ira kaufman wrote:
if i may give an opinion.many head gasket failures i've repaired have had a
crack in the sealing ring.i'd venture to opine that after the many heatings
and coolings the metal in the sealing ring is tempered and can't reliably take
even the original torque let alone an increase without cracking.
The sealing rings are generally made of an austenitic stainless steel
alloy, and as such are NOT subject to tempering in the manner which you
imply might occur.
Further--although it doesn't really apply here--getting a metal hot
generally tends to anneal it--and thus reduce any tendency to crack due
to the presence of any strain hardening.
Not to deny that the cracking you have observed doesn't occur, it does,
I've seen it much as you have. Such cracking is generally the result of
'fatigue'--too many cycles at too high a level of stress. If stress
stays below the so called 'endurance limit' of a metal part, then it
will not crack and fail. It generally takes between eight and twelve
MILLION stress cycles at a given level with no failure before it is
established that a metal is not being stressed beyond its endurance limit.
Needless to say--you ain't gonna get even into the neighborhood of that
many cycles torquing and re-torquing a head. What WILL get you there is
the number of pressure rises due to combustion events the gasket ring
experiences in an operating engine. What generally WILL cause the
observed cracking is too LITTLE clamping force on the sealing
ring--generally as a result of inadequate torquing of the head
fasteners, inadequate clamping force due to failure to lube said
fasteners prior to torquing them, improper height of the liners relative
to the block deck, and/or improper (or non-existent) torquing of the
head fasteners as an engine warms up for the first time.
In plainer language--if there is adequate clamping force between the
head and the top of the liners, the friction between the sealing ring
and the head and liners acts to reduce the (varying) radial stress on
the sealing ring sufficiently to prevent it from fatiguing and failing.
Greg
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