> I've heard of mechanics referring to `stretch bolts' used to fasten
> engine heads, maybe they're actually studs, but what I heard that
> they were specially-engineered bolts, to be used ONE time only, as
> the amount of stretch was part of the overall torque calculation,
> and once they were stretched once, they must not be used again be-
> cause they would BREAK.
Not so much that they will break, but that they may break, and more
importantly, they will not torque correctly.
You torque stretch bolts all the time. Whenever you tighten a bolt as
soon as the materials are touching, you are both compressing the
material, and stretching the bolt. There is for any material (thusly
any bolt as well) an increase in tension by stretching. As you continue
to turn the nut tighter, you stretch the bolt, increasing the holding
force of the bolt. At some point, the bolt or whatever will begin to
fail. In the case of metals, it goes into non-elastic elongation. Ie,
stretches permently. As soon as you get this, you loose lots of your
holding force.
You're used to this two ways. One, is the proper torqueing of bolts.
Engineers figured out that maximum. The other is experience. If you've
been using wrenches for a while, you've come to know that feel when a
bolt is as tight as it's going to get, and when you've gone too far.
All the torque stretch type bolts are is a refinement of this. Where
some carefull engineering has maximized the holding force capabilities
of a bolt. But in doing so, this maximization can only be achieved
once. After that, though they are often times re-used, they are not
working or holding as well as they can. Most times, it's still adequate
though.
> There may be other applications for stretch bolts, such as rod caps.
> I hope this is not a case of too little information causing too
> much trouble, on my part, but then it sounds like it could be worth
> your time to find out for sure. It sounds to me that these stretch
> bolts are quite different from the usual graded variety.
>
> Anyone out there have the definitive answer on this ?
>
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