks and ensures a secure, long-lasting seal.
Bottom Entry Design: Specifically designed for cisterns where the water supply pipe enters through the bottom of the tank.
Quiet & Fast Fill: Engineered to provide a quiet and efficient refill of the cistern, helping to minimize noise in the household.
Exceptional Low-Pressure Performance: A key selling point of the Torbeck valve is its ability to operate effectively and reliably even with low water pressures (as low as 0.1 bar), making it versatile for various plumbing systems.
Adjustable Height: The valve is height-adjustable, typically ranging from 8.5 to 12 inches (approximately 215mm to 305mm), allowing it to fit into most cistern sizes, including compact designs.
Anti-Siphon Design: Incorporates safety features to prevent contamination of the fresh water supply, ensuring compliance with UK water regulations (WRAS approved).
Small Footprint: The compact design allows it to fit into tight cistern spaces without snagging on other components.
Removable Filter: Includes a removable filter for easy maintenance and cleaning of debris. 
This valve is a workhorse component in the plumbing industry, offering a blend of proven design and the professional-grade durability of a brass connection.
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The Torbeck B661AT-3/8B 3/8" Brass Tailed Bottom Entry Fill Valve is essentially the European version of the popular B661AT model. It offers the same reliable performance but features a specific, smaller inlet connection size.
Key Features and Specifications
3/8" BSP Brass Shank: The defining feature is the durable, high-quality 3/8-inch BSP(British Standard Pipe) brass threaded tail connection. This size is standard in many European countries, while the UK primarily uses 1/2". The brass construction prevents cross-threading and corrosion.
Bottom Entry Design: Configured for installations where the water supply pipe enters from the base of the toilet cistern.
Excellent Low-Pressure Performance: Torbeck valves are highly regarded for their robust operation even with very low water pressures, functioning effectively from 0.1 bar upwards.
Quiet & Efficient: Designed to provide a fast, quiet refill cycle and a reliable, positive shut-off seal to prevent constant running water.
Adjustable Height: The valve is height-adjustable to fit a range of cistern sizes, typically around 8.5 to 12 inches (215mm to 305mm).
Anti-Siphon Mechanism: WRAS-approved and compliant with water regulations, it features an anti-siphon design to prevent contamination of the main water supply.
Compact Footprint: The slim design allows it to fit into most cistern configurations without interference.
This valve combines the trusted mechanics of the Torbeck design with the durability of a brass fitting, making it a professional and reliable choice for compatible European plumbing systems.