Intake Manifold Leaks

["Glenn A. Merrell" <gmerrel@domain.elided>]

Hello Tim,
    This problem occurs when the intake manifold and head surfaces are
not parallel, plane and square, left to right head and intake to head
left to right.  The situation occurs when initially installing the
cylinder heads and the intake manifold.  If the head intake surfaces and
intake manifold mating surfaces are not plane, at the same grind angles
and free of scratches/dings/cuts, then no amount of gasket sealant or
type of gasket material will make a permanent seal, sooner or later,
things will loosen up and leak.  The reason for this is things get out
of square by milling and changing to thicker gaskets that original
(0.040 old to 0.055 current, or 0.080 extra thick head gaskets), heating
and cooling cycles loosen the intake bolts faster because of the out of
square condition,  the first little leak then compromises the gasket
with dirt/coolant/fuel, and a continuous leak starts.
    It is very important to make sure all the surfaces are true, smooth
and parallel. Milling the head seats the head a few thou down closer to
the crank, "decking" the block takes another -0.005 or -0.010.  Basic
geometry shows that the angle and position of the head to intake
manifold then changes, and if the intake manifold is not ground to match
at the mating surface, surely a leak will start.

    What I do before installing the heads is set the heads on the
gaskets onto the block on two studs.  Then I bolt up the intake manifold
with gaskets, not full torque to fully compress the gaskets, but 1/2
torque and even all around.  Next I run a feeler gauge under the four
corners of each cylinder head.  If the gaps are all the same, perfect!!
BUT if not, something is out of square, most likely the intake manifold,
but possibly the cylinder head milling was improper.  In a situation
like this, no amount of torque will keep things together, as the intake
mainfold is actually being "stretched" to conform and fill to the space
between the cylinder heads.  It takes a bit of machining to make things
mate up correctly, then things will not leak.

In My Humble Amateur Opinion,

Regards,
Glenn  Merrell, VTR Webmaster ProTem
Triumph Stag Register USA VP
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