Note what is said here http://www.autobodypro.com/tektips/a...nutsnbolts.htm
"One-time-use, or torque-to-yield (TTY), fasteners take advantage of the torque yield principle. Most torque specifications build in a 25 percent safety margin--they can be overtorqued by 25 percent before damage occurs. TTY bolts do not have this safety zone. They are designed to be tightened just into the yield point but not to the extent that the metal is distorted. The advantage is they deliver 100 percent of their intended strength, versus the 75 percent strength provided by regular bolts. The drawback is that they can only be used one time. Once they are removed from an application, they can no longer support the load they previously did, and they must be replaced with TTY bolts. Installing a conventional fastener in their place will result in a 25 percent weaker clamping force. Because TTY fasteners are used in a variety of applications, such as subframe mountings, it's important to identify these during the estimating process--not just for the additional cost involved but also to alert the technician."