Park Tool SBC-1 Spoke Ruler
This handy shop aid is ideal for sizing spokes, ball bearings and crank cotters. To use, just hang the head of the spoke in the oblong hole and read its length on the scale. For crank cotters and ball bearings it works just like a gauge for drill bits. Spoke lengths are in both metric and fractional markings. Made of anodized aluminum with embossed markings to allow easy reading and long life.