Interesting article I found in MOTOR Magazine (November 2009):
<i><b>By design, torque-to-yield bolts are tightened very close to their yield (stretch or
break) points. These bolts can very easily break during removal and installation unless
special procedures are followed.</b></i>
<b>Hit Me!</b>
<i>I am attempting to remove the cylinder head from a 4.2L Vortec 6-cylinder engine in a
2005 CMC Envoy. When I started to remove the head bolts, three out of the six I attempted
broke off. At that point, I decided to stop. Am I doing something wrong? Is there a special
procedure/or removing these bolts? And do I need to replace all of them when I finally get
the last one out?
John Stepp Tucson, AZ
</i>
Like most recent engines, the Vortec is equipped with torque-to-yield (TTY) head bolts.
You've probably installed bolts like these before. Instead of specifying a torque value for
bolt installation, the manufacturer requires that these bolts be tightened using a torque
plus angle procedure. After the bolt is tightened to the initial torque value, it's tightened
an additional number of degrees to reach the final value.
Tightening bolts in this manner brings them very close to their yield points. The yield
point is when a bolt begins to stretch or, if taken even further, breaks. Slightly stretching
bolts gives them the resilience they'll need to keep the gasket properly compressed over
thousands of heating and cooling periods in the engine's service life.
As you might imagine, a bolt that's been stretched (even slightly) is going to be more
fragile than a bolt that has been tightened but not stretched. When it comes time to
remove TTY, special precautions must be taken. GM issued a technical service bulletin on
this topic on Oct. 21, 2008 (No. 05-06-01-026B).
It applies to a wide range of GM models, including:
2004-07 Buick Rainier
2002-09 Chevrolet TrailBlazer and GMC Envoy
2004-09 Chevrolet Colorado and GMC Canyon
2002-04 Oldsmobile Bravada
2006-09 HUMMER H3
2005-09 Saab 9-7X
These models are equipped with 2.8, 2.9, 3.5, 3.7 or 4.2L Vortec inline engines with
VINs 8, 9, 6, E, S and RPOs LK5, LEV, L52, LLR, LL8.
All of these engines have TTY head bolts and may also have TTY main bearing cap bolts.
(Important to Triumph owners!)
<u>Over time, the head bolt threads may seize to the corresponding engine block internal
threads. When you attempt to remove the bolt, it breaks at its weakest point before the
seized threads break free</u>.
GM's solution to this problem is remarkably simple.
<u>Before attempting to remove the bolts, use an appropriately sized punch and hammer
to rap on the head of each head bolt. The vibration should free the seized threads and
assist in successful removal. </u><i><b>Do not use air tools of any kind during your removal
attempt</b></i>. <u>Using hand tools only, carefully but confidently remove the remaining bolts.</u>
In a perfect world, that would be the end of the story. But despite careful preparation and
your best efforts, some head bolts still may break. The strength of these bolts may have
been compromised by the torque-to-yield procedure and they just weren't strong enough
to make the return trip out of the engine.
Don't despair; GM offers an additional lifeline.
A broken bolt extractor kit (Part No.EN-47702) is available to assist in removal of the remaining bolt segment.
The kit includes the following components:
• One 1/2-in. reverse-twist drill (EN-47702-6);
• One double-ended drill pilot insert (EN-47702-1) to ensure a straight drilling procedure;
• Drill pilot inserts for larger diameter head or main cap bolts (EN-47702-2) to ensure a straight drilling procedure;
• Bolt extraction #3 EZ Out (EN-47702-3) to be used after the drilling procedure;
• M11x2 bottom tap (EN-47702-5) to chase the head bolt threads after the completion of bolt removal;
• M10xl.5 bottom tap (EN-47702-4) to chase the main bolt threads after the completion of bolt removal.
<font size="2"><i>Despite your best efforts, some
torque-to-yield cylinder head
bolts still may break during removal.
If they do, special tools like this
reverse-twist drill bit may be needed
to remove the remaining broken bolt
segment from the block.</i></font id="size2">
The bolt threads may loosen when the bolt breaks. In this case, you may be able to
remove the remaining bolt segment by backing it out with a pick tool or the reverse-twist
drill bit. It's worth a try before you get into drilling the broken segment for the EZ Out.
After all of the broken bolt segments have been removed, some careful preparation is
necessary before reinstalling the cylinder head. In addition to cleaning the gasket mating
surfaces, the threaded engine block holes must be propped for installation of the TTY
bolts. Utilize a thread chase tool like those found in the EN-47702 kit to clean the
threads. Follow this cleaning with dry compressed air to ensure that the threads are clean
and dry prior to installation of new TTY bolts. Do not place oil or threadlocker on the
bolts. <i>Never</i> reuse TTY cylinder head or main bearing bolts.
Threads contaminated by trace amounts of debris, antifreeze or oil will cause the bolts to
creak or snap while being tightened. These bolts will fail due to excessive torque. In
extreme cases, the threads may need additional cleaning with a nonresidue cleaner like a
brake clean product followed by drying with clean and dry compressed air.
jclay
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