Several months ago I went on a business trip to the enormous Boeing factory in Everett, WA, where I was able to crawl on and around several 747s, 767s, and 777s airliners in various stages of assembly.
I noticed that the engines aren't attached until the rest of the plane is pretty much assembled. To keep the airplanes from tipping while workcrews are in the fuselage, enormous weights are hung from chains on the wings, dangling above the floor like bizarre Christmas ornaments. Each weight is a solid slab of steel the size of golf cart and is painted with florescent, reflective yellow paint.
Most interesting is the boldface label stenciled on each side of the weight: "7,800 LBS. REMOVE BEFORE FLIGHT."