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Surface Flatness
To maintain constant contact between the gasket and the head and block, deck surfaces must
start out flat and remain flat after the fasteners are torqued. The maximum initial
"out of flat" as measured with a straight edge and feeler gauge should not
exceed .0025" in any direction. To find out if the surfaces are staying flat during
engine operation, it's necessary to "map" the used gaskets. Wipe them off and
trace the outline on a piece of paper. Measure the thickness of the gasket body within
1/4" of every bolt hole and at a point midway between bolt holes. In addition, try to
find an area of the gasket body that has not been compressed and measure the thickness
there as well. The compressed areas should be 003-.004' thinner than the uncompressed
areas, and all compressed thickness' should be within .001-002" of each other. If the
compression is inadequate or uneven, the clamping force can be changed by raising or
lowering the torque on the fasteners in those areas 5 ft lbs. at a time. This will fine
tune the load on the gasket and result in less flange bending. Examine the gaskets again
at the next tear down. If they are evenly compressed: the head and block are staying flat
while the engine is running. This reduces cylinder bore and valve seat distortion which
makes more power.
Clamp Load !Torque Procedures
Insufficient clamp load is the cause of many head gasket problems if the gasket is not
tightly clamped between the head and the block combustion gases and engine fluids can leak
past the gasket.
The key to sufficient clamp load, is to have adequately stretched head bolts or head
studs. Proper clamp load will not be reached if head bolt assemblies (or stud/nut
assemblies) have too much friction. This is because torque wrench readings will reach
specification before adequate bolt stretch has been achieved. Clamp load normally declines
moderately over time as the head gasket " relaxes" and bolt stretch is reduced.
We measure this as "torque loss" of the bolts. Clamp load can decline
excessively due to extreme engine temperature, excessive head motion or detonation. For
proper installation:
· Use hardened washers under bolt heads (or nuts) to prevent galling of the
cylinder head and to reduce friction.
· Tighten head bolts (or nuts) with a smooth motion. Sudden or jerky movement of
the torque wrench gives false readings, resulting in clamp loads as much as 20%
below normal.
· Follow the recommended torque pattern and tighten bolts or nuts in at least three
steps, up to the specified torque.
· Fastener torque specifications shown often are for use with moly, anti-seize, or Gray
Bolt Prep (GRA2) Motor oil or EP grease is not recommended. Bolts which enter the
engine's water jackets should be sealed with Teflon sealer or Gray Bolt Prep. If more
clamping force is required to correct a sealing problem, the torque can be
raised 5 to 10 ft. lbs. at a time until the problem is corrected. Just remember that
excessive torque can strip the threads, break the fastener or permanently warp the
flanges. Approach the problem carefully and slowly. Don't try to use excessive
torque to make up for warpage or defective surfaces -- it won't work!
With higher installation torque, it's more important to chamfer the bolt holes in the head
and the block. This will prevent the threads from pulling up and distorting the block deck
surface which can keep the gasket from being compressed properly. If you have doubts about
how much chamfer is needed, examine the gasket around the bolt holes. If there are signs
of gasket crushing within .050-.100" of the bolt hole, increase the amount of chamfer
slightly.
· In severe service, such as racing, retorquing is recommended to restore maximum clamp
load. After a complete engine warm up and cool down, retorque the bolts or stud/nuts to
specification.
In circumstances where retorquing after running the engine is not possible, the next best
method is retorquing before the engine is taken off the engine stand. After torquing the
fasteners, wait at least 10 minutes, back off each fastener, one at a time, 1/8-1/4 turn
and pull them back up to the torque specification. This will allow for gasket relaxation,
thread engagement variations and ensure consistent clamp load.
Retorquing bolts that use Gray Bolt Prep will require approximately five ft./lbs.
additional torque to overcome the locking action of the compound. Retorquing will not
adversely affect the sealing capability of the material.
Appearance of Used Head Gaskets
Learning to "read" a used head gasket can help you solve some problems and
prevent others before they happen. A "plug light" with a magnifier is an ideal
tool for close examination of the gasket.
· Examine the combustion armor for carbon tracks which indicate either combustion leakage
or that the armor was hanging into the combustion chamber or cylinder chamfer.
· Examine armors for discoloration of the stainless steel. This is a sign of excessive
casting temperatures.
· Look for signs of seepage around coolant holes. A gasket which was sealing properly
will have distinct impressions of the castings around coolant holes. If impressions are
not distinct or if the gasket coating has been washed away, there was probably a
coolant leak due to inadequate clamp load or excessive head bending.
· Measure the used gasket with a micrometer to see if it had been properly
compressed as described in surface flatness section.
WATER CIRCULATION
Most head gasket combustion seal failures occur because of excessive heat in the gasket
sealing areas. To correct the situation, more coolant flow must be provided to these
areas. This can be accomplished through the use of a more efficient water pump, higher
pump speed, restricting the coolant flow out of the engine (increases jacket pressure) or
revising the coolant holes in the castings and/or the gaskets. Remember to make only one
change at a time so the effect of a particular change can be determined
· Many OEM and aftermarket water pumps do not flow sufficient coolant evenly to both
sides of the engine. We highly recommend the newer design high performance pumps
introduced by manufacturers such as Edelbrock and Howard Stewart.
· To maximize the life of head gaskets, the use of electric water pumps is not
recommended. They often do not flow sufficient coolant to prevent hot spots in the engine,
even if the "gauge" temperature is normal.
For an even more extensive look at gasket usage and tips on doing it like
the "Pro's", Use this link to go to Fel-Pro's web site to
read more.
Note: This link opens in a new window. (Fel-pro's site is one of the best we've seen for
information and tips. Let's hope we see more companies doing what Fel-Pro have done.)
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