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Can you help me understand the general "torque-to-yield" concept, which is often used for head bolts. In the case of my Tacoma, the head bolts are specified to be torqued to a spec of 27 ft/lbs and then turned another 180 degrees.

As I understand, and I am asking if this understanding is correct, the torque spec represents a state that can support the elasticity (preserving/restoring the original length/shape of the fastener) and the additional turns (in my case 180 degrees) goes beyond that to "seal" the clamping force by disfiguring the fastener. Is this a correct understanding? Are there other instances of using this concept beside head bolts?

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A torque to yield (T2Y) fastener can be used in most any situation, though head bolts seem to be the most prevalent. The reason T2Y fasteners are used is, even with regular bolts, you are trying to get to a certain stretch length of the fastener. You can usually achieve it through a torque value, but a torque value is a general indication of the stretch, not an exact measure of stretch. With T2Y bolts/fasteners, the stretch is measured through the angle added at the end. A T2Y fastener deforms during this process, which is why they should not be reused. The deformation of the fastener actually gives a lot more accurate stretch and load then does a regular fastener.

I think you basically have the concept correct, but maybe have the fine points a bit off (or aren't explaining them correctly). The original torque value is usually to get all bolts to a clamping force which is equal across the entire fixture and the same for each bolt. Once the initial load is set, then the addition of the angle (in your instance, 180° turn) is used to put a stretch on the bolt causing the additional clamping force onto the fixture. The reason for the initial torque sequence is so all fasteners will be starting at the same point. If you tried to apply 180° from where you've hand tightened them, how do you know each will have the same results? You wouldn't.

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  • Also, I just broke my angle torque gauge, well not really broke it but the POS paper gauge under the plastic dial got unglued so it turns together with the dial, which is no good. My question is, do you think turning 180 by eye-estimating (e.g. two approx 90 deg turns) is precise enough or do I basically need to be more exact?
    – amphibient
    Apr 4, 2022 at 17:03
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    Especially with head bolts, the need for accuracy is fairly important. If you, for whatever reason go past the threshold of where the fastener completely deforms, there's no going back. I don't know what those tolerances would be for any given fastener (and I'm sure they are all different), but once you've done your two final torque stages, you'll never know if you were on target or not. If you're +5° on one bolt, then -5° on another, that's quite a bit of variance, yet would be easy to hit without an angle gauge. You've had so many issues with this thus far, do you want to chance it? Apr 4, 2022 at 18:12

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