Piston slap occurs when the piston is forced rapidly against the side of
the engine cylinder wall. The more clearance between the piston and
cylinder wall, the louder the knock. Controlling piston slap is a
complex process. Too little clearance between the piston and the
cylinder wall and the parts will score and fail. Too much clearance and
you get a knock. It doesn't help that usually the piston and cylinder
are made of different materials and have different expansion rates.
Several features are used in piston design to reduce slap. To keep the
piston close to the cylinder yet allow room for expansion, the piston
skirt (the part that slides against the cylinder) is tapered - it is
bigger at the bottom than at the top. The top of the piston expands
more, where the extra clearance is, because of higher heat at the top of
the piston. The bottom always remains close to the cylinder.
Pistons are also made oval shaped. The large part of the piston is close
to the cylinder, while there is clearance on the smaller sides. As the
piston expands, heat is transferred into the smaller sides, so the
piston becomes more round. Thus, the large sides of the piston always
stay close to the cylinder and piston slap is avoided.
There are several other piston features to counter piston slap, such as
offsetting the piston pin position, but I think you get the idea. Too
much clearance between the piston and the cylinder and we hear that
Knock, knock, knock.
In the past, the sound of piston slap meant trouble. Worn cylinders,
damaged piston skirts, or cracked pistons were common causes, and all
meant expensive repairs. Now things have changed.
Engine designs have changed to make them more compact, lighter, with
less internal friction, and higher revving. To do all this, piston
design had to change, and some of the major changes are shorter piston
skirts and straight piston skirts. The short, straight skirts allow the
piston to rock more in the cylinder, and we hear it as piston slap.
You may want to use a thicker grade engine oil or check with GM if they
have revised piston kits for the 5.3L (new gudgeon pins and rings etc.).
Generally I would not worry too much about it unless it starts to
consume an unacceptable amount of oil and or the piston slap still
occurs when the engine is at its normal temperature. If your local
dealer says "its normal for it to happen" Ask them if they do have a
revised piston kit.
It the noise is gone by the time the engine warms up,
it's totally normal. The dealership will probably do
nothing for the problem as there have been bulletins
released about this noise, and about the fact that GM
will not be fixing this problem unless it's really bad.
There are parts for the problem, but I have yet to
hear one of these engines that are bad enough to
warrant replacing the pistons. We have gotten to
the point in the dealership that I work at where we
will have a foreman do a cold start check and then
basically let the customer know that nothing will
be done for this noise.
It's just a fact of life with short skirt pistons. And
it's not a particularly new problem, it's been around
for years on different engines.
There were some other problems that could cause
a "lifter" type noise, specifically an o-ring that seals
the oil pump pickup to the front cover, but that
problem would have showed up at an early mileage.
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