| Author | Message | ||
| '75 308 GT4 (Peter)
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I did not take any length-measurement of my studs when I had the heads off of my block. Hypothetically speaking then, say the thickness of the head with the gasket is around 9cm long. The stud dia. and thread pitch are: 10mm X 1mm. No lube on the threads. | ||
| Bill Sebestyen (Bill308)
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Peter, In addition to the length of the stud, (head thickness plus gasket thickness), and thread pitch, if you could also supply the diameter of the stud, and let me know whether the threads are libricated or not (probably not), I could run the numbers and calculate the elongation based upon assumed material properties. From this we could take an educated guess as to where on the stress strain curve the material will be when fully torqued. I'm sure it will be within the elastic range. Craig, If you get the chance, I'd like to see your formula to convert torque to rotation. I understand your explanation of the shear joints. Do you base the holding power on a coefficient of friction and if so what do you assume for its value? Bill | ||
| Craig Dewey (Craigfl)
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Peter, I'm sure that Ferrari is requiring a clamping force that assures the stud to be in elastic tension. There is a direct formula that converts torque to degrees rotation but I'm at work and don't have it here right now. Although there is a formula, friction between the nut and threads and the nut and washer can influence the theoretical calculation. This is why Ferrari specified a lower torque first step (29 ft-lbf) and finishing with the turn of the nut method to produce a higher clamping force(equivalent to a higher torque). If you wanted to see what this "extra" clamping force would be, you only need to know the pitch of the threads and the diameter and length of the stud. You would calculate the increased length of the stud as 3/4 turn times the pitch and find the force needed to stretch the stud to this increased length. I'm sure this would raise the net effective nut torque to much higher than the 29 ft-lbf starting point. Additionally, If there is not enough clamping force between the head and the block, internal forces generated by the cylinder compression or firing the mixture, may stretch the stud enough to cause leakage past the gasket. And Bill... Structural high strength bolts are used in essentially shear type connections. These connections can be clearance bolt holes or reamed holes. In either case, the strength of the connection is a result of its resistance to slipping determined by the clamping force of the bolts. Since the applied forces in this type of connection are 90 degrees to the tension forces generated from tightening, larger forces can be tolerated before exceeding the yield of the material. The safety factor in the design is actually related to the connection slippage and not the bolt stress so a suitable safety factor can be obtained with the bolts at 90% Yield. | ||
| '75 308 GT4 (Peter)
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I've thought about this some more. I have a hard time believing 29lbs-ft would put a head stud at the limit. Would the stud NUT be the culprit? There's not alot of meat on a nut. | ||
| '75 308 GT4 (Peter)
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Okay Bill. Speaking of these 328 head studs, requires tightening to 4KgM plus 120* rotation, that's 29lbs-ft (very light compared to my head nut torquing requirement). Continuing turning 120* more is already forcing the stud into which state, elastic or plastic? Either state, can 29lbs-ft really be sufficiant to hold down a cylinder head? If its in an elastic state (as you describe, can be totally recoverable), then the studs can be re-used (because it would've returned to its original length). If the 29lbs-ft puts it into a plastic state, I would have a hard time trusting those studs. Ferrari obviously did their homework, because I haven't heard a story where a 328 blew the heads clear from the block, but still... | ||
| Bill Sebestyen (Bill308)
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Edward, I agree with you that torque to yield bolts should always be replaced in a critical application. Once the yield point has been reached, plastic, non-recoverable, deformation occurs. Torque to yield is not the same as torque to an initial value plus rotation. The difference is that when properly specified, initial torque plus rotation limits stretch to the elastic region. With regard to the recommendation or replacing nuts after use for critical applications, it could be these are sacrificial in nature. I noticed when I replaced the nuts on my cam bearing caps that the nuts seemed to be a little soft and sloppy. If this is the case, and I didn�t have them tested for hardness, the soft material may be intended to save ware and tear on the studs. The down side of a softer nut material, other that a decrease in holding power, is that the nuts may be subject to gauling, which could adversely affect the torqueing process. Peter, Stretch or deformation, with regard to engineering metals, can be classified as elastic and plastic. Elastic deformation is fully recoverable. If you apply a load to a metal and don�t stretch it beyond its yield point, then release the tension, the rod will return to its original length. If you stretch it beyond its yield point, the rod will retain a portion of its stretched length. This behavior is best shown on a stress-strain diagram. Different materials have slightly different shaped curves. The one shown below is typical for a high-strength steel.  In the above plot, a material sample, in the form of a rod, 0.25 inches in diameter and 2 inches long, is pulled to failure in a test machine. The material in our example is roughly comparable to ANSI 1090 cold drawn steel, very high strength. Starting with no load and then gradually stretching the rod, we see that it requires more and more force. For a given increase in elongation, there is a proportional increase in force required. A material behaving in this manner is said to obey Hooke�s law. When this rod is stretched about 0.05 inches, we find the corresponding force is up to 3500 lbs, the yield strength of this particular material. Up to this point, if the load were removed, the rod would return to its initial, no-load length and there would be no damage. Engineers select materials and bolt sizes or structural members to always remain within this region, called the elastic region. Craig stated that high strength bolts, used in structural connections, are tensioned to 90% of the yield point. This seems a bit high IMHO, as I always thought bridges and buildings generally used a factor of safety between 2 and 5, which would limit tension loads to between 20-50% of yield, but I am not a civil engineer. Getting back to our discussion, suppose our rod was stretched beyond the yield point, what happens? Well, upon exceeding about 0.05 inches elongation, the rod begins to plastically deform. If we were to remove the load, having first entered the plastic region, we would find that the rod would be longer that it was before we started. Other interesting things are happening in the plastic region. As we continue to stretch the rod, we find that the corresponding load continues to increase, but at a slower rate. Eventually we reach a level of elongation, at which the corresponding force reaches a maximum. This point is called the ultimate strength of the material and for our example, the corresponding load is 5000 lbs. If we continue to stretch the rod beyond this point, we see that the corresponding force actually decreases and at about 0.570 inches of stretch, the rod fractures. Loading a member to a high percentage of its yield strength is generally only permissable for static, or non-moving structural applications, like bridges and buildings. For applications subject to cyclic loading, like connecting rod bolts or axles, engineers must select materials and sizes that will limit loads based upon fatigue considerations. This means that instead of stressing components to a high percentage of the yield strength, they must be designed to operate well below the yield strength to account for the millions of cycles of loading expected during the component�s lifetime. The more cycles expected, the lower the allowable loading. Bill | ||
| Edward Gault (Irfgt)
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That is what my 82 308 manual says also. I do not see anything wrong with reuseing the rod nuts even though I purchased new ones for my engine when I did the rebuild. I could see no advantage in using new ones though as the locking feature still worked on the old ones and new ones were not that expensive. Newer engines that require new bolts are available in a set along with the head gasket to make it more convenient. Some of the GM engines that have TTY bolts are a pain to order as they have different threaded ends to hold accessories to the head and you have to specify which location the bolt is in. I dislike using TTY bolts as it always feels as though they are going to break off. | ||
| bernie dennison (Bernied)
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Reguards reuse of head bolts on the 328. The two manuals that I have for this engine are very specific when there is a need to replace a bolt or nut; such as, the rod nuts are to be replaced on rebuild and not the rod bolts. There is no mention of replacing either the head bolts or nuts on rebuild. bernie | ||
| Edward Gault (Irfgt)
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There are no automotive head " torque to yield" bolts that are reuseable that are known to exist. I have checked through all my sources and can find no instances where you can reuse one of these bolts, in fact they are emphatic about the importance of replaceing them. I also cannot find any case where there are any use of torque to yield studs in automotive use. My 308 uses studs with no mention of any special torque proceedure or any need of retorqueing at a later date. | ||
| Craig Dewey (Craigfl)
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For "high strength bolts" in structural connections (Buildings, bridges, etc), bolts are tensioned to near 90% of the yield point(elastic limit) of the steel. In this case, loading may produce even higher loads so bolts are not reused after the original tensioning- which is usually specified by the "turn of the nut" method(specific rotation). I would expect that head bolt/studs are not tightened to such extremes and therefore are reusable, unless of course the elastic limit was accidentally exceeded. | ||
| '75 308 GT4 (Peter)
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Stretch is still stretch, and the bolt will not shrink back. I'd say its safe in this instance, because how many times are you going to pull the heads off this engine? I re-used the studs on my block and I sleep well at night. Under other circumstances (like the propane tanker-trucks I service, the man-way covers use B7 studs) I always replace them. 4KgM is very light, the head nuts on my engine were at 10KgM (which is 72.5lbs-ft). | ||
| Bill Sebestyen (Bill308)
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Unless there are explicit instructions to replace the bolt after tensioning , when using the torque plus rotation method, there should be no reason to not be able to use the bolt again, provided you did not exceed the elastic limit of the bolt material. Bolts are almost always used within the elastic limit. Exceeding the elastic limit may be ok in some no critical applications, but you better know what you are doing. As soon as you begin to tension a bolt, you begin to stretch it in accordance with Hooke's law. Hooke�s law states that, within the elastic limits of the material, deformation produced (stretch) is proportional to the stress (load per unit area � in this case ,the cross sectional area of the bolt shank). The normal method of torqueing a bolt, equates bolt stretch to a final torque value. Stretch is highly dependant on friction, especially at high torque values, so this is not the best method to tension a bolt, but it is sufficiently accurate for most applications. With this method, the best practice is to perform the final torque slowly, but without stopping. If one stops, at say 95 ft-lbs, when you really want 100 ft-lbs, you must back off on the bolt before torquing to the final value for accuracy. If you try to go from 95 ft-lb to 100 ft-lbs, you will find that the bolt will not turn when 100 ft-lbs is applied. This is because of the difference between sliding and static friction, where static friction is always greater. Whether a bolt is lubricated or not, also has a significant impact on the final bolt stretch. In this case, the decrease in friction will cause the bolt to be stretched more than if it was not lubricated. The torque to a modest initial value, then tighten to a specified rotation, is a refinement of the standard method. This method is affected by friction only during the initial torqueing, where friction effects are not as great, so overall accuracy is better. The initial torqueing provides a starting point for the final bolt stretch. Since the thread pitch is constant, a more accurate stretch of the bolt is attained. The best method is to directly measure bolt stretch with a tool like a micrometer. Unfortunately, this method is not practical, except in some through bolt applications. Bill | ||
| Edward Gault (Irfgt)
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If the proceedure is given in torque as well as additional turning then you cannot retorque using the same bolts. They must be discarded after one use. Do not attempt to retorque the head bolts. | ||
| Craig Dewey (Craigfl)
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The most accurate way to tension bolts is to specify a rotation in degrees. This is because the pitch of the thread is constant and rotating the nut/bolt will give the bolt an exact stretched length, which is really what you are after- a consistant clamping force. Torque is not always the most accurate method to give you a specific clamping force because friction becomes an issue. But, torque seems to be the easiest thing most people use in order to try to get the proper clamping force. I suspect in the high torque range non-linear friction/torque is more of an issue so they start with a torque and finish with a more accurate degree rotation. | ||
| bernie dennison (Bernied)
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Head bolt tightening on late 328 is given as 4 KgM plus 120 degrees of rotation. Does anyone know what this relates to in torque? Earlier heads have a torque listed without the rotation factor. Why would Ferrari ever give a torque plus rotation rather than a total torque value? thank you, bernie |