Older Block Outdoor Unit Rattles: Storms Loosened Fan Guard

A rattling guard on an older block, loose again within months of the last time it was secured properly. The position takes wind-driven rain repeatedly through the year. The fasteners are all original.

Two things have to be separated here: whether the fixings were tightened properly, and whether they were ever capable of staying tight. Looking at the threads answers the second one. Corrosion between the faces means the fastener has been gripping through a layer that keeps changing, so it was never going to hold for long however carefully the work was done. That shifts the answer from technique to material.

A fastener grips by pressing metal faces together, and corrosion products between those faces are soft and keep changing. Each wetting adds a little more, each drying leaves it flaking, and the clamping force falls away with it. Vibration then moves the fastener easily, because nothing solid is left holding it in place. Neither the vibration nor the tightening is unusual here; the interface between them has stopped doing its job.

Corrosion-resistant fasteners go in, which changes the outcome rather than repeating it. Retightening the original ones is legitimate as a stopgap and should be described as exactly that, because they will loosen again on the same cycle as before. Where a position takes weather directly, the fastener specification counts for much more than the force applied to it during fitting.