As I've heard it described, it's because the studs are long, there is a 
certain amount of twist in them, so you need to *hold* the torque setting 
briefly. Since the 'break' type wrenches release immediately when the 
torque setting is reached, your heads end up under-torqued.

Mind you, this is just what I've heard, I'm not an engine builder by trade.

At 08:44 AM 11/6/2004 -0700, jimt wrote:
>On 11/6/04 6:54 AM, "Vince Jahn" <vjahn4533@CHARTER.NET> wrote:
>
> > Hi All,
> >
> > I was just about ready to put my rebuild heads back on my 85 westy, and was
> > reading in the Bentley manual where it recommends not to use
> > a "Break" type of torque wrench. Does anyone know why?  I have both, and
> > would prefer using the "Break" type because I believe it's
> > more accurate since I can dial in the 37 ft lbs.
> >
> > Thanks much,
> >
> > Vince
> >
> >
>You will find that debated in many groups.  Any torque wrench is only as
>good as its calibration.  I recently took a couple wrenches down to  a place
>that had the calibrated checkers for their wrenches and found mine to be
>accurate enough.  Off by about 2 lbs at 75 foot pounds for example on one
>wrench.  That wrench has seen a lot of use over the years and is still good
>enough for working on anything I am working on.
>
>����������������������������������������
>jimt
>Planned insanity is best.
>Remember that sanity is optional.
>http://www.tactical-bus.info (tech info)
>http://www.westydriver.com