Losing a crank arm preload cap on the trailhead or the workbench is one of those small parts problems that stops a build in its tracks — the crankset won't hold preload without it, and the plastic caps aren't cross-compatible across every Hollowtech II generation. Shimano's FC-T661 Crank Arm Fixing Bolt (part number Y1KS13000) is the replacement preload cap for a wide swath of Hollowtech II cranksets, covering road, trekking, cyclocross, and mountain applications from the same design era.

The bolt threads into the left crank arm and sets bearing preload before the two pinch bolts on the crank arm get torqued down. Shimano uses plastic construction here intentionally rather than metal — the softer material prevents overtorquing, which protects the bottom bracket bearings from being crushed under excessive preload. Installation requires the splined preload tool, either Shimano's TL-FC16 or the Park Tool BBT-9 / BBT-10 that most home mechanics already have on the pegboard.

Cross-reference before ordering. This is the correct bolt for a long list of cranksets including Ultegra FC-6700 and FC-6703, 105 FC-5700 and FC-5703, Tiagra FC-4600 and FC-4603, Sora FC-3500 series, along with FC-CX70, FC-CX50, FC-RS500, and FC-RS400 on the road and cyclocross side. On the mountain and trekking side, it fits FC-T661, LX, Deore, and Alfine FC-S501 cranksets from the same generation. If you're running a newer Hollowtech II crank like Ultegra R8000 or 105 R7000, verify the part number against Shimano's tech docs — later generations use a different preload cap design and this one won't be a match.