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F0355
South Africa
298 Posts |
 Posted - 05/02/2009 : 20:38:20
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Interesting subject, and one that was covered well by David DuBois, Bremerton, WA, USA on the MGA section of the BBS site and the use of lubricants to threads on high tensile bolts:
If you get a set of ARP high tensile strength bolts and nuts, they give two different torque specs, one using engine oil as a lube and the other using ARPs lube (that looks very similar to moly-lube anti seize). The torque spec using the ARP lube is roughly half the spec when using engine oil.
Torque is an inexact method of pretensioning fasteners to 75% - 80% of yield strength. The most accurate method is measure the bolt stretch. When a bolt is torqued a given amount, the spec is saying that for X amount of torque will cause the bolt will stretch to the proper amount for the bolt to be pretensioned to 75% - 80% of yield strength. This is fine as long as the spec also specifies if the torque is done with no lube, or with a specified lube. In the world of submarine work, all critical assemblies (those that are intended to keep water out of the people tank) always specifies exactly which lube is to be used when torquing a given bolt - use the wrong lube and the assembly is disassembled, everything cleaned and new fasteners are used with the specified lube and the whole assembly is redone. In the general automotive world (not racing) torque is normally done using engine oil unless some other lube is specified (like ARPs lube).
To show how the torque required to produce a given amount of stretch on a bolt, Bob Grunau of the Ontario MG T Register torqued a high tensile strength bolt, fresh out of the box to a given torque, then measured how much it had stretched, while another bolt was passed around for people to look at. He then too the bolt that had been passed around and subjected to the oil on poeple's skin and torqued it to exactly the same amount as the first "clean" bolt. The result was that the "lubricated" bolt had stretched more than the first one had. Not only did this demonstration show how show how lubrication affects the pretensioning of a bolt when using torque as a method of measuring, it also demonstrates why torque is not the best method of pretensioning fasteners
And later:
.....in spite of all I know about the theory of fastener torquing, I too use a torque wrench and engine oil for lube. I have experienced first hand what can happen with a fastener that is over torqued and breaks when the vehicle is at speed (new engine required) and as a result will only use a properly calibrated torque wrench and use only engine oil for lubrication unless something else is specified by the manufacture or supplier of the fasteners. The information that I used above is really only important when using high tensile strength fasteners, such as rod big end bolts, Head studs and such. Manifold studs are not what I would refer to as high tensile strength fasteners, being a grade 5 at most. My real point is to use only engine oil unless otherwise specified by the supplier or manufacture, otherwise over torqued and broke fasteners will likely result.
This discussion all came about because of this posting by Andy Preston, California, USA:
Don't do what I just did. I used anti sieze compound on the manifold studs on my MGA 1600 cylinder head. I thought that this would make them easier to remove sometime in the future. I tried to torque them to 25 foot pounds and sheared one stud and stripped another. I summize that the anti sieze compund acts like a lubricant and so you can't get the correct torque.
I tested a sample 5/16" nut and bolt and it torqued to 40 foot pounds before it sheared. I added anti sieze compound to my test nut and blot and the bolt stripped before getting to 20 foot pounds.
Peter Steyn
Johannesburg, RSA
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