Re: Stag Cylinder Head Studs

[Glenn Merrell <gmerrel@domain.elided>]

Hello Folks,
    Okay, there has got to be at least one mechanical engineer out there
on this digest.  So, after reading up in my Machinist Handbook on
fasteners, threads and torques, I'll give it a go.  Let's take a look at
the reasoning to retorque the cylinder head studs.
a.  On the Stag engine, the cylinder head studs are not perpendicular to
the block, where the head bolts are perpendicular to the block.
Expansion and contraction forces are not equal on the lower threads that
penetrate into the block to the upper stud threads.  There are 8 threads
into the block on the stud fully inserted, but only 3.5 threads engaging
the upper stud by the head nut.  There is more force per square inch
taken up on the nut thread area than than distributed across the threads
into the block; that is, assuming similar force on top and bottom parts
of the stud (which is actually different because of the diagonal
insertion into the block at the bottom), the bottom of the stud has a
larger thread surface contact to spread out the force, the upper stud
has less thread area by more than a 2.2:1 ratio bottom threads to top.
The nut takes more than two times the force due to less area.
b.  The cylinder block is cast iron, the cylinder head is cast
aluminum.  The cylinder head studs are hardened steel.  From my basic
physics, I recall that aluminum expands something like 3:1 to steel for
the same heat applied, and contracts in the same ratio.  Every time the
engine goes through a heat/cool cycle, the expansion/contraction
dynamics occur on all of the components.  The studs, threads, and nut
actually stretch, but being of hardened materal, do not return to their
normal position.  "N" number of heat/cool cycles later, the nut will be
loose on the stud unless other factors keep the nut from backing off.
Hence, locknuts, nylocknuts, etc are employed to resist movement after
proper torque has been applied.

The more temperature cycles the engine goes through, the faster the
loosening.  Retorquing is always done within 100-500 miles of a new
gasket set or head removal, then at an average point for the average
driver creating the average number of temperature cycles...or in other
words, a scientific wild A@# guess or wide range of mileage to be
considered safe without failure of the component.  If fasteners were
perfect, we would not have cylinder heads blowing the gaskets.  Why
retorque after 100-500 miles?  First, proper mechanical application of a
threaded fastener is; once it has been torqued, the expansion forces
stretches the fastener, but does not distort the threads.  The threads
actually hamb together.  This stretch factor is then accounted for by
retorquing the fastener back to original spec, plus a bit.  This creates
a proper mesh of the ID/OD threads to jamb tight.  In a perfect
application, that should be the last word, the fasterer should hold
forever.  But, threads, once distorted, now have a different torque and
holding rating then when new.  When taken apart, new fastening hardware
should be used, the old ones discarded and replaced with new.  This is
because once a thread is properly torqued, it has distorted and actually
matched its mating thread slightly.  You can only distort a thread so
many times before it fails from over torque, and the more times you
distort it, the less holding force it has, giving it a tendacy to move
under loads.
Now that whe have torqued properly, lets talk gaskets.
So why do gaskets fail?  As stated, the steel cylinder  block and the
aluminum cylinder head expand at different rates, but what does the
gasket do?  If properly applied, the gasket sticks to the steel block on
the bottom side, sticks to the aluminum on the top side, and the
fiberous core flexes with the expansion and contractions.  This is why a
gasket is used in this type of application.  Cylinder head gaskets are
not used to take up the "space" from groves and imperfections in the
metal, but to allow the surfaces some movement from temperature
cycling.  When the engine is machined, there is less than a half of a
thousants of an inch across the surface of the block or head face for
tolerance. The head gasket is thick to allow this normal movement
without infringing on the integrety of the seal..
Back to the original question, why do gaskets fail?  There are four
fluids involved passing through the gasket; coolant, fuel, oil, and
air.  Fuel being combusted runs through a wide range of pH properties
during its brief life, eroding the aluminum at the gasket junction. This
is why there is a metal ring at most block holes to the head holes which
allows both a physical and electrical bond between the two surfaces.
The metal rings also help hold the gasket together so it does not
separate top to bottom layers.  Early coolants had some similar
coorosive properties and is why deionized water was recommended when
filling.  Oil too, when reaching its end of life, has a higher coorosive
character that eats the metal surfaces.  Clearances are pretty slim
between some of the water jacket holes and cylinder to cylinder
distances.  If, during assembly, any contaminate like oil or water was
left on either block or head surface, the gasket may not adhere
properly.  This is why the service experts do not recommend any sealant
on head gaskets, but clean the surfaces with a non residue cleaner like
brake cleaner or alcohol.  This is also why the repair manuals recommend
sanding clean both surfaces, and throughly cleaning both surfaces before
applying the gasket.  Add a loose torque area and the gasket separates
in at the contaminate between gasket and metal, or if adhered, in the
middle, top from bottom layers, creating a small space.  If this
separation just happens to occur in the gap between the water jacket
hole and the cylinder  or oil hole, the high pressure high temperature
coolant or combustion gasses see a huge pressure differential and follow
the path of lower pressure.  In the stag engine, also up the stud holes
in the head.

    For those of you rebuilding your engines, if the gasket has become
fixed to the mating surfaces of the block and head, you will notice that
dissassembly destroys the gasket, literally pulling it apart.  As you
curse the scrapping and sanding effort, note that the gasket was most
likely working, but improper torque allowed the gasket to fail in the
middle layers.  If you take the head off and it is loosely sitting
there, it was either perfectly matched to both surfaces, or totally
useless.  Or, you may see that the gasket worked in some areas, but did
not adhere to the area where it failed due to contaminates.
    Some gaskets have a coating that is activated by petroleum, and they
recommend a thin wipe of fuel with a clean rag prior to fixing, then
they become slightly sticky.  Others have coatings that are heat
activated, and adhere when things heat up for the first time.

Not keeping proper torque on the fasteners will only allow the gasket to
fail more rapidly around that loosened area.  Undertorque will allow a
blowout/blow through, overtorquing will distort the cylinder head.
Torque spec's in the ROM were guestimates based on typical application
of the fastener and thread pitch and size,  used for the initial
fastening of all new components.   Experience shows that maybe +5 ft
pounds maximum from ROM specification may be more correct on first
retorque and then that same value on subsequent retorques.  More may
collapse or distort the head.

So, begging to differ, please retorque your cylinder heads, just don't
move things about expecting the gasket to keep together and keep a good
seal.  Move the head, replace the gasket.  Alwasy follow the retorquing
instructions in the ROM.

That is the best I can do folks.  How about some professional opinion
from an M.E.?

Regards,
Glenn Merrell
Triumph Stag Registry USA VP
membership inquiry's to:
Mike Wattam <101714.1343@domain.elided>

"Keep Your Stag Cool, Install a NEW Composite Cowl Today"
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