Block Outdoor Unit Rocks: Mounting Bolt Corroded and Seized

Knocking from an outdoor unit with one corner sitting slightly lower than the other three. Three of the mounting bolts tighten normally when tried. The fourth will not move in either direction at all.

A corner that has dropped while its own bolt is immovable is a specific combination. It is worth reading carefully rather than simply tightening everything again. A seized fixing has stopped being adjustable, so the corner it holds cannot be brought back into line. Everything the assembly does from then on pivots about the three that still work. Trying each fixing for movement, rather than merely checking they feel tight, is what surfaces this.

Threads corrode and eventually bind, and once that has happened the fixing is locked wherever it happened to be at the time. If that position leaves its corner low, the unit sits unevenly and rocks about the fixings that still function properly. Tightening the other three does not help in the slightest, because the problem is a corner that cannot be raised rather than one that has come loose.

The seized fixing is cut out and replaced, which restores adjustment and lets the unit sit level once more. Trying every bolt for movement during any inspection is the habit worth building here, because a seized one gives no warning at all and reads as reassuringly solid. Anti-seize compound on replacement fixings in an exposed position is a small addition that prevents the whole thing repeating.