Typically TTY bolts are torqued to a spec, and then tightened an angular amount more than that. So, if all you have is a torque spec they are not being torqued to yield. On high performance assemblies, like racing engines, the bolts are frequently replaced with studs. And instead of tightening them to a torque, they are tightened to a given stretch, based on the thickness of the piece you are clamping down, using a dial indicator. A time intensive thing to do. TTY, is a less expensive, and less time consuming way of getting an approximation of the tighten by stretch method. You torque to a preload, and then turn a given angle more. With how much that angle is, calculated based on the the pitch of the fastener. Generally I use new fasteners for things which need to be torqued down. The torque reading is an indication of the amount of preload in the fastener. Since the pitch, or threads per unit length, is known they have calculated backwards to arrive at how much stretch or preload you have at a given torque. There is a critical part of this assumption: The fasteners are clean and properly lubricated. Dirty, or dry threads resulting higher turning friction, and thus you can track the torque, before you actually get tot he stretch you want. If you are reusing fasteners you should use either a direct display electronic torque meter, or an old style beam torque wrench. Both of those will show chatter if the fastener is dirty or under lubricated. The click type torque wrenches don’t. So, starting with new clean fasteners, and running a tap into any tapped holes, and blowing them out with some brake cleaner and then compressed air, goes a long way in assuring things are clean. A small amount of Loc-Tite blue is a good lubricant in most applications.
