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Thread: Question(s) about Torque-to-yield bolts (engine rod bolts)

  1. #1
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    Question(s) about Torque-to-yield bolts (engine rod bolts)

    I understand the theory of Torque-to-Yield bolts, but I am confused about some points I have begun to consider in regards to a FORD factory service manual and my understanding of TTY bolts. Maybe someone can help me out??

    Some info
    The issue is the connecting rod cap bolts & nuts
    1. FSM says tighten to 50 lb/ft and turn more +90 degrees.
    2. FSM says bolts are not reusable if rods are disassembled new bolts are required
    3. New FORD bolts are $20 each $240 total.
    4. The rod bolt is a somewhat normal bolt with a "T" shaped head and conventional nut & thread and a raised knurling on the shank to help guide/center the cap during assembly.
    5. The bolt is approx 5/16" x 24 x 2"(it may be metric but very similar) and assuming every 90 degree of turning the nut past snug stretches the bolt approx .001" for arguments sake (1/96")
    6. General rule of thumb for engine rebuilding says if rod cap bolts are removed and replaced, the rod 'big end' must be re-sized/re-machined.(which requires removal of engine, dis-assembly, re-assembly, and re-installation, + many new expensive parts, gaskets, seals, bearings, etc)

    My Goal is to not spend lots of money repairing my possible mistake in disassembling the rods to inspect the bearings.

    My questions:
    1. If I mark the bolt & nut, remove & replace the same nut to the same bolt and align the marks, has anything really changed. Assuming that no additional material has been worn away from the thread engagement of the nut & bolt, and no wear from the nut face turning into the rod cap, was disassembling & reassembling to the exact original position, harmful to the bolt? Has the clamping force changed?

    2. If I reassemble with the torque wrench set to the recommended 50lb/ft and turn +"some number of degrees" method, I currently see only approx 45 degrees additional turning necessary to realign to my original marks.
    .a. I believe the bolt has a permanent size distortion of +.000125 in length.
    .b. If I stop tightening at the original marks, is the clamping force just as it was before it was loosened & removed?
    .c. If I goto +46 degrees(1 degrees more) or +50 degrees (5 degrees more)I believe I will begin stretching the bolt in the yield zone of the bolt again, is there any reason to believe +.0000014"or +.00007" extra stretch will harm the bolt?
    .d. I thought TTY bolts were supposed to be measured with a micrometer and tightened until a specific amount of stretch was measured/achieved, not calculated by turning the nut, can i reliably believe the first use of the bolt totally maxed out the available 'stretch'?

    3. Does anyone have any idea about the rated tensile strength of any factory FORD connecting rod cap bolt?
    .a. Is there any reasonable reason to believe FORD uses(or doesn't use) the same grade of bolt across their entire production line except for specials like the SHO v6 or the Shelby /Mustang V8? Somehow I think 140,000 psi comes to mind for stock bolts and high performance aftermarket bolts are 180,000 psi rated.
    .b. Can someone provide a formula for calculating the stretch force being applied to 2 of these bolts when the estimated rod+piston weight of 8.5oz, is being pulled down by a 3" throw on a crank shaft spinning at some number of RPM's with +1 - +10psi atmospheric pressure applied to the bottom of the piston 12.6 sq in surface area, (2"-20" of vacuum on the top of the piston)? At what RPM is the clamping force of the 2 bolts being exceeded by the "weight at xRPM's" of the moving piston+rod, and the cap-to-rod connection beginning to come apart?

    Most of these questions should end with a Yes/No/Why question for follow-up. Can you explain??

    I know there is a lot of "question" here, and probably there is not enough info to answer with 100% accuracy, but hopefully someone or multiple someone's can make some good generalizations with typical types of bolts found in the general automotive industry, and help clear some of this up for me. (Maybe I will be lucky and a retired FORD engineer is here on this list?)

    Thanks in advance
    Byghtn
    Last edited by byghtn; 03-24-2015 at 01:48 AM. Reason: more clarity -- maybe, + grammar, + punctuation

  2. #2
    Administrator Kelly Bramble's Avatar
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    I think that you understand what torque-to-yields bolts are and why they are used so I’ll just try to answer the question that’s burning in your mind...

    Can you reuse the bolts and nuts utilizing one of the smart re-installation methods outlined and not have any problems near or far term?

    Short answer.. Maybe.

    I’m not sure any of your re-torque methods will get you back to the desired torque and in the desired elastic range of the bolt originally specified by engineering for the installation. Also, it is not uncommon for the threads to yield and on removal you end up with a sheared or broken bolt.

    Personally, I think trying to second guess the engineering without detail data is --- well not engineering.

    So, it’s only $240 to do the job to factory specifications?
    Last edited by Kelly Bramble; 03-24-2015 at 08:07 PM.

  3. #3
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    Kelly, Thank you for your answer...

    Quote Originally Posted by Kelly Bramble View Post
    I think that you understand what torque-to-yields bolts are and why they are used so Iíll just try to answer the question thatís burning in your mind...

    Can you reuse the bolts and nuts utilizing one of the smart re-installation methods outlined and not have any problems near or far term?

    Short answer.. Maybe.
    ..I am at Maybe++, I'm looking for some good - 'negative' reasons besides "just because the manual says so"..

    Iím not sure any of your re-torque methods will get you back to the desired torque and in the desired elastic range of the bolt originally specified by engineering for the installation. Also, it is not uncommon for the threads to yield and on removal you end up with a sheared or broken bolt.
    ..The nuts went back on very smoothly, there was no grit or shavings apparent, so I do not think the threads were deformed, nut or bolt, however, I did not try to run the nut up past the original seated position with the cap removed, that would have been some good information to have observed +/-..

    Personally, I think trying to second guess the engineering without detail data is --- well not engineering.
    ..I agree it is not 'engineering', but maybe someone knows something about a similar situation to practical industrial application such as 'First installations typically use some "X" factor of available stretch' that should have been less than 100%..

    So, itís only $240 to do the job to factory specifications?
    ..Item 3 = $240, but you missed item 6 = some ungodly amount,(probably above $1000), well beyond the $$ of a low mileage junk yard motor, just because I removed & replaced the rod cap nuts..
    I'm not trying to be argumentative, it's more of a debate if you will allow me...

    I have the following opinions(brief explanation).
    1. The bolt should not have been tightened & stretched to 100% the first time around. 100% clamping force maybe but not 100% available stretch before clamping force begins to drop off and the actual 1-piece bolt becoming a 2-piece bolt.
    2. Tightening to the exact same position should have made no difference.
    .a. Tightening to less than the original position could be massively different...
    .b. Tightening to +1 or +2 degrees past the original tightening would be like the factory tightening to 91 or 92 degrees initially, can the mass-production equipment really get exactly 90 degrees 100,000 times before re-calibration, there has to be some amount of acceptable error in this regard. I suppose it is possible the factory went to +100 or +110 degrees, or what ever the maximum allowable error was, but that should not be the typical.. IMHO.
    .c. Accounting for wear in the thread or the face of the nut, +1 - +2 deg of extra tightening should easily account for that.
    .d. If the bolts were that sensitive, there would be massive failure rates upon initial application.
    .e. There are lots of other repairs that have flexible parts or difficult to re-create circumstances where this idea would be difficult to re-use these fasteners with any degree of certainty. Head gaskets could have variability in thickness thereby making the "ORIGINAL" set-point difficult/impossible to determine. Rubber & plastic parts may have deformed beyond repeatability of reliable 'LOCATION RESETTING", or they may deform differently the 2nd or 3rd application of clamping force. This example is 'steel-on-steel' and except for point "c" above, this should be a repeatable re-assembly..
    3. Other information sources have indicated that the main weakness in this engine is not this rod bolt, but the actual rod & piston,(but then-again, maybe they are having rod failures due to bolt stretch?) I am looking for good information for my own insanity--I regularly 'over do' what ever project I'm working on, and this is another example of over-research...maybe. I have this idea and I can not find any reasonable reason(in my opinion) not to reuse these at least 1 more time. I can fully understand not going +90 degrees, but +1-+2 degrees seems completely possible, I'm looking for a sound engineering reason(and explanation) not to reuse these bolts.

    I am also still looking for an equation that would help me to calculate the 'bolt load' at "x" RPM, with the crankshaft moving through the range of 45-135 degrees, (+ other factors & elements that can be easily measured or calculated)... so I can make arrangements not to get close to that mathematically calculated point of destruction, or possibly reverse calculate the original bolt strength.....Have you noticed that most cars from a particular manufacturer have the same "RED LINE" indication on the tachometer? All models of all engines explode at the same point??

    Again,
    Thanks for your answer, and I'm hoping for more.

  4. #4
    Administrator Kelly Bramble's Avatar
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    First and foremost I think it's your money, engine and decision... I have no money lose, no customers to get mad at me or other reason to push $20 bolts on you.

    Now, let me respond to a statement you made.
    You said: "besides "just because the manual says so"."

    I guarantee you that there has been an engineering analysis, FEA simulations, testing, and years and years of field proof to demonstrate that to get the most life out of the engine one needs to torque these fasteners a very particular way. There are no evil design engineers or tech manual writers trying to scam you into buying new bolts every new build.

    Another point of view is to consider a scenario where you have two piles of identical parts. And what I mean is that these parts are built perfectly identical (which is impossible BTW). Then, have one machine built from one pile in Japan and the other machine built in the jungle of Borneo. These will not be the same machine in terms of quality as one will last longer than the other. Too often the quality of the build can and does affect the quality of the end item machine.

    Now to be fair to you, reciprocating engines of years past (50’s and 60’s) did not use Torque to yield bolts. I am personally guilty of reusing head and crank bolts on a 63 Buick 225 V6 as well as a 1970 Pontiac 400 V8. Both engines did seemingly run fine however I do know that these engines where not built as good as they could have been. Be aware that engines built in the 50’s where 60K mile engine’s and in the 1960’s American recip’s where 100K mile engines.

    The whole torque thing on these cap bolts is about quality of build and usable life of the engine.

    In this case – if you have the dough, I recommend doing the job right if not take your chances and keep your foot off the gas pedal.

    I'm positive that you will make the right decision for you...

  5. #5
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    As Kelly states, it is your engine and your risk; however, should you decide to reuse the original bolts then there one issue in your post that I can address and that is the manner in which the reinstalled bolts should be tightened.

    When reinstalling a previously stretched bolt, it is the same as installing a silghtly longer than original bolt; as result, you still need to torque the bolt to the specified 50 lb-ft and it should rotate a small amount past the reference mark you made before removing the bolt. Then to achieve the original bolt full strength you also need to rotate the bolt to the specified 90 degrees to achieve a plastic yield equal to that of the original installation. Neglecting to utilize the original specified values will result in a bolt clampng load below that of the original bolt installation.

  6. #6
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    Quote Originally Posted by Kelly Bramble View Post
    First and foremost I think it's your money, engine and decision... I have no money lose, no customers to get mad at me or other reason to push $20 bolts on you.

    Now, let me respond to a statement you made.
    You said: "besides "just because the manual says so"."

    I guarantee you that there has been an engineering analysis, FEA simulations, testing, and years and years of field proof to demonstrate that to get the most life out of the engine one needs to torque these fasteners a very particular way. There are no evil design engineers or tech manual writers trying to scam you into buying new bolts every new build.
    ..I agree with this and I did not intend to discount the actual engineering & science of TTY bolts, I was commenting in relation to my points 2.b & 2.c..

    .....

    Now to be fair to you, reciprocating engines of years past (50ís and 60ís) did not use Torque to yield bolts. I am personally guilty of reusing head and crank bolts on a 63 Buick 225 V6 as well as a 1970 Pontiac 400 V8. Both engines did seemingly run fine however I do know that these engines where not built as good as they could have been. Be aware that engines built in the 50ís where 60K mile engineís and in the 1960ís American recipís where 100K mile engines.
    ..Yes I agree with this as well, one used to have to de-coke the engine every 20K mi, wouldn't that be grand with the driving habits of today..

    The whole torque thing on these cap bolts is about quality of build and usable life of the engine.

    In this case Ė if you have the dough, I recommend doing the job right if not take your chances and keep your foot off the gas pedal.
    ..I don't have the dough and that is the point and purpose of my last paragraph..

    I'm positive that you will make the right decision for you...
    ..I'm not sure if it is the RIGHT decision, but it is the decision that I must be accept at this point in time..
    Thanks for your continued interest and input.

  7. #7
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    Quote Originally Posted by JAlberts View Post
    As Kelly states, it is your engine and your risk; however, should you decide to reuse the original bolts then there one issue in your post that I can address and that is the manner in which the reinstalled bolts should be tightened.

    When reinstalling a previously stretched bolt, it is the same as installing a silghtly longer than original bolt; as result, you still need to torque the bolt to the specified 50 lb-ft and it should rotate a small amount past the reference mark you made before removing the bolt.
    ..My original question is "Is the bolt setup actually different if it was disassembled & reassembled to the exact same point? Does the physical process of disassembly (relaxation) & reassembly (retension to the exact same physical location) actually change the bolt setup?"..
    Then to achieve the original bolt full strength you also need to rotate the bolt to the specified 90 degrees to achieve a plastic yield equal to that of the original installation. Neglecting to utilize the original specified values will result in a bolt clampng load below that of the original bolt installation.
    ..In my original question points 2.a, thru 2.d-- It appears that any tightening past 50 lb/ft + approx +45 degrees results in permanent deformation/elongation
    and my follow-up question 2.a thru 2.e, can you explain what is wrong with these statements?.
    .
    Better yet, can someone direct me to an appropriate load cell to test this theory or maybe even donate some time and access to such a load cell???
    I think this would be problematic because the original bolt distortion revealed in my original question indicates that the bolt has already permanently stretched some due to being originally turned +90 degrees, going an additional +90 degrees will surely result in compounded deformation of the bolt, possibly beyond usability. My thoughts are to go +1 or +2 degrees past the original setpoint assuming there was always some small tolerance for over stretching in the first application(meaning the original installation actually went to +91 or +92 degrees instead of stopping exactly at +90degrees), +90 degrees past the original setpoint would be difficult to believe to be available..

    Thank you for your thoughts JA

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