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Old 13th September 2021, 09:05   #11
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Quote:
Originally Posted by MSS View Post
Kaiser and Lovel,

This is a genuine question. Are you saying that these are not in fact stretch-to-yield (more correctly stretch-to-beyond-yield-point) bolts used on the K-series?

I have seen some previous threads on this topic and note that they were not conclusive.

Personally, I would have thought that to use bolts which stretch into the plastic region would render the whole assembly unstable following temperate cycling that occurs in an engine and therefore the application must be within the elastic region. But this thinking may be somewhat overly simple.

I wonder if the bolts operate close to the yield point but within the elastic region during normal tightening and temperature cycling but may go into the plastic region due to overheating and then become incapable of being reused due to permanent (i.e. plastic) stretch.

Stretch to yield is the wrong term for these bolts, it should be torque to yield.


TTY bolts are designed and used correctly by taking the bolt from its elastic state to its plastic state by taking it past the yield point between the two states, to attain the required clamping force, expansion and contraction due to temperature changes of all the components within the clamping area, metals with different thermal characteristics including the bolts, is taken into account at the design stage to ensure that when the assembly is subject to max and minimum working temperatures, the clamping force is within spec.


Another more descriptive term for these bolts is TTT, Torque Turn to Tighten, torque initially, then turn ( x degres )to tighten.


Even after being fastened to the correct setting taking it into the plastic region, the bolt will still have elastic properties.



If the temp rise goes over the design max, which has a safety factor built in, a hose bursts and goes unnoticed, then look out for HG problems.
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Old 13th September 2021, 09:20   #12
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I deliberately used that particular term because we are in fact debating whether the bolts operate below or above the yield point i.e whether the deformation of the material (i.e. the stretch or elongation) is elastic or plastic.
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Old 13th September 2021, 09:28   #13
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Quote:
Originally Posted by MSS View Post
I deliberately used that particular term because we are in fact debating whether the bolts operate below or above the yield point i.e whether the deformation of the material (i.e. the stretch or elongation) is elastic or plastic.

If a used bolt is the same length as a new one it hasn't been over the yield point and has stayed in the elastic region.


If it is longer, then it has been over the yield point and into the plastic region.
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Old 13th September 2021, 09:33   #14
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There are a number of experiments where a rod of metal is stretched in a machine and the stress and strain recorded. Quite a number can be found on YouTube.
There is always quite a noticeable deformation taking place, before the rod breaks, and there is a noticeable narrowing as well.
The stretch caused by thermal expansion is most likely only a fraction of the stretch caused by the thread, and nowhere near the yield point of the bolt, in the k4.
I think the confusion arises by some people using the term yield as a simple reference to the bolt "giving" and being designed to do so.
The "yield" point is however quite specific in engineering terms, as the point of no return. I think this could be the cause of some misunderstanding.
I have never seen a k4 bolt stretched. I have however seen a few V6 bolts broken, but they are much shorter. I would not re-use a V6 head bolt, but quite happily a k4.
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Old 13th September 2021, 09:49   #15
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Not really sure where to begin with that - credit where it's due for some useful information, but it didn't start off well with the description of 'one of the most unreliable power trains ever made'.
Things only went downhill from there!

No mention of liner heights or the fact an elastomer HG must be used if they're low or uneven is my biggest bugbear - basic stuff that absolutely must be followed for trouble-free K Series repair.
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Old 13th September 2021, 10:21   #16
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Quote:
Originally Posted by kaiser View Post
There are a number of experiments where a rod of metal is stretched in a machine and the stress and strain recorded. Quite a number can be found on YouTube.
There is always quite a noticeable deformation taking place, before the rod breaks, and there is a noticeable narrowing as well.
The stretch caused by thermal expansion is most likely only a fraction of the stretch caused by the thread, and nowhere near the yield point of the bolt, in the k4.
I think the confusion arises by some people using the term yield as a simple reference to the bolt "giving" and being designed to do so.
The "yield" point is however quite specific in engineering terms, as the point of no return. I think this could be the cause of some misunderstanding.
I have never seen a k4 bolt stretched. I have however seen a few V6 bolts broken, but they are much shorter. I would not re-use a V6 head bolt, but quite happily a k4.

Thank you. I understand the theory, the experiments and deformation (necking) that occurs when rods are placed under tensile stress - I did the experiments regularly when I was a slip-of-a-lad studying my City & Guilds General Course in Engineering and Shipbuilding. I now teach this stuff to engineering students as a hobby!

What I am trying to get to the bottom of is whether the bolts when torqued-up in accordance with MGR's specification are below or above the yield point i.e. in the elastic or the plastic region. My guess is that they are in the elastic region but not far from the yield point and can enter the plastic region under certain circumstances - perhaps temperature cycling, reuse etc.Therefore the term torque-to-yield is used loosely (although not unreasonably) but does not mean that the bolts are torqued up to the yield point on the stress-strain curve i.e. they do not permanently stretch under normal circumstances.

Alternatively, that the bolts are in the plastic region when torqued-up to spec and the above term is being used precisely. But they go out of spec due to further stretching as a result of the above types of circumstances.


If the above were not the case, why would MGR's repair manual for the k4 specifically document that the length of the bolts should be measured and the bolts discarded if above a certain length? The possibility of a change in length implies that at least some bolts can be found to have entered the plastic region.

PS marinabrian attached the relevant page of the k4 repair manual specifying the need for length measurement in a previous thread on the topic during 2016.

Last edited by MSS; 13th September 2021 at 11:26..
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Old 13th September 2021, 10:54   #17
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Quote:
Originally Posted by kaiser View Post
There are a number of experiments where a rod of metal is stretched in a machine and the stress and strain recorded. Quite a number can be found on YouTube.
There is always quite a noticeable deformation taking place, before the rod breaks, and there is a noticeable narrowing as well.
The stretch caused by thermal expansion is most likely only a fraction of the stretch caused by the thread, and nowhere near the yield point of the bolt, in the k4.
I think the confusion arises by some people using the term yield as a simple reference to the bolt "giving" and being designed to do so.
The "yield" point is however quite specific in engineering terms, as the point of no return. I think this could be the cause of some misunderstanding.
I have never seen a k4 bolt stretched. I have however seen a few V6 bolts broken, but they are much shorter. I would not re-use a V6 head bolt, but quite happily a k4.

Oddly enough when a bolt enters the plastic stage it can produce strain hardening to different degrees, cause by the strain causing dislocations within the metals structure, cold working.



As the strain is increased the strength of the bolt increases to its maximum strength, after this is reached and further strain is applied, bolt strength decreases to eventual possible, but not always necking and then fracture.


This phenomena can cause fastenings taken over their max strength to fracture by being subject to thermal stresses or vibrational stresses.


There was an engine, can't remember which at the moment, in that it was known and expected for one particular bolt to snap in use, even though it was set up correctly by hand or machine, the bolt was very close to an exhaust port.
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Old 15th September 2021, 19:39   #18
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I watched this and annoyed Mrs sworks by disagreeing at the tv as it went on……. There’s a lot of inaccuracies regarding the K series in my opinion
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