8/26/2008
Pointers for Servicing Leaky Gaskets
This information should be considered by engine builders delivering a rebuilt cylinder head to a DIY/retail customer or an installing garage that has had the head rebuilt.
Question: When replacing leaky gaskets on an engine, do I just scrape or peel off the old gaskets, or do I have to do something else?
Answer:
Removing gaskets used to be a fairly simple procedure. Most engines
used to have cast iron blocks and heads, with iron manifolds and
stamped steel covers. Iron castings could withstand a lot of abuse, so
if the old gaskets were stuck in place, you could scrape them loose, or
grind off the residue. You didn’t have to worry too much about damaging
the sealing surface.
Today’s engines are different. Most are
either “bi-metal” engines with aluminum cylinder heads on cast iron
blocks, or all-aluminum. Stamped steel valve, oil pan and timing covers
are still used, but so are cast aluminum, magnesium and plastic covers.
Plastic intake manifolds also are common. These softer materials can’t
take the abuse their cast iron ancestors could, so gasket removal
requires more finesse rather than brute force.
Care must be used
to avoid scratching, gouging or damaging these parts when gaskets are
removed. A small scratch or gouge may not seem like a big deal, but it
does not take much of a surface imperfection to create a leak path on
many of these engines.
Gaskets have also changed. Many engines
today have soft-faced composition head gaskets with a solid or
perforated steel core surrounded by graphite or a non-asbestos
material. Some have a slippery “non-stick” coating that improves cold
sealing and also makes the gasket fairly easy to remove. But others
have a sticky silicone coating that adheres to metal surfaces and is
difficult to remove. Many late-model engines also use “Multi-Layered
Steel” (MLS) head gaskets. These gaskets are made of three to five
layers of steel and have a very thin rubber coating on the outside to
improve cold sealing. The rubber tends to stick to the surface and can
be difficult to remove.
MLS head gaskets also require a very
smooth, almost polished surface finish to seal properly, which means
you have to be extremely careful not to scratch or gouge the surface
when replacing a head gasket on one of these engines.
Most
cork/rubber valve cover, oil pan and timing cover gaskets have been
replaced with molded silicone rubber gaskets (which come off easily but
should not be reused), and some covers have no gaskets at all. They are
sealed with RTV silicone (which has to be scraped off and replaced with
fresh sealer or a gasket when the parts are reassembled).
Many
intake manifolds are sealed with gaskets that have raised or printed
silicone beads on a metal or plastic carrier. Others have no gaskets at
all and just use sealer.
One of the safest ways to remove old
gaskets is to spray them with aerosol gasket remover. Give the chemical
time to work (usually about 20 minutes), then carefully scrape away the
gasket residue with a scraper tool (not a screwdriver) or razor blade.
The
trick to using a gasket scraper correctly is to scrape at an angle that
is almost parallel to the surface. By keeping the angle small, the tip
of the scraper will slip under the gasket and shear it away from the
surface without digging in. If you try to use it like a chisel, you’ll
probably end up gouging the surface and damaging the part.
Using
an abrasive pad in a drill to whiz off gasket residue on aluminum or
plastic parts is risky because the abrasive may remove material that
leaves a depression that may cause a replacement gasket to leak.
Question: Can an engine have a bad head gasket, but not leak coolant into the crankcase?
Answer: Yes.
If the leak is between cylinders, the gasket may not leak any coolant
into the crankcase or cylinders. But it will leak compression between
the adjacent cylinders. This will usually cause a steady misfire and a
significant loss of power. A compression test or a power balance test
can be used to confirm the problem.
This type of head gasket
failure may be the result of overheating, detonation or improper
torquing of the cylinder head bolts. Any of these may crack or crush
the steel armor that surrounds the combustion chamber.
If a head
gasket is leaking coolant, the loss of coolant will eventually allow
the engine to overheat - which may cause further damage to the head
gasket and possibly the cylinder head, too. An internal coolant leak
can be confirmed by pressure testing the cooling system to see if it
holds pressure.
Adding sealer to the cooling system may
temporarily seal a head gasket that is leaking coolant, but such a
product is no help whatsoever if the head gasket has a compression leak
between cylinders. Replacing the head gasket is the required fix.
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