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[terry paget]

I bet they don't. I wonder if they are still available for 18 year old cars. I know we on the forum take liberties. When changing wishbones I used to, but don't any more. Changing a diff I am alarmed at how many of the bolts involved are stretch bolts which Haynes says must be replaced.

[Nick W]

All bolts stretch, that's how they work. But how many suspension bolts are torque to yield, which is what you meant? Personally, I only replace suspension bolts if they're worn, rusty or damaged. TTY head bolts are a different matter, and things like big end, main bearing caps and flywheel bolts are definitely worth considering if I get that far into an engine.

[LC0112G]

Most bolts (like 99.999%) where the tightening spec is X Nm + Y degrees are torqued past yield - or "Stretch Bolts" as Nick says. They should therefore be replaced with new ones if they're ever undone. A bolt which only has an X Nm tightening spec is probably not torqued past yield, so can be re-used unless there is info to the contrary.

In practice, people only tend to replace bolts that are in hard to get at locations where failure might have catastrophically expensive results - like inside engines (con rods, head bolts etc). I'd be surprised if an indy garage replaced old bolts with new ones on things like diff mounts, suspension arms or drive shafts.

[terry paget]

Are stretch bolts made of different specification steel to ordinary bolts, or merely tightened beyond a 'yield' point? If so, what was the point of doing it? Are they weakened by the experience, and liable to fail if thus treated again?

[LC0112G]

Are stretch bolts made of different specification steel to ordinary bolts, or merely tightened beyond a 'yield' point? If so, what was the point of doing it? Are they weakened by the experience, and liable to fail if thus treated again?

There is no such thing as a "stretch bolt". The term is used, erroneously, to indicate a bolt where the manufacturers intend it to be tightened very close to, or past it's yield strength.

All bolts stretch as they are tightened. They behave like springs and stretch according to Hookes Law. Doubling the load doubles the stretch but critically removing the load allows the bolt to contract back to it's original length again. This applies ONLY up to the yield point. If you tighten past yield point the bolt will start to behave non-elastically, and when you remove the load it will have been permenetly lengthened - It won't contract to the original length. If you then tighten it up again to the same load, it will permenetly deform again. And again and again. There is no way of accurately knowing how much it can be permenetly stretched before it will snap - might be 10 tightenings, might be 3.

The load in the bolt is very difficult to predict from the tightening torque because it depends massively on the friction parameters of the bolt thread, nut thread and any lubricant. There can easily be a 50%-200% variation in the load in the bolt just by torque tighteneing. However, angle tightening does produce a known extension in the bolt - turning the bolt 360 degrees stretches it by one thread pitch. If you know the material the bolt is made from and it's dimensions, you can then calculate the load in it required to create that stretch. And vice verca, you can calculate how much angle is required to impart the required amount of pre-load. That's why things like head bolts have angle tightening specs, so the load (on the head gasket) can be accurately controlled.

Metric High Tensile bolts are typically marked 8.8, 10.9 or 12.9. The number before the decimal is the tensile strength of the material in MPa. The number after the decimal is the percentage of this tensile strength that you can use before yield starts to occur. You can down load calculators from "Bolt Science" if you're really interested.

[Enceladus]

It's worth adding that some or maybe most of the Omega suspension bolts are mated with elliptically distorted thread nuts. If the bolt & nut assembly is corroded then it's more difficult to disassemble and the process of doing so can potentially damages the threads. If you then reuse the bolts and nuts then it might be difficult to re-torque them accurately.

It's a judgement and experience call. My experience is that the bolts are usually in a better condition than the nuts. So I reuse the bolts if they're undamaged and renew the nuts where I have some available.