Along those same lines, it's interesting that the flex plate has 6 or 8 bolts to attach it to the crank but only 4 bolts attach the plate to the converter, and those bolts only have 4 or 5 threads.
Bolt torque discussion
- Thread starter Duane
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nm9stheham
Well-Known Member
Going back to post #5, for a head bolt, the objective is the clamping force.
- The torque or stretch is just an indirect indicator that you got there.
- The bolt size and material is to make sure you can get what you want with adequate margin and thread strength in the fastener and the matching threads.
The reason I asked about the stretch was for the topic of the different length head bolts. If you looked for stretch to be a constant in proportion to fastener length (example: twice as much stretch for a 2x fastener length), then the longer the bolt, the more the stretch is needed to get to the same bolt stress level... and thus the same clamping force. If you need the extra torque to get to the same proportional stretch, then that is what you do. You theoretically should not have to, but the torque to stress relationship has a lot of variables that are impossible to always 100% quantify.
So different torques could make some sense for the different head bolt lengths IF they are trying to get the same proportional stretch and thus to the same clamping force. Even if the higher torque is used to get to the same proportional stretch (and thus the same bolt stress and clamping force), then the total force on the threads in the block is the same, regardless of the torque levels.
Hope this makes some sense..... and yes, I imagine it is boring for some!
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So quick question on YTT modern bolts, I need to relocate oil pick up support bracket. I have read equally compelling arguments for both sides of the reuse them vs replace. What is the FABO brain bolt trust thoughts? I was looking at simply replacing them as I figured it was cheap insurance. I am just not that familiar with the modern bolts. Apologize for my ignorance but just not much experience on all things modern.
nm9stheham
Well-Known Member
Oh, that one is easy to see. Seriously.... the total torque (rotational force) is the same at both the crank bolt circle and the TC bolt circle, but the TC bolts are 5-6 times further out from the center of rotation. To exert the same torque, the TC bolts have to exert 5-6 times less force on the flexplate than the crank bolts. So they can be smaller, have fewer threads, and be fewer in number.Along those same lines, it's interesting that the flex plate has 6 or 8 bolts to attach it to the crank but only 4 bolts attach the plate to the converter, and those bolts only have 4 or 5 threads.
nm9stheham
Well-Known Member
TTY (torque-to-yield)? In my world, YTT mean's "yesterday's technology tomorrow" LOL. I am not getting the connection between TTY bolts and the support bracket bolts? TTY bolts are a particular bolt design and application technique, not a matter of being modern or ancient.
I would not imagine pickup support bolts being a TTY design...maybe I don't know. So they could be re-used over an over... as long as they were never overtorqued, or the threads were not worn. But replacing them seems like they would be cheap.
MopaR&D
Nerd Member
Most fasteners on engines are spec'd to a torque well below the plastic deformation range (where the metal starts to "stretch" farther than it can spring back). Very high stress areas such as heads and connecting rods need all the clamping load they can get so that's why you see TTY bolts more often in those apps. It should be verified with ARP but I don't think the vast majority of their fasteners are meant to be loaded to the point of yielding; their specialty is making extremely strong fasteners that don't need to be stretched to get the most clamping force; since the metal itself is stronger the bolt/nut can be torqued farther before the metal starts to stretch. OEMs don't do this because it's more expensive than making smaller bolts that are stretched to their absolute max tension and for that I personally replace any fasteners on newer engines that have any indication they might have been stretched (like if the instructions call for set torque plus a 1/4 turn, or if the spec'd torque seems high for a fastener of that size). Also important to keep in mind for rod bolts as mentioned above, those are under very high reciprocating (back-and-forth) loads and usually get torqued to where they start stretching so after a set number of torque-downs the bolts will become weaker.
I can try to dig up my old notes from a Machine Design class I took, we actually learned how to calculate the clamping force put on two parts held together by a bolt or stud depending on thread type, fastener size etc. If you know how much clamping force you need to hold 2 parts together there is a way to calculate the minimum bolt size needed based on the material (Grade 5, 8 etc.).
The most interesting thing I learned was how a bolt or stud actually works... when the parts are clamped together, it removes all external forces meaning the bolts themselves only ever "feel" the stress from being torqued down; for example the pressure of combustion trying to push up on a cylinder head won't put any more force on the head bolts/studs than they are already under UNLESS they aren't torqued far enough or something happens that allows some of that clamping force to go away (failed gaskets, heat cycling). When you look at an engine with a blown head gasket where a head fastener failed, it's usually the threads in the block not the bolt/stud itself that failed. The threads are the weakest part of any fastener.
Hope all that rambling helped at least a little bit LOL
I can try to dig up my old notes from a Machine Design class I took, we actually learned how to calculate the clamping force put on two parts held together by a bolt or stud depending on thread type, fastener size etc. If you know how much clamping force you need to hold 2 parts together there is a way to calculate the minimum bolt size needed based on the material (Grade 5, 8 etc.).
The most interesting thing I learned was how a bolt or stud actually works... when the parts are clamped together, it removes all external forces meaning the bolts themselves only ever "feel" the stress from being torqued down; for example the pressure of combustion trying to push up on a cylinder head won't put any more force on the head bolts/studs than they are already under UNLESS they aren't torqued far enough or something happens that allows some of that clamping force to go away (failed gaskets, heat cycling). When you look at an engine with a blown head gasket where a head fastener failed, it's usually the threads in the block not the bolt/stud itself that failed. The threads are the weakest part of any fastener.
Hope all that rambling helped at least a little bit LOL
You have to be careful on what type of oil that you put on them... different oils have different lubrication properties and can affect the clamp load for a particular torque...
80% of the torque value is influenced by the friction of the fastener....
Like I stated, surface friction on the threads and under the bolt head has over 80% influence on torque of a bolt...
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Torque the bolts to the specified torque that is recommended, no need to compensate for bolt length..
We allowed for 5 torque cycles/clamp loads on a bolt before throwing it away...
Example: for a connecting rod... One clamp cycle is used in machining to clamp the cap to the rod and machine the crank bore... Then the rod and cap are split so you can install the piston in the engine... That is two torque/clamp load cycles on the bolt to get the engine out of the factory...
We allowed two clamp loads for the field - one for service, and one for rebuild... However, they did not take in account for another cycle if you plastic gauge/check your clearances...
So the engine factory gets one repair for themselves... A rod and piston can only be reused/recycled once in the factory before it is scrapped....
A typical connecting rod bolt will stretch an average of .001' - .0015" per clamp load... I have seen it get up to .0025" sometimes...
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To properly do a bolt stretch study, you need to have both ends of the bolt ground flat and parallel and center drilled... Then use a ball micrometer to measure the bolt from center to center on the ground ends... That way you are sure that you are measuring the axis and are not thrown off by not being on the center of the bolt, any mismatch/off center will affect your readings...
And yes, you need to use micrometers, not calipers... One of my coworkers did not follow my advice on that and had to redo his study as the calipers are not sensitive enough to give an accurate reading for bolt stretch... Calipers can only measure to the nearest thousandth, micrometers can measure accurately to the nearest ten thousandth... You need that accuracy to get proper readings...
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When you torque a bolt, it has some elastic stretch and some permanent stretch... The elastic stretch goes back when you loosen the bolt, the permanent stretch is just that, permanent...
We used to monitor torque vs angle when we torqued to yield... You stretch the bolt and monitor torque vs angle and when the curve bends over, you stop when the slope of the curve goes 10% past the peak... That is how you get maximum yield and minimal stretch...
That is how you keep from putting too much permanent stretch in a bolt and achieving maximum clamp load... Once you get into the permanent deformation range, you are not getting any more benefit for clamp load....
Some of you guys would crap when you saw us hooking our 2 1/2 inch drive impac to the crane to tighten bolts on downdays in the mill. Crane to hold it and change position and two guys to run it. 4 1/2 and 4 5/8 sockets were common.
You should only use lube if specified by the manufacturer... If you use it and it is not taken into account, it will throw off the clamp load and torque readings...
nm9stheham
Well-Known Member
If I can add to what has been said above...... Yes on the ARP approach... they are not meant to be used past the yield point.... the tension/stress would probably tear the mating threads out first! They are used for more clamping force; this helps hold head gaskets in place better, keeps rod ends more stable in shape under load, and reduces the movement and distortion of the main caps under load.
TTY bolts are not actually for maximum clamping force, but for more consistent clamping force.
- Torquing non-TTY standard bolts to a set torque level gives a pretty wide range of resulting clamping force. OK for old factory stuff.
- Lubes on threads helps to make a more accurate translation between torque and clamping force, which is why ARP supplies it.
- TTY makes sure a specific section of the bolt enters the deformation region when tightened with a certain procedure, and the materials and design of that 'yield region' of the bolt are selected to make it do that without getting too close to failure. The tightening method puts the bolt more consistently around a given clamping force point than the simpler torquing procedure. Think of it like a pressure relief valve on an oil pump.... you can have a lot of variations in speed on the pump but the pressure stays pretty constant. Same thing for a TTY bolt versus installation variables.
The clamp load is more than the load seen by the fastener... If the load on the bolt exceeds the clamp load on the fastener, then you will get failure/breaking of the bolt...
The bolt acts kind of like a spring, the clamp load is what keeps it together...
nm9stheham
Well-Known Member
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When we developed torque specs, we would take all of the parts involved in the joint and torque at least ten "sets/samples" to failure with a DC electric wrench that monitored torque vs angle and the fastener engineer would use that sample size and information to determine the torque specs for that particular joint... I was involved in this and was right there doing the testing on my parts with him...
We would look at the graphs and determine where the ramp bent over 10%...
nm9stheham
Well-Known Member
Yep, I am pretty sure I understand all that KK; tnx. Did you mean 'maximum tension (stress) with minimal stretch' at the end of paragraph 2?
What I wanted to try to understand is the 'why' behind thinking that the longer head bolts needed to being torqued to a higher value, as Duane brought up. What is the objective of more torque with a longer head bolt? Are they trying to get to a more consistent stress/tension/clamping value? And are the looking at proportional stretch (strain) to judge the right value of torque, rather than just torque?
Give me some sizes and I can give some good recommendations.
Ok now let me throw a question out there. If you have been building engines for over 40 years and never had a bearing issues or an issue with blown head Gaskets but have cracked not one but three heads would you continue your practice of torquing heads to the suggested torque values. And if so why wouldn’t you try something to see if it helped.
Duane
Well-Known Member
They are all 3/8 course thread and fine thread on the nut end.
Factor in they are fastening a motor plate with no stock motor mounts.
Duane
Well-Known Member
They are all 3/8 course thread with fine threads on the nut end.
Remember these bolts are fastening a motor plate with no stock motor mounts.
Duane
Well-Known Member
One of the previous posters said to think of a bolt or stud as a spring. If you were trying achieve a certain amount of tension from that spring, a longer spring would need more stretching before it got to the same tension as a shorter spring.
ARP has a good explanation on their website.
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