Possible/probable Consequences of over torque !

[hardtimes]

I'm taking down/checking out a flathead engine. Started to take first rod cap off to check for clearance/wear, etc....
I have several makes of older torque wrenches. So, I started to loosen first nut and the old torque wrench started to max out. So, I tried two other brands that I have...same/same ! What the heck...
My buddy had loaned me a high $ snapon adj clicker type..which is new.
I leaned into the two nuts with the snapon...and it confirmed what the other wrenches showed...AND ...went to 90 ft lbs, before the rod nuts creaked loose !
Any engineers on the board? What, if any, are the harmful effects of such 'overtorqueing' Anybody , all opinions welcome.

[Kahuna]

Distortion of the item, or breakage

[rotorwrench]

The con rods made by FoMoCo in the flathead era are tough as hell. You might try torquing one to normal torque and then see if it takes the same torque to remove the nut too. Many years of cold and heat soak plus just time in general could possibly have some effect. If the threads have been stretched, it should be difficult to turn a new nut back on there. If a person knows of an average length of the fastening ends, they could check for elongation. If a new nut goes on easy and they pass for normal rod length, bend, bore diameter, and twist then pass a Magneflux inspection, I wouldn't be affraid to use them.

[Sixseven]

Unbolting can have a different torque than initial bolting on install. During a fresh install, threads and shoulder surfaces are clean and possibly lightly oiled. Torque is a result of friction between surfaces in contact; static and dynamic. Where static friction is the force required to start motion between two surfaces in contact while dynamic friction is the force during motion. Static friction is always greater than dynamic friction but the difference is the least on a freshly installed fastener that is lightly oiled. Static friction greatly increases over time so restarting the movement of a tightened bolt increases significantly.

In short, I wouldn't put a lot of stock in measured torque required to unbolt a fastener, particularly after the passage of time.

This argument is not to be confused with torque limits suggested when unbolting head studs. The often referred to limit of 70 to 85 ft-lbs for unbolting a head stud is based on experience (empirical data by others) that applying greater torque on stuck head studs will break the stud. This is an example of high static torque where the imposed force (torque) on the stud has a resistance far in excess of stud strength so the stud fails before the stud moves.

[hardtimes]

Sixseven,
Thanks, explained this way, it even makes sense...kinda !
Well, I did try to put the two nuts back onto the same rod bolts and yes they went on nicely ! With my old eyes, I could not discern (cheated with magna glass)any distorted threads. I had just twisted off four head studs,on same block, where the metal cross section of the stud....looked clean solid metal thru/thru ! I then had to ponder 'static resistance', have a refreshment and ponder some more.
It just doesn't seem correct that if rod nuts , on a clean/NON RUSTED/oiled /properly torqued bolt (45lb), would take 90 ft lb of torque to loosen....even after 50 years of resting in place Funny part was that there were the proper gizmos in place , where you bend a tab up against nut land to ensure no back off. What the heck, I guess that that's just a part of the world being flat,eh !

[Sixseven]

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What I meant by 70 to 85 ft-lbs to remove the stud is that if the stud is frozen, experienced rebuilders will only use that maximum amount of torque in an attempt to remove the stud so they don't break it. On an engine that has been assembled within the last ten years, possibly longer, the studs should back out with far less effort. I found a great series of articles by an experienced rebuilder once that described good techniques for removing frozen studs and best reassembly practices. If I can find it I will post the link.

Ken

[Tinker]

I am certain i am not an engineer. But please explain why you would need a torque wrench to tear down a motor?

[Tinker]

I do know if you use anti-sieze or sealant when re-installing you should be carefully of your torque values, they will change. I might be so bold to say as time passes the torque values would increase due to strain, but the clearance would suffer by a thousands too. But maybe not. Engineer status not verifiied. Great question you have asked and interesting post made outside of mine.

[Pete]

"I might be so bold to say as time passes the torque values would increase due to strain"

If you can arrange that, we will have perpetual motion.

[Tinker]

If i figured that out i would have a 1000 acre ranch! I guess that is true with an engine that never runs......

[mrtexas]

I don't think your experience is unique. When I changed the rod bearings on my 37 engine it took considerable torque to loosen them, way more than to tighten them.

[Sixseven]

Originally Posted by Tinker

I am certain i am not an engineer. But please explain why you would need a torque wrench to tear down a motor?

You don't need a torque wrench to tear down an engine at all.

It has been recommended by experienced rebuilders as a tool for limiting stress on frozen head studs. The point being that you can measure the force you exert on a frozen stud and stop before you break the stud. If the stud is not broken there are other preferable methods of stud extraction that are more desirable than working off of a broken stud.

[Sixseven]

[QUOTE=Pete;816125]"I might be so bold to say as time passes the torque values would increase due to strain"

It is true that an increase in torque value is directly related to an increase in strain, but they do not increase with time. The stress on a head stud increases when the temperature of the metal gets hot and expands. When the metal cools to ambient temp, the stress on the stud is reduced. The explosion from the burning fuel also puts an increased load on the stud, this is a cyclical load based on time which is each occurrence of the four pistons per side firing.

To answer the original question about over-torquing a bolt, recall that torque introduces a longitudinal stress on the bolt (simplistically stated it stretches the bolt/stud). The total stress on a head bolt for instance could be calculated as follows:

(approx) total stress = Initial torque stress +temperature induced stress + cyclical stress from engine firing.

If a stud or bolt is over-torqued the total stress is increased during engine operation. The initial over-torque action may not cause the stud/bolt to fail, but under operation of the engine, the total stress could deform the bolt/stud (depending) to a point it fails or elongates. Bolt failure or elongation might result in a blown gasket among other things.

Furthermore, the significantly increased torque required to remove a frozen head stud does not necessarily translate to increased torque stress on the head. This would be the initial torque stress that I introduced above. The stud is initially tightened to provide pressure to hold the head in place. The increased torque required to remove a frozen stud is significantly due to the static friction required to loosen the stud that is held into place from (1) binding caused by rust and (2) the increased friction of the dirty mating surfaces between the block and the stud. Binding (point 1 above) is primarily caused by the creation of rust as it expands into the voids of the screw threads.

[hardtimes]

Got it, thank you! Now I think that I know the reason why I snapped the four studs off. Hm, learn something new every day.

[66miles]

I really feel for you hardtimes. I just changed out my head gaskets, and a couple of the studs/nuts gave a noticeable CRACK sound as they first moved (I had the wrench set at 70 for removal). Makes a guys skin crawl, especially knowing how old those studs are, and how they might have been abused by previous backyard mechanics over the years. I got lucky though!

[JWL]

There is a term used to describe the experiences of "breaking-loose" a fastener. As a young engineer I was taught to call it "stick-slip". It is the reason a proper re-torque is often suggested to be done on a fastener which is first loosened.

What is wrong with some of the previous discussion regarding heat stressing and expansions is that the studs or bolts also expand at the same time the head is expanding so as to limit the heat induced "stretching" cycles. Further, the clamping value of the properly torqued bolts/studs FAR exceed the combustion pressure and there is not any cycling as a result.

[Tinker]

I guess I will be using a torque wrench when tearing down an engine now. Thanks for all the explanations. I definitely learned something.

[Mart]

I suppose one thing that could possibly increase with time would be rust or corrosion between the nut and the stud, making the torque to remove higher.

Some good points raised above, interesting to read, thanks.

Mart.

[VeryTangled]

Hi Everyone, good thread.

I was waiting to jump in here, and I guess it's time.

One of my jobs is teaching in an auto shop at a community college. (I know, I know, those that can, do... We're all doomed with the quality of both students and instructors that are part of our educational system!)

We have a room full of Snap-On tools that would make your jaw drop. Included is about twenty torque wrenches, nearly all of them the manual click kind, a typical example is the QD3250, which is a 1/2 inch drive, 50 to 250 ft/lb, that we'd use for lug studs for example.

Here's a link to Snap-On's page on the QD3250.
http://buy1.snapon.com/catalog/item....re&dir=catalog

On that page there is a warning: "Do not use to break fasteners loose." BTW, it also says they need periodic re-calibration to stay accurate.

Our shop policy is that torque wrenches never be used for loosening, only for tightening. (Also wrenches are reset to their lowest number before storage, it de-tensions the spring inside.) I just called our shop manager and he immediately said NO WAY when I inquired about the subject of loosening. I asked if we had any documents on the subject, I think there's something at school, but I'm not there now. I didn't find a manual for the QD3250 online yet.

So... I can see where some torque wrenches might not be harmed by loosening, I'm not certain that it is harmless for all torque wrenches.

Another interesting thing: I just looked at this Snap-On .PDF (http://buy1.snapon.com/catalog/OBJEC...ges/Torque.pdf), and it lists the accuracy of their various torque wrench product lines, some of them have an accuracy listing as CW only, some CW and CCW. I assume this is clockwise and counter-clockwise. That would seem to indicate that some can be used for loosening (or is that tightening reverse-threads, would the wrench care?). The accuracy on the QD model line (referenced above) is +/- 2%CW and 6%CCW. The TQ series is listed as clockwise only.

I offer this as a possibility to discover the torque as you disassemble: Set a number on your wrench (below what you estimate the tightness) and see if the bolt will tighten, if it doesn't, go a little higher (5ft/lbs) and try again. You know you're at the tightness, when the fastener moves. You just hit a value that was MORE than what the tightness was, so your fastener was just below the value it took to move it.

What do you think?

-VT/JeffH

[Bubsyouruncle]

Great discussion!

Now I know why I broke 3 studs back in 1958. Learned my lesson. Haven't pulled another one since. Haven't run a tap down into the block, either, since then.

VT - I notice that everybody is talking about the wench clicking. I have a couple thoughts about that. There are such things as left hand threads. If you can't use the wrench counter clockwise, how do you torque one of them?

Other thought: if you are concerned about damaging your wrench, use one of the old fashioned wrenches with just a scale. Since you are removing, all you are concerned about is not breaking the stud, not getting the nut properly torqued. The beam wrenches really don't care about that sudden decrease in torque.