The following documentation is about a much maligned aluminum jack stand structure known as Big Red ("BR"). I am not debating whether BR has acceptable properties for its intended purpose such as horizontal stability or load holding capacity. Each individual must make that judgment based on their experience, knowledge, perception and gut feeling, and being new to the game I'll admit it's been a difficult decision whether or not to work under a car with precious little ground clearance!
In the end the desire to perform my own work and have that feeling of a job well done has overcome my fear of the work area itself, but I have taken what I consider to be the necessary precautions by building my own jacks.
Now back to BR, or what I call the Little Jack Stand That Could!
I took BR to a testing lab here in Seattle that has on site a hydraulic press capable of producing 300 tons of force (600,000 lbs). Now even BR cannot withstand that kind of pressure but I think you may be surprised at what the little fellow can handle. Kindly read on.
The operator, the machine, and BR. Note that BR is not crapping his aluminum pants even as his fate awaits. I must admit he looks small and helpless.
A paltry 1,070 lbs
A few minutes later and BR is napping under 10,470 lbs
BR yawns at 15,380lbs
BR starts to break a sweat at 21,810 lbs
The little fellow finally throws the towel in at a whopping 23,550 lbs!!!
Even so he still looks pretty good after having 23,550lbs on his head. Notice the aluminum has started to deform underneath the pin.
This would seem to indicate you could stack six 3,300lb Z's nose to tail on a single BR if you had to.
The conclusion I've drawn is that I will use a couple BR's as backups to my wooden stands and feel exceedingly safe.