Prototipo Works

Prototipo Works - Under Flush Bidon Cage Boss Blank 5.0mm V2 (Stealth Black)



A new addition! The same under flush bolt design as our original under flush, but with the following changes:

  • A smaller head diameter of 5.0mm to increase compatibility across more frames.
  • A shorter head height of 1.5mm to nestle into a select number of bidon mounts in circulation.

This under flush bolt utilises a 2.5mm Hex.

Product Summary: 

  • Prototipo Works M5 316 Stainless Steel Underflush Bidon Cage Boss Blank.
  • 2.5mm Hex, 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. 

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.

The 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 5.1mm or greater (our under flush bolt is 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 5.6mm opening hole diameter or greater (our alternate under flush bolt is 5.5mm in diameter).

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


  • Warning! Choking Hazard, Small Parts.
  • These special unique underflush bolts are for blanking purposes only! Not designed to take loads.
  • Product appearance may vary slightly.
  • Use at your own risk!

Special Notes: 

  • Will visually disappear under flush on a majority of mass produced major OEM produced bicycles (see assembly notes above).
  • May not visually disappear under flush or install correctly on bespoke, low volume custom bicycles

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