On a two-stage snowblower the auger is held to its shaft by a few soft bolts called shear pins. When the auger meets a frozen newspaper or a rock under the snow, a pin snaps, the auger stops, and the gearbox behind it is spared. Small outboard motors used to do the same with a pin through the propeller. Owners are told to keep spares on hand, because the snapping isn’t a fault; it’s the part doing its job. The mistake the parts guides warn against is replacing a broken pin with an ordinary, stronger bolt from the drawer. The machine runs fine until the next rock, and then what breaks is the thing you can’t fix in the driveway. A shear pin is the one part whose success looks, to anyone watching, exactly like a breakdown.