Recommended Additional Upgrades
The RCB S1-FL 17mm is the absolute flagship of the Racing Boy master cylinder lineup. It takes the high-volume performance of the standard 17mm S1 and adds the “FL” (Forged Luxury/Folding) refinements.
If you are building a “dream spec” bike with high-end 4-piston calipers (like the RCB R1), this is the most advanced control unit you can put on your handlebars.
What sets the S1-FL 17mm apart?
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The 17mm “Big Bore”: This isn’t just a daily commuter part. The 17mm piston is a high-volume pump. It is designed specifically to push enough fluid for large 4-piston calipers or twin-disc systems. If you use this on a small stock caliper, the lever will feel too stiff; but on a 4-pot R1 caliper, it feels like “pure power.”
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Forged Strength: The “FL” series uses a forged aluminum body. Forging compresses the metal, making it much stronger and more rigid than cast or even some CNC parts. This means zero body flex even under extreme racing pressure.
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The Folding (Collapsible) Lever: This is the signature “FL” feature. The lever is hinged so that in the event of a crash, it folds upward instead of snapping off. This can be the difference between riding home and being stranded.
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Enhanced Leverage: The S1-FL lever is slightly longer than the original S1, giving you more mechanical advantage. This makes the 17mm bore feel easier to pull, providing “one-finger” braking precision.
The “Perfect Build” Synergy
In the world of RCB modifications, the S1-FL 17mm is usually the “brain” of the system, and it is almost always paired with:
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Caliper: RCB R1 4-Piston Monoblock.
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Lines: Braided Stainless Steel lines (to handle the high pressure).
Performance Verdict
The S1-FL 17mm is for the rider who wants zero compromises. It offers the most direct “radial” feel, meaning you feel every vibration of the brake disc through your fingertips. It transforms the braking experience from a “guessing game” into a surgical tool.
Is this for a high-performance street build, or are you preparing a bike for the track?
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