Problem is, you're anchoring in the wrong direction. When you torque them
you're bottoming them, which presses against the thread faces from the
bottom up, when you torque the head on, you're pulling on them in the
opposite direction. the threads at the bottom don't help "anchor" the stud
until there is enough stretch in the stud to pull them away from the bottom.
Until then all the tension has to be held by the top few threads. In
practice, the stud won't stretch that much and the threads will fail long
before you get there.
-----Original Message-----
From: N197TR4@cs.com
To: Bill Babcock; rocky@tri.net; bradlnss@lightspeed.net; FOT@autox.team.net
Sent: 3/6/2004 5:50 AM
Subject: Re: re-torqing cylinder heads hot???
Something not addressed here, and perhaps it is not relevant......???
The block side threads are necessarily coarse going into gray iron or
cast iron castings.
Not very scientific on my part, but my head tells me to moderately
torque the blockside fastenings as they are more 'anchors' than anything
else.
The theory of not torquing studs in a head goes like this:
If you torque the stud in the block it bottoms and the pressure of
engagement is on the back side of the threads, especially towards the
bottom
of the hole assuming that all the threads are uniform. When you stretch
the
stud with head clamping tension it pulls the threads the opposite way,
especially towards the top of the block. You can easily see that for
some
tension value there will be threads that are engaged on the backside
(towards the top) and threads engaged on the frontside (at the bottom),
and
some hovering in the middle with no engagement at all. This actually
makes
it more likely that you'll pull threads from the top of the block, or
crack
the metal around the threads.
You never have this problem with bolts, though you do have the problem
of
not getting full engagement if the bolt is not exactly the right length,
and
the higher likelihood of shearing during torqueing since the bolt is
being
turned as it's pulled (a stud is only being pulled).
There simply isn't a good reason to torque studs into a block.
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