8/10mm Flare Nut Wrench

A flare-nut wrench is the wrench that hydraulic-line work requires once you've rounded one compression nut and learned why a regular open-end wasn't enough. The 8/10mm Flare Nut Wrench gets purchase on five of the six flats of a compression nut while still letting the hydraulic line pass through the slot at the wrench's open side. Where an open-end wrench loads two flats and four corners, the flare-nut wrench distributes the load around the fitting, which is how compression nuts stay un-rounded over many service cycles.

The 8 mm and 10 mm sizes cover the larger end of the bicycle hydraulic-brake fitting range. Some hydraulic systems run line diameters wide enough to reach 10 mm at the compression nut, particularly on MTB builds and on a number of current road hydraulic systems where the line dresses with a larger fitting. A shop that services mixed road and MTB hydraulic builds reaches for this wrench more often than for the smaller-sized flare nut, because the larger fittings are where rounding happens fastest under the higher torque those nuts get.

The wrench body is low-profile to reach fittings tucked between the caliper body and the frame or fork, which is common on integrated road bikes where the line dresses tight against the cable port. Forged steel construction holds the flat-to-flat tolerance over many bleed cycles; trivalent chrome plating wipes clean between jobs without flaking, which matters on a tool that touches brake fluid.

Specs

• Function: flare-nut wrench for hydraulic-line compression nuts
• Sizes: 8 mm and 10 mm
• Six-point profile (five flats engaged through the slot)
• Construction: forged steel
• Trivalent chrome plated per ISO 1456:2009
• Low-profile body for tight caliper-to-frame clearance
• Article number: 183/2

Built in Zreče, Slovenia

Unior has been forging hand tools in Zreče since 1919, and is the official technical partner of multiple World Tour and downhill teams. Flare-nut wrenches sit in the line of forged hand tools the company has been making for over a century, and the workshop-grade tolerances on this 8/10 are the kind that matter only the first time a customer's caliper compression nut tries to round on you, after which you don't go back to anything that isn't right.

Pro tip from our mechanics

A compression nut wants to be torqued to the manufacturer's published spec, not by feel. Over-torque crushes the olive (the soft-metal sealing ring inside the fitting) and the next bleed comes out contaminated with smashed olive material; under-torque leaks. The 8/10 doesn't accept a torque-wrench drive directly, so the routine is: snug the nut by hand with the flare-nut, then verify with a torque wrench that does accept a flare-nut crowfoot or a torque-compatible flare wrench like the 1760/2 in the same kit. Our wrench-selection guide covers the trade-offs on torque control and which Unior wrench fits which fitting on the system: Choosing the right wrench for hydraulic disc brake service →