Stretch Bolts

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.

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.

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

Does make you wonder if it was simply a guess that they are stretch bolts

- by the maker's recommendation to fit new ones.

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.

Actually I think you've already assisted in that matter in one of your most recent comments, Dave. I'd forgotten about the different methodology between tightening stretchers and regulars. Seems like a perfectly good way to tell 'em apart.

Can't say I've ever heard of pre-threadlocked bolts before. Another method they sometimes use is stainless steel wire woven though the bolt- heads. Bentley did that for all sorts of fixing points around the car where IMV it was totally unnecessary. But I'm no engineer....

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.

and correctly tightened up.

It's ironic where bolts should be used the once, that tensile strength of steel actually increases after a 'stretch'.

By a form of work hardening? So basically it's OK to re-use 'em. Thanks, Fred.

Not if they are stretch bolts. Follow the correct tightening procedure and a used one will snap.

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.

Thanks for the interesting explanation; helps fill in some blanks.

As per Peter's response.

Most stretch bolts can be used a number of times and should be marked accordingly. IIRC Ford flywheel bolts can be used 3 times.

I think as Mr Cheerful has alluded to, this is really as issue where new bolts come with threadlock compound. Where after a single use these bolts will have a damaged coating.

There's also some information here:

Lockwire? It's widely used in aviation.

Interesting. Only genuine stretch bolts I've used are specified on later versions of the Rover V8. Earlier one used conventional bolts.

The early type can be re-used if they are the same length as a new one - ie not stretched.

The stretch type *must* be replaced. Not really surprising as a used one measures longer than a new one. If it doesn't, it means it hasn't been tightened to the point where it stretches.

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 f*ck-arse about like this.