Noisy engine? It might be "piston slap"
by Jim Kerr

Knock, knock, knock, knock. No it's not the neighbour at the door.
It's the disturbing sound of piston slap coming from your car or
truck's engine. Is it a problem? Maybe - it depends on whom you talk
to. During the last couple of years, I have had hundreds of inquiries
about piston slap. Let's look at what causes the knock, and why it is
much more common now than in the past.

In simple terms, 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.

Closer manufacturing tolerances have helped reduce piston clearances
and slap, but some engines need more piston clearance to allow for
piston expansion. During the first few minutes of operation, the
piston can expand several thousandths of an inch, yet clearances are
typically in the one to two thousandths of an inch. Fortunately, the
cylinder also expands, or we would find pistons seized.

On vehicles built in the last decade, piston slap that occurs for a
few seconds on cold start is quite normal. My own vehicle, with only
30,000 km on it has piston slap for about 5 seconds when first started
on a below freezing winter morning. Service information from General
Motors states "A cold Piston knock which disappears in 1.5 minutes
should be considered acceptable". From experience, I have found that
piston slap that occurs only during cold starts and lasts only for a
minute or less causes no problems. Just don't place a load on the
engine until the pistons have expanded and the clearance has been
reduced.

Speaking of clearance, we normally find piston to cylinder clearance
in the .0005 inch to .002 inch range. A human hair is typically about
.002 inches thick, so you can see the clearances are very small. Some
manufacturers are using special coatings on piston skirts to reduce
friction. This enables them to reduce clearances even less and prevent
piston slap. Ford V8 overhead camshaft engines use coated pistons; so
does the Corvette Z06 high performance engine, as well as other
manufacturers.

A good example to show the advantages of coated piston skirts is the
Corvette. Clearance specifications for the coated pistons are from
minus .001 inch to plus .001 inch. You read correctly; minus
clearances! The piston is actually larger than the cylinder on the
skirt sides of the piston. That coating has to be slippery! As the
engine warms up, expansion in the cylinder block gives more clearance.

The correct engine oil can help reduce slap. Good oil takes up some
clearance and is not easily scraped off the cylinder wall during cold
starts. Sometimes, changing oil brands or viscosity can reduce a cold
start piston slap.

If you suspect your piston slap is excessive, then there is an easy
method to locate which cylinder has the problem. Before starting the
engine, remove one spark plug wire and short it to the engine block.
When the engine is started, that cylinder has less pressure pushing
the piston sideways. If the knock changes, or is gone, then that is
the cylinder with the problem. If the knock is still there, try
another cylinder at the next cold start. It takes a little time, but
it is much better to locate where the problem is before disassembling
the engine.

So is piston slap a problem? Not for most engines built in the last
decade and if the noise is there only for a few seconds during cold
start. If the knock is in an engine of older design, the knock
continues, or it is there during acceleration, then engine work is in
the future. The worst part about piston slap is trying to explain that
a cold start knock on newer vehicles can be normal, especially when
you try to sell your vehicle!

Jim Kerr is a master automotive mechanic and teaches automotive
technology. He has been writing automotive articles for fifteen years
for newspapers and magazines in Canada and the United States, and is a
member of the Automotive Journalist's Association of Canada (AJAC).

Using Internet Explorer 5.0 or greater
LISTEN to GM Piston Slap right here

http://www.pistonslap.com/photos.htm

Lg

my $0.02... until they're broken, they don't get that much noisier.

However:
the exhaust system starts to rust out and spring pinhole leaks...
aftermarket muffler put on changes the tone...
the underhood insulation falls off (like my Jimmy) and the engine
appears to get louder.

For a typical car, it's a gradual thing where the parts that are "wore"
but not broken all add up to a noisy engine:

fuel injectors,
a bearing in one of the pulleys,
the power steering pump,
a pinhole exhaust leak,
the underhood insulation ripped out,

and so on... combined together it makes for a "loud" sound instead of a
"nice" mechanical sound.

Parts wear, clearances (gaps) between parts get wider. Modern engines
are often much more susceptible to increased noise as they get older
because they use things (for example) like short "slipper" pistons that
tend to rock in the bores more than older long-skirt pistons. The
benefit being less friction and better efficiency, at the cost of more
noise as the wear takes clearances to the ends of the tolerance range.

probably because we get fatter and the only way they can complain about the
extra weight is to make some noise.
hehehe
old john

I've not noticed older engines becoming noisier...unless a exhaust leak
develops.  I have a 1997 Grand Caravan that is as quiet and as smooth
running as the day I bought it nearly 8 years ago.

I have a theory as to 1 reason why cars get noisier with age.  I think
that some noise-deadening material becomes less effective with age.
Possibly by settling/compacting or maybe absorbing moisture.  Is this
possible/likely?

David Jory