Bolt recommendation

[gtstorey]

Originally Posted by NOT A 45

Tensile strength, and thread pitch are used to determine how much rotation (angle) is required to reach a bolts yield. When we refer to yield it means the maximum clamping force of the fastener. A torque to yield bolt can in fact be reused however, to reach it's yield additional rotation is required. You would have to be a engineer to calculate how many additional degrees would be required, and also with each use the yielded clamping force is reduced. This is why they are recommended to be replaced after the first use. Achieving a fasteners yield is the only reason for the torque angle method of tightening.

I'm not sure that is true. Once the bolt yields, it has a narrow window that the clamping force doesn't go up with additional load being applied, which is why it's used. You "might" be able to get another use out of it and not reach the point of strain hardening, at which point the clamping load would increase with more load added and the bolt would start to become more brittle. After some point the bolt would really start to stretch, the bolt cross section would reduce and the clamping load would go back down until the bolt breaks. (It has been 30 years since I had mechanics of materials class, but the stress strain diagram is actually pretty simple)

Of course this is based upon Torque to Yield bolts being in the "yield" range and not being a misnomer, but it matches what I I have read behind the true purpose of TTY fasteners which is to apply and exact clamping load by being in the yield range.

https://en.wikipedia.org/wiki/Stress...93strain_curve

[NOT A 45]

Originally Posted by gtstorey

I'm not sure that is true. Once the bolt yields, it has a narrow window that the clamping force doesn't go up with additional load being applied, which is why it's used. You "might" be able to get another use out of it and not reach the point of strain hardening, at which point the clamping load would increase with more load added and the bolt would start to become more brittle. After some point the bolt would really start to stretch, the bolt cross section would reduce and the clamping load would go back down until the bolt breaks. (It has been 30 years since I had mechanics of materials class, but the stress strain diagram is actually pretty simple)

Of course this is based upon Torque to Yield bolts being in the "yield" range and not being a misnomer, but it matches what I I have read behind the true purpose of TTY fasteners which is to apply and exact clamping load by being in the yield range.

https://en.wikipedia.org/wiki/Stress...93strain_curve

I think were saying basically the same thing. The bolts yield is not a specific number, but rather the maximum clamping force a bolt has. The bolt has to be stretched further each time it's used to reach its yield however that maximum clamping force is reduced each time it is stretched.

I remember way back in 97 I attended a class on the new LS1. I watched a video of them re using head bolts. They used the same bolt 9 times and showed it gradually needing more angle to reach its yield. The 10th time it snapped.

[gtstorey]

It would be interesting to see the actual clamping load as they turned the bolt more. They were probably actually increasing the clamping load after the first few times as the bolt "strain hardened" and then the clamping load would have dropped off as it started to fail.

[NOT A 45]

Originally Posted by gtstorey

It would be interesting to see the actual clamping load as they turned the bolt more. They were probably actually increasing the clamping load after the first few times as the bolt "strain hardened" and then the clamping load would have dropped off as it started to fail.

They graphed it for each use. The second use was close to the first but took an additional 15 or 20 degrees to reach. Each additional use the clamping load went down dramatically and each time requiring more degrees to reach yield until finally it broke.

I think they were trying to drill into our head not to reuse the bolts since we were just coming from the small block era where you could use them over and over again.

After seeing that, I've never re used a torque to yield head bolt!

[NOT A 45]

And I also learned once you've reached the yield of the bolt, turning it further actually reduces the clamping load.

[gtstorey]

The "yield" point maybe actually somewhere into the strain hardening portion and beyond the typical engineering definition of yield. Or maybe the window of yield vs strain hardening is so small for the bolt material that it is indistinguishable.

[NOT A 45]

Originally Posted by gtstorey

The "yield" point maybe actually somewhere into the strain hardening portion and beyond the typical engineering definition of yield. Or maybe the window of yield vs strain hardening is so small for the bolt material that it is indistinguishable.

Basically what I came away from the lesson was accurately applying a even clamping load across the cylinder head or other critically important components. With the old type of just torqueing the bolts to 75 lb/ft it doesn't account for any friction in the fastener so you end up with un equal clamping force. I have found this to be true using my torque angle torque wrench installing a set of heads. After the small initial torque to mate the surfaces, my tool on the angle pass displays the actual turning torque after applying the degrees of bolt rotation. Those numbers are all over the place! Generally like 15 ft/lb from one bolt to the other.

[barkeater46]

True enough ! When Volvo took over Mack trucks and we got the Volvo engine, all critical fasteners were toque/turn, but were reusable 4x. The clincher was the bolts were to be center punched on the head to show how many time they were torqued. LOL. They have since done away with reusing critical bolts, one time only. Many other bolts are also one time use, but solely because they are patch lock coated or nitrate treated.

[gtstorey]

That is my understanding of why you want to be in the small flat spot between normal elastic range (straight rise over run portion of the diagram) and the curved portion of strain hardening. In that portion you actually have some different amount of rotation that won't increase the clamping load so they all end up clamping the same amount. At least theoretically.

[Rock-It Man]

Originally Posted by NOT A 45

Basically what I came away from the lesson was accurately applying a even clamping load across the cylinder head or other critically important components. With the old type of just torqueing the bolts to 75 lb/ft it doesn't account for any friction in the fastener so you end up with un equal clamping force. I have found this to be true using my torque angle torque wrench installing a set of heads. After the small initial torque to mate the surfaces, my tool on the angle pass displays the actual turning torque after applying the degrees of bolt rotation. Those numbers are all over the place! Generally like 15 ft/lb from one bolt to the other.

There's going to be variation in torque readings at a given angle, from bolt to bolt. That's because bolt torque is not a direct measurement of clamping force. Bolt torque varies with thread friction. That's why measuring the turn angle is more accurate; it correlated directly to how much the bolt stretches, aka strains.