Prototipo Works

Prototipo Works - Under Flush Bidon Cage Boss Blank 5.5mm (Stealth Black)



Prototipo Works M5 316 Stainless Steel Underflush Bidon Cage Boss Blank. 3mm M5 Metric thread. Excellent finishing touch to all builds.

Used to neatly and professionally blank bidon cage bosses when not in use. Designed to visually disappear under flush in most bidon bosses.

Designed and engineered in Melbourne, Australia.


Features Narrow head allows it to disappear in most bidon mounts
Taper Tech thread base for easy installation
Precision CNC machined from 316 Stainless Steel
Precision laser etched Prototipo Works logo
Sold as pair 
Finish Natural CNC machined finish with Stealth Black Oxide Coating
Used  in motorsport and military applications to reduce glare
Our  strong 316 Stainless Steel cannot be coloured through anodising. The Stealth Black Oxide coating’s primary purpose is not to provide additional abrasion resistance like our PVD coated parts but provide a unique black finish perfectly fitting to any considered bicycle build
Our  Stealth Black Oxide coating, uniformly coats our 316 Stainless Steel parts all over creating a light protective and dull cosmetic black appearance
Th e Stealth Black Oxide coating may dull and grey over time pending the riding environment and cleaning conditions its subjected to.


  • Prototipo Works strongly recommends the use of a lithium based anti seize assembly lubricant during the installation of our hardware. An anti seize assembly lubricant helps prevent seizing, galling, cold-welding, and the fusing of metal on metal threaded components. Ideal for salty environments, humid environments, in the presence of water, moisture, sweat and ingress.

  • Please ensure all mating threads and installation tools are the correct size, are clean, free of damaging burrs and inserted straight.


  • Bidon cage boss hole opening diameter must be at least 5.6mm or greater (our under flush bolt is 5.5mm in diameter) with at least a 3mm unthreaded depth before the thread begins into the frame.

  • Where appropriate, we also offer a suitable version for a 5.1mm opening hole diameter or greater (our alternate under flush bolt is 5.0mm in diameter).

  • If unsure if it will fit correctly on your bicycle, please contact us.

Special Note: 

Prototipo Works engineer custom products and therefore appearance may vary slightly. 

Please ensure all mating threads are the correct size, are clean, free of debris and damaging burrs. Please ensure adaptor is aligned and threaded onto the valve straight and as intended.

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