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By unknown | Feb 25, 2009 | COMMENTS [ 0 ]

The term "stretch bolts" doesn't sound good. Bolts shouldn't stretch. How can they do their job if they start stretching?

The term "stretch bolts" doesn't sound good. Bolts shouldn't stretch. How can they do their job if they start stretching?

The reality is that any bolt whose purpose is to clamp things has to stretch a tiny bit to create the tensile stress within itself, which provides (and maintains) the clamping force and the friction on the threads that prevents it from coming loose. Let's take the example of cylinder-head bolts.

Their heads press on the cylinder head and their threaded ends screw into holes in the engine block. Thus they clamp the cylinder head to the block, sandwiching the head gasket between the two. When refitting the cylinder head to the block, after replacing the gasket for instance, the bolts are screwed in tight, to ensure the cylinder head sits evenly on the gasket. Then you tighten them. Once the bolt heads are in contact with the cylinder head, any further rotation of the bolt causes the threaded portion to advance deeper into the block while the head is held back. This can only happen if the bolt stretches a little and in this way a tensile stress is created in the bolt because it is trying to return to its unstressed length.

But as the tensile stress increases, there comes a point where the bolt will yield. Any further increase in tensile stress will make the bolt deform permanently. Now it will no longer return to its unstressed length if the stress is removed, though in an installed position it will still try to become shorter and therefore will retain the tensile stress.

Thirty-odd years ago, the stretch bolt was introduced. It has steadily gained in popularity among manufacturers, if not among mechanics. Careful design and choice of materials are used to ensure that such a bolt has a yield stress (the tensile stress at which the bolt starts to yield) somewhat lower than the traditional bolts. Stretch bolts are installed by first nipping them all down to a fairly low-torque value, calculated to bring them close to their yield stress. Thereafter they are tightened a prescribed number of degrees on a second round of tightening. This will take them past their yield point. Friction, and its associated danger of insufficient clamping force, has been taken out of play.

Unlike the older cylinder-head bolts, stretch bolts don't have to be (normally shouldn't be!) re-torqued after the engine has worked for a certain length of time. Mechanics who grew up before the throw-away era are less keen on stretch bolts because these are meant to be used once only. Once a bolt has deformed permanently, it should never be used again unless the manufacturer stated otherwise. - Motoring Reporter


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