Re: was: Weepy Head Gskt Fix! is: stretch bolt defence clamping methods,

[Kevin Ford <cheryl_ford@bc.sympatico.ca>]

Avi wrote:
> 
> Mik,
> stretch bolts are used in HEAVY DUTY engines the ones that cost $500,000
> (and more) AN ENGINE.........including some small ships engines........the
> stretch bolts are installed with an hydraulic set-up...(and they get reused).
> (in essence all threaded fasteners are a VARIATION of so called "stretch
> bolts" including your "Raceware"), the difference in your OEM "stretch
> bolts" is that the bolt is taken a lot closer to the yield point, in other
> words using it to the MAX, that is also the reason why they are not
> recommanded for REUSE, since they (the bolts) might be too close to the
> yield point (after the first use), BTW, you can reuse OEM stretch bolts but
> you MUST measure the bolt to find out if it did not exceed the yield point.
> 
> "The highest possible clamping forces with very small variations can be
> achieved with threaded fasteners using the angular tightening method and
> the yield point controlled tightening method" (from automotive engineering
> handbook).
> 
> Avi
> one of this days you might tell me how you can pull a head STRAIGHT-UP from
> a VW 4cyl without taking the "studs" off......(engine in car...)
> 
> >As for the "stretch-bolts" being "BEST" there more in line with
> >"Least costly for manufacturer" category!

Avi,  if you are describing "stretch bolts" as torque to yield bolts,  
it is not generally advised to reuse them.  When the bolts are torqued 
to their specified rating, one point of the bolt actually reaches the 
"plastic" (yield) point (does not come close, but goes past it)  and 
begins to neck down (it may be on a microscopic level).  This point 
begins to work harden (like a tensile test piece).  The next time the 
bolt is used, the necking down continues, and a different (and less 
predictable) tension is achieved.

"Normal" or conventional head bolts & studs do stretch when torqued, 
but they are well within their elastic range.  

In car engines, the head bolt fasteners can be evaulated in a 
good-better-best fashion like this:  torque-to-yield head bolts, high 
quality ("race") head bolts, studs.  This is a common topic on the 
Buick turbo list, and you can research more on this on the Buick Turbo 
Regal web site. These guys really know their stuff (and how to make 
serious power) , and have plenty of good data to back up what ever they 
are saying.

I have not seen an Audi I5 (street) engine which was making enough 
power to start to encouter head lift and leaking/blown head gaskets, so 
I stick to the stock head bolts. ( some are torque-to-yield and some 
aren't)  But anything is possible.;)

Kevin Ford
Chase, B.C., Canada