4. Always install fresh U-bolts with your new leaf springs. Used U-bolts can be weakened from rust, stretching, damaged threads, and so on. It’s cheap insurance to keep your axles under your 4x4. Don’t forget to cut off the excess threads once they are torqued to spec. A 4 1/2-inch angle grinder loaded with a 0.045-inch cutoff wheel makes quick work of overly long U-bolts.