[Supras] ARP-2.2 mm Cometic head bolt retorquing

[berniek at technicaldevelop.com]

Jim and group:

    Thanks for the interest in this matter.

    The torque wrench used is a simple beam type, with 120 lb-ft range in
each direction, and a handle pivoted in the center to keep the
effective beam length separate from any torque applied by "twisting of
the wrist".  While the "cracking" torque of the nuts may not mean much
under ordinary conditions, The RA finish of the nut flanges and the
washers was very good, and a good amount of assembly lube was used. 
Since the car had only been driven 50 miles or so, The nut "cracking"
was not the type of sudden, unexpected event seen in a high mileage
engine.  It was in fact, barely discernable.  After that, I did loosen
the nuts about 45 degrees and retorque, and wound up with the wrench
beam in the same position as it had previously been.

Of course you are correct about the ratio of static to dynamic friction,
but I believe you will find it to be less in an instance of lubricated
surfaces then in the classical physics experiment using a felt covered
board with a weight on top, pulled by a spring scale.  When surfaces are
lubricated, friction can remain high at low rates of relative movement,
until a hydordynamic wedge of oil develops, as in a conventional sleeve
bearing.  I don't recall exactly where in Shigley (mechanical design text)
it is, but there is a graph of a dimensionless number relating to friction
force as a function of bearing speed, luricant viscosity, and the like. 
The curve begins at a high value, falls rapidly, and climbs back up as
bearing speed is raised, and viscous shear becomes the dominant friction
source in sleeve or thrust bearings.

>
> Hmm, I'd be concerned with what you've done on the retorque.  Firstly,
> there's a reason why every torque wrench has stated in the manual that
> it should not be used to remove a fastener.
>
>>From wikipedia: "For a given pair of surfaces, the coefficient of static
> friction is larger than that of kinetic friction."
>
> http://en.wikipedia.org/wiki/Coefficient_of_friction
>
> Basically, the reading is inaccurate and useless, and this typically
> leads to damage of the wrench as it is stressed well beyond
> it's intended capacity.
>
> Also, when you stated that after loosening three of the nuts
> that "virtually no tightening was required, even up to 90 lb-ft"
> suggesting you did not loosen them enough to get enough movement
> when tightning to get an accurate torque reading.  So now you
> have potentially three nuts undertorqued.  I recommend you properly
> retorque the head by loosening each fastener approx 1/16th to 1/8th
> of a turn and tightening back up to your torque of choice in the
> proper order stated in the manual (alternating pattern from the
> inside out).  Make sure each fastener has some movement as it is
> tightened and as the desired torque reading is approached.  I prefer
> to use a recently calibrated beam wrench over a clicker for this
> process.  Finally, as ARP recommends I would torque each fastener
> to full torque 3-4 times to properly mate their threads.  Hopefully
> you did this during the initial build up.
>
> ARP has some excellent articles on their website about fastener
> torque and clamping force.
>
> On Sat, Oct 27, 2007 at 10:38:57PM -0400, berniek at technicaldevelop.com
> wrote:
>> Rockey and group:
>>
>>     Just got done pulling the top end of the engine apart and first
>> removed the cam cover on the exhaust side after about 50 miles and maybe
>> 10 temperature cycles.  The ARP nuts were tight.  They cracked slightly
>> when loosening them with a torque of about 80-90 lb-ft.  When
>> retorquing, virtually no tightening was required, even up to 90 lb-ft.
>> I tried about 3 nuts, two in the center and one at the front end, and
>> they were all the same.  After seeing that, I did not bother to remove
>> the intake cover (difficult due to fuel pressure gauge  in the injector
>> rail and dual injector connectors.  Left the stock ones in place "just
>> in case" and spliced in the Greddy connectors).
>>
>>
>>     I still wonder, though, if in time the indentations the gasket will
>> make in the head and block from thermally induced relative movement will
>> make retorquing necessary in the future.  Guess I might take a look at
>> this again at perhaps 10K miles.  In the meantime, on to 14 PSI boost.
>>
>>
>>     As always, comments would be appreciated.
>>
>>
>>           BernieK
>>
>>
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>