Stretch Bolts

On Sat, 20 Jan 2018 12:11:26 +0000, Kumquat May wrote:


Well, there are certain higher standards for aircraft components, military issue hardware and critical care medical equipment and that is perfectly understandable. But cars? Nawww. The most dangerous nut in a car by far is still the one behind the wheel.
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London black cabs used to have every single nut and bolt lock wired at one time. Probably dating back to before self locking nuts etc became available. Plenty makers stuck with tried and tested ways long after there were alternatives.
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On 18/01/2018 18:27, Cursitor Doom wrote:

In that case, the wrong bolts would invalidate your insurance - which could make an even bigger hole in your wallet if detected.
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Surely if you have an expensive near new high performance car, you'd service it by the book?
But are you saying something like a pad change involves replacing stretch bolts?
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On Fri, 19 Jan 2018 00:39:30 +0000, Dave Plowman (News) wrote:

I would but it's not mine.

Not suggesting any such thing, Dave. I think the confusion arose through a typo by Mr. Cheerful. These are the bolts that hold the brake *calipers* to the hub carriers.
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Ah - right. So:-
'I'm not minded to pay a fiver each for one inch, M12 bolts (so on this car 50 quid all told just for the brake calipers alone) when I cannot see what's wrong with regular bolts just dabbed with Threadlock and done up a little tighter.'
doesn't actually refer to you?

Do they have to be removed to change the pads or discs?
If so, and they are one use stretch types I'd certainly replace them.
I tend to think the makers know best in this sort of safety critical situation.
Of course if it's not your car any bodge isn't going to effect you.
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On 19/01/2018 11:10, Dave Plowman (News) wrote:

Most modern calipers are secured to the mounting bracket by two bolts (usually 8mm thread by about 30mm), these bolts usually come with a new set of pads.
Caliper mounting brackets are usually held to the stub axle by some much larger bolts (around 12mm fine thread and about 25mm long) These only need removal when replacing discs.
Both sets of bolts are usually secured with threadlock.
I doubt whether either lot are stretch bolts, but anything is possible.
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I'd be surprised if they were - other than on a racing car etc where every ounce counts. Stretch bolts are usually reserved for where the very best fixing is required, and space is tight. Like say on a cylinder head. For things like a calliper fixing you'd normally just use ordinary high tensile bolts of an adequate size.
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On 19/01/2018 14:36, Dave Plowman (News) wrote:

Exactly, and unless they are stretch bolts or specified as one use only, then I would be quite happy to clean and reuse them with some fresh threadlock.
The original question of whether to re-use stretch bolts was a red herring IMO
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Does make you wonder if it was simply a guess that they are stretch bolts - by the maker's recommendation to fit new ones.
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On Fri, 19 Jan 2018 11:10:00 +0000, Dave Plowman (News) wrote:

As I said it's not my car but the owner is a very good old friend of mine who trusts me totally and I don't want to see him (seeing as on he's on a fixed income and hasn't had a raise for 10 years) pay a penny more than he absolutely has to, that's all. I look out for my pals.

A trifle unkind, Dave. A trifle unkind.
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Not really. Advising someone they can happily use inferior fixings for the brake callipers is not something a responsible person would do.
Such things should be replace with bolts of equal strength or higher.
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On 19/01/2018 18:41, Dave Plowman (News) wrote:

and correctly tightened up.
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On 19/01/2018 17:50, Cursitor Doom wrote:

It's ironic where bolts should be used the once, that tensile strength of steel actually increases after a 'stretch'.
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On Fri, 19 Jan 2018 19:11:17 +0000, Fredxx wrote:

By a form of work hardening? So basically it's OK to re-use 'em. Thanks, Fred.
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Not if they are stretch bolts. Follow the correct tightening procedure and a used one will snap.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 19-Jan-18 11:42 PM, Cursitor Doom wrote:

No and maybe.
The increase in strength is due to plastic yield. This shifts the initially equal +/- elastic limits so it can take a higher tensile load before further yield.
Some factory service manuals will specify a max bolt length, if the bolt is shorter than the limit it can be re-used. If it is longer then it has yielded too much. The plastic hardening curve has a near flat portion where additional extension takes nearly no increase in load, this would give no reserve for the odd overload due to detonation etc.
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On Sat, 20 Jan 2018 08:02:59 +0000, Peter Hill wrote:

[snip]
Thanks for the interesting explanation; helps fill in some blanks.
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On Sat, 20 Jan 2018 11:37:55 -0000 (UTC)

There's also some information here:
https://mechanics.stackexchange.com/questions/32397/always-replace-torque-to-yield-bolts
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On Sat, 20 Jan 2018 12:04:32 +0000, Davey wrote:

torque-to-yield-bolts
Hmm. Maybe, but....
"It is very important to follow the proper torquing sequence and specifications when installing T-T-Y head bolts. Always clean the threads where the bolts pass through, any thread damage, corrosion or rust will create excessive friction giving you a false torque reading, robbing you of valuable clamping force. Using engine oil, lightly oil the threads and under the heads/washers on T-T-Y bolts unless otherwise specified by the vehicle service manual. Clean, oiled threads prevent binding, allowing for accurate and consistent torquing. Be careful not to over-oil the bolts, especially if they are threading into a blind hole. Too much oil will hydrolock the bolt and give false torque readings."
AFAIC, this is venturing into FUD territory. Like any busy person has the time to fuck-arse about like this.
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