Bolt torque discussion

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Off topic but I watch guys use locktite on bolts (often convertor bolts) and not remove the old locktite or clean the oil from them and then complain about locktite not working. Bolts mush be cleaned.
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. :BangHead:
 
Well theoretically all recommended torque spec are based on the optimum stretch for the material, but as I understood it the longer a fastener, is it would need slightly more torque to be optimum.
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.
I'll respectfully submit that there is no 'optimum stretch' per se, just what you need to get the stress (and tension) in the bolt to where you want it to get the desired clamping force on the gasket.

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!
 
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.
 
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. :BangHead:
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.
 
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.
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.
 
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
 
Thanks for the reply. I posted this thread as a carryover from another thread where there was indeed plenty of disagreements on the torque when using studs.
Rather than hijack the other threads and out of courtesy I started this thread. Eg arp recommends a stud torque of 140 lbs with oil for 1/2 inch studs 45 lbs higher than stock.

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....
 
Clearly one disparity is the generalized torque specs are based on an assumption about what is being threaded.
Threading into a casting, I've got to think the cast material and shape is going to be a big factor.

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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I have read on this forum that some guys torque the longer head bolt studs to a higher torque than the shorter ones to get the longer bolts properly tensioned.
Another guy questioned whether the block itself would be damaged by this increase over stock to the block.

Torque the bolts to the specified torque that is recommended, no need to compensate for bolt length..
 
Before I call it a night for the go by the book guys proper torque to yield puts x amount of stretch on each bolt. Sooner or later the elasticity of that bolt is lost. How do you know when this occurred unless you Mic each bolt and mark each one for size. Some of my studs are over 30 years old so just something to think about

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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How do you know when this occurred unless you Mic each bolt and mark each one for size.

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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Interesting.... Just a wild guess on the thinking behind this, but it sounds like this particular concept of properly torqued is to try to achieve a set amount of stress in the bolt/stud. Are they looking at bolt stretch and trying to get the stretch to be in proportion to the length, or ????

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.
 
I watch some guys doing head bolts, main bolts, and head bolts not lubing the washer and bottom of the nut where it’s probably most important. When I torqued my aluminum rods in my 572 I called the and they gave me a number. I was told to slightly snug the bolts and go to full torque on one pull and move to the other side. That was different.

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...
 
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.
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 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).

Hope all that rambling helped at least a little bit LOL

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...
 
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.
I'd love to see it LOL

Edit: Correction... I wanna help!
 
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Clearly one disparity is the generalized torque specs are based on an assumption about what is being threaded.
Threading into a casting, I've got to think the cast material and shape is going to be a big factor.

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%...
 
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....
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?
 
I ordered a custom set of studs to fasten my front motor plate to the front of the timing cover.
The long 4 bolts go into the block,2 go into the aluminum thread cover, no recommendations from arp as this is custom.
What would you torque these studs to and would you torque the ones threading into the aluminum the same?

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.
 
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?
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.
 
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