A hook wrench engages slotted or notched lockrings; the rings whose engagement points are pockets cut into the outer face rather than splines or prongs. The 255/2 is the Unior hook wrench sized for the lockring patterns most commonly seen on bicycles: fixed-gear cog lockrings, some bottom-bracket lockrings, threaded-headset lockrings. One tool, several jobs.

Unior makes 14 hook wrench sizes across the industrial-tool catalog. The 255/2 is the size selected for bike-specific use: long enough to give leverage on a tightened lockring without being so long that it doesn't fit between hub flanges or against a frame. The hook itself is shaped to drop cleanly into the lockring's notch and stay engaged through the turn.

Where the 255/2 earns its place

The wrench is a multipurpose tool; three or four jobs on a typical service bike, depending on the build:

Fixed-gear cog lockring. Track and fixed-gear hubs use a small reverse-threaded lockring over the cog that the 255/2 backs off. The Singlespeed Chainwhip 1659/2DP has an integrated lockring tool, but the 255/2 is the dedicated version if you do fixed-gear work in volume.
Threaded bottom-bracket lockring. Older square-taper and ISIS bottom brackets often use a slotted lockring on the non-drive cup. The 255/2 fits the most common notch geometries; oddball patterns may need a dedicated BB lockring tool instead.
Threaded headset lockring. Quill-stem headsets on classic bikes use a slotted top lockring above the adjustable cone. The 255/2 backs it off without resorting to a hammer-and-punch workaround.

The tool isn't a one-size-fits-everything solution; lockring patterns vary by manufacturer and era, and some notches are too narrow or too deep for the 255/2's hook. When the hook drops in cleanly, the tool works. When it doesn't, you've got a different lockring pattern and you need a different tool.

How to use it

Identify the lockring's notch; the slot or pocket cut into its outer face. Slip the wrench's hook into the notch with the handle oriented for the swing arc you need. Turn counter-clockwise (the usual direction for backing off, though some patterns reverse this; check the specific component if uncertain). Apply force through the handle; lockrings are typically tightened to lower torque than cassette lockrings, so they back off with a firm hand-pull rather than a breaker bar.

If the hook doesn't seat, don't force it. Forcing a slightly-mismatched hook wrench rounds the notch and makes the next attempt harder. Either the lockring is a different pattern (check Unior's other lockring-tool sizes) or the notch has already been rounded by prior service.

Compatibility

Fixed-gear and track cog lockrings (most common notch patterns)
Slotted bottom-bracket lockrings (older square-taper, ISIS, some current designs)
Quill-stem headset lockrings
Other slotted/notched rings on the bike that match the 255/2's hook geometry

Specs

Slotted-engagement hook design
Bike-appropriate handle length for between-flange clearance
Trivalent chrome plated to ISO 1456:2009
Article number: 255/2

Built in Zreče, Slovenia

Unior has been forging hand tools in Zreče since 1919, and sponsors professional road and mountain-bike race teams. The hook wrench is one of those tools that started life on industrial benches; Unior makes 14 sizes across the broader catalog for non-bike applications; and the bike-specific version is the size that earned its place through workshop use rather than catalog design. The 255/2 is in the cycling collection because mechanics keep reaching for it.

Pro tip from our mechanics

If you're working on older bikes or doing fixed-gear conversions, the 255/2 ends up in the same drawer as your bottom-bracket and headset tools. The fixed-gear-cog application is the one most riders never think about until the cog needs swapping for a different ratio; on a track bike, that's a 90-second job with the 255/2 and a chainwhip. For multi-job bench utility, the 255/2 is the kind of tool that sits there unused for weeks and then handles three different jobs in an afternoon. The cassette-replacement workflow covers the fixed-gear lockring sequence: When and how to replace your cassette →

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