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I don't want to sound too much like an engineer here, but the cement blocks are designed for a wide distribution of load with soft dirt under them. The jacks are producing a point load on a beveled edge, many thousands of PSI beyond normal. I'm surprised it doesn't just crumple.
One other thing. When I work under my trucks, sometimes I am practically capable of shaking them and rocking them trying to break stuff loose or move a tranny or something. It needs to be secure.
My Dad almost died under a 89' mustang doing the u joints because it rolled off of the stamped steel ramps used up front and the 3ton stands in the back tippedover at the same time. He had blocks under the tires in case the stands failed but when the stands tipped the blocks were now in front of the tires. The only reason he is still alive is the tires were still on. That said, I used stacked three hole blocks at all four corners with 4X4s and 3/4 ply between the front two and back two stacks( blocks were 4 high at the back and three high at the front) and that was an empty frame. He almost clobbered me when he saw it and now I own 2 sets of 12 ton stands
