From digest.v6.n228 Sun Feb 16 16:16:38 1997
From: Randy Hubbard <raceware_at_wwnet.com>
Date: Sun, 16 Feb 1997 14:37:47 -0500
Subject: Re: S-T-R-E-T-C-H BOLTS
The replacement bolts your dealer supplied are referred to as
s-t-r-e-t-c-h bolts, because they are PERMANENTLY stretched when
properly installed. When stretched they actually "neck down" or have a
reduced diameter and area where the metal has been stretched. This
reduced area has experienced Plastic Deformation, and will not hold the
same torque (tension) if applied again, it will in fact stretch more,
(loosing tension) and often fracture. This is why stretch bolts must
ALWAYS be replaced, and never reused.
Stretch bolts are used by car manufacturers for several reasons, usually
none of what is reported in the hotrod magazines... First with stretch
bolts automated engine assembly is much easier as the tightening of the
bolt can vary by as much as 1/2 turn in most cases and provide the same
clamp force. This is due to the fact that when a fastener is tighten
into plastic deformation, it will not change it's effective tensile
strength, (and clamp force) by very much over a given amount of
elongation, (stretch). So when the pneumatic machine tightens the bolts
in your engine, it can vary up to 1/2 turn and still have the same clamp
force... at least in theory. Having watched the engines being assembled
using this method, and viewing real-time direct readouts of clamp force
verses degree rotation of the bolt, I can assure you reality and theory
are not in 100% agreement!
The second reason car companies use stretch bolts is that ALL composite
head gaskets take a thermal set in operation. Solid annealed steel
gaskets may have minimal gasket compression after engine operation, but
usually exhibit some, though less than any of the composite gaskets.
Forget what you've read about permanent torque head gaskets, as they
don't *really* exists. Perhaps less torque LOSS gaskets would be a
better term than permanent torque...
Now the reason for this lesson in engine design... is to expose the
popularity of stretch bolts. Anyone who's been in the auto industry for
20 years or more probably remembers the days when it was mandatory to
re-torque head bolts on new cars after 1,000 miles or so. This was
because of the above issues with gasket compression after operation.
Well "stretch bolts" were intended to be the cure-all and end-all of
head gasket re-torquing... as the car companies were paying dealers to
re-torque the head bolts on new cars, which costs lots of money.
Unfortunately, stretch bolts may not have totally lived up to their
In order for a stretch bolt to be cost effective, it is often made from
low grade steel with a class rating of 10.9 and a typical tensile
strength of 150,000 PSI. Some companies use even LOWER strength stretch
bolts! For anyone familiar with Society of Automotive Engineering specs,
you'll realize that a metric class 10.9 rated bolt has about the same
tensile strength as an SAE grade 8 bolt available from your local
hardware store... Not exactly Rocket Science quality fasteners. The
clamp force between the head and block is limited by the yield point
strength of a particular bolt. The yield point is the point at which
plastic deformation (stretch) occurs with little increase in clamp
force. Low strength bolts limit the available clamp force which can
allow head gasket leaks.
By using a stretch bolt, manufacturers are attempting to maintain the
same clamp force when a head gasket is initially installed and AFTER it
has been thermally cycled and compressed. In reality, this doesn't often
occur as the gaskets do not compress uniformly over the entire gasket
surface, and there are other factors especially the structural rigidity
of the head and block, that enter into the equation.
Due to the many variations that can and do occur, stretch bolts have a
checkered history. When used in a NON-gasketed installation, stretch
bolts seem to meet basic clamping requirements, as long as they are not
subjected to loads that exceed the humble tensile strengths of their
design. In gasketed applications, our experience is that they are a
"cost effective" measure for the manufacturer, but not the best
engineered fastener design for maximum performance and reliability.
As an example, it's not un-common for engines equipped with stretch
bolts to experience head gasket failures. Typically it starts as an oil
or coolant leak and can eventually lead to compression loss to the
coolant system and chronic engine over-heating. This is usually a result
of the stretch bolts having insufficient clamp force to keep the head
from lifting on the gasket due to cylinder pressure. Over time the head
gasket deteriorates to the point where a leak is visible or over-heating
occurs. Regardless of how competent a technician is, if the head bolts
have inadequate clamp force, the problem will re-occur, and usually
sooner than later. The solution is to install better quality fasteners
with adequate strength to meet the demands of the application.
Your Mileage May Vary -- Literally!
On the 'Bahn,
Randy, a.k.a. BoltMeister
NEW** VW/Porsche/BMW Interest At: http://www.wwnet.com/~raceware **NEW
> *** My dealership sold me torx head bolts for my '83 528e when I did a
> valve job at 217K. They are one time usage stretch bolts that are not
> normally tightened with a torque wrench but tightened to a torque angle. I
> have a page out of a tech bulletin but it does not show the number.
> Interestingly, with these bolts, you do not have to re-torque after a heat
> cycle. Tighten once - forget.
> The part number was 11-12-1-726-478, list price $31.32 in 1993.
> Norm Grills (grills_at_airmail.net) Plano Texas BMW CCA # 57310
> ONLY BMW's - 635, 528, 325, K100RT BMW MOA # 19040
> Webmeister -
> Lone Star Chapter BMW CCA \
> Lone Star BMW Riders \ http://web2.airmail.net/grills/
> E24 FAQ /
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