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Replacing a Broken Brake Bleed Nipple


Replacing a Broken Brake Bleed Nipple

The center hole in the nipple acts as a guide when left-hand drilling a broken nipple out.

John H Glimmerveen

When a motorcycle is new, the component parts are recently assembled. If the mechanic at the factory or the dealer's assembled everything with assembly lube, or even Loctite, disassembly will be relatively easy.

For the classic motorcycle mechanic, the bike may have rusted in a barn for thirty years and many of the assembled metallic components will have seized. In most cases this seizing is a result of electrolysis.

In Chemistry, electrolysis is defined as the migration of positive and negative ions through an electrolyte solution. In practical terms, electrolysis associated with metal parts manifests itself as the joining of two metal parts by their surface oxides.

One particular item prone to seizing is the brake bleed nipple. In most applications the bleed nipple has a small diameter hole in the center to allow fluid to pass through once it has been loosened for bleeding. Unfortunately, this center hole makes the nipple weak and can lead to fracture during turning if the threads are corroded through electrolysis.

Removing a broken nipple is relatively simple. As the nipple has a center hole to guide drilling, the mechanic can use a left hand drill which in most cases will unscrew the broken nipple.

The tools required include:

  • Left hand drill set-ideally in 1/16" increments from 1/16th" to 5/16"
  • Electric hand drill
  • Small heat gun
  • WD40 or its equivalent
  • Brake parts cleaner
  • Safety glasses

In most cases, it is advisable to remove the caliper from the bike to repair a broken brake bleed nipple. This way the caliper can be located in a soft jawed (aluminum strips) vice. However, the procedure can be done on the bike too.

Although the bleed nipple may be broken, often times the seal at the base of the nipple is still secure, therefore the system will contain fluid. To drain the fluid simply loosening the hose at the fitting and drain it into a suitable container.

Before attempting to drill out the broken nipple, it is good practice to soak it with WD40 twenty four hours before commencing the drilling process.

With the fluid drained and the nipple soaked in WD40, the nipple can be drill out. However, the caliper should be warmed first with the heat gun to expand the aluminum around the broken steel nipple.

The first drill to use will be slightly bigger than the center hole in the nipple (typically 1/8"). Increased resistance will be felt when the drill hits the hole's base: do not try to drill any further. If the nipple did not unscrew out during the first drilling, increase the drill size by 1/16" and repeat the process.

Should the nipple still not come out with subsequent drill size increases up to a maximum of the tapping size for the thread type and pitch in use, it may be necessary to take the caliper to a specialist engineering shop that will use spark erosion (Electric Discharge Machining, or EDM) to remove any remaining metal from the broken nipple.

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