I hung a pair of Lascalas (123 pounds each), 15 years ago on 3/8" threaded rod. They don't go any where! The only problem I had is that after a while, even with jam nuts on both sides of the cabinet wall, the nuts come loose from vibration; maybe once every two or three years I have to retighten them. Of course they are in the same room as the church organ speakers, so there is a whole lot of shaking going on! Everyone who has seen them, thinks that 3/8" rod was over kill, but all agree that they won't come down. Just to give you an idea how strong it is, I can rest my etire weight on the top of the speaker (200 pounds) and nothing moves. 5/8" or 3/4" will be more rigid, but 3/8" is so strong, that there isn't really a benefit to going larger. But, hey, its not that expensive, so if its what you want to do, go for it. If you fasten both ends correctly, they will be there until you take them down.