In article <2stamgF1nocp2U1@uni-berlin.de>, Mike G
<mikgibbs@tiscali.co.uk> writes
>
>"Peter Twydell" <Peter@nospam.demon.co.uk> wrote in message
>news:rMw97kF1gWaBFwKx@ntlworld.com...
>> In article <31303030343237394169513841@zetnet.co.uk>, Guy King
>> <guy.king@zetnet.co.uk> writes
>> >The message <IYM1+fDEtTaBFw+P@ntlworld.com>
>> >from Peter Twydell <Peter@nospam.demon.co.uk> contains these words:
>> >
>> >
>> >> I do know that some engine rebuilds require new bolts, but I've never
>> >> been 100% sure why that is either, unless it's something to do with
>> >> their being softish and the threads getting worn when they're
>extracted?
>> >
>> >Some bolts are "stretch" bolts which elongate during the tightening
>> >process and this is allowed for in the tightening specifications. I
>> >suspect that if you were cunning you could devise another tightening
>> >spec which worked with old bolts.
>> >
>> >However, I've re-used bolts on various engines which allegedly need jew
>> >bolts and not come unstuck yet.
>> >
>> OK, stretching makes sense if there's a nut on the exposed far end,
>> doing the pulling and clamping parts together. ISTR seeing a TV
>> programme years ago that showed the then-new K Series engine using this
>> technique. What's the process if there's no tensile force on the bolt,
>> though? How can a "standard" cylinder head bolt stretch when going into
>> a blind hole??
>
>There's always tensile force on tight bolts. Otherwise they wouldn't be
>tight, and up to a point, all tight bolts are stretched. Blind hole or nut,
>the same applies.
>Same with cyl head bolts, they stretch because the torque required to twist
>and break them, is greater than the resistance given by the thread.
>Stretch bolts are designed to stretch to a point from which they don't
>recover when removed. Restretching, weakens them. Therefore they shouldn't
>be reused.
>Mike.
>Mike.
>
>
OK, I get it now. I was forgetting the effect of the torque on the
threaded portion and considering only the head. Am I right in thinking
that what you're saying is that once the head is in contact with the
surface, the rest of the bolt is forced to turn in the thread when the
head is turned further, thereby moving down the thread and stretching ?
(Not elegantly expressed, but it's late and I'm still working)
--
Peter
Ying tong iddle-i po!