A Slap In The…


What causes piston slap noise on a newly
rebuilt motor? I had my 440 rebuilt at a
very well-known Mopar shop, they go
back to the ’60s. It was bored .30 over,
9:1 C.R. pistons, 452 heads and a 0.484˝
Mopar cam. It has very good oil pressure.
60 PSI hot, at 1000 RPM idle with 10W30
synthetic. When I called them, they said
there is no warranty and you can’t satisfy
everybody all the time. I had several other
experts listen to it and they said “piston
slap” and would not answer any questions
when I said who built it. I’m not complaining
about performance, it propelled my
3800 lb. B-Body to 12.0s in the quarter

mile. This piston slap special sounds just
like a diesel. The first time I started this
motor, it was loud and obvious.

As far back as I can recall, factory cast
pistons had the pin hole offset by a small
amount to ensure that there will be no
audible slap. This design insures that the
piston cannot slap back and forth in the
bore. Of course, this increases friction
by a bit, reducing HP output. For these
reasons, aftermarket pistons do not have
this feature, this be a positive or negative
depending on where you sit.
Now we’ll discuss those aftermarket
pistons. They do not have the cast-in
expansion-control steel slippers present
in the OEM castings, so even the hypereutectic
versions, which feature an alloy
with reduced thermal expansion, need
increased skirt-to-wall clearance values
vs. OEM specifications. True forged pistons
require even more, and the required
clearance, needed to prevent scuffing,
increases with the bore diameter.
If the clearances are correct, however,
the audible slap should almost totally
disappear when the engine reaches
operating temperature; if it does not,
the only conclusion is that the fit is too
loose. Generally, with today’s excellent
CNC-machining tolerances, the clearance
is “built into” the pistons. In other
words, if you order 0.030˝ O/S pistons for
a 440, you’d receive pistons designed for
a 4.3500˝ bore. Virtually all good shops

still measure the clearances, and some
builders have their own idea about what’s
ideal, this would vary depending on the
Long term, for a street-driven engine,
the rather extreme noise you describe is
not good. At all.

I have no idea about Michigan’s civil
laws, but here in New York, we have an
“Implied warranty of merchantability.”
If you buy something that doesn’t fulfil
its purpose, the seller must make good.
While I have absolutely no idea who
built your engine, I am surprised that a
reputable shop would turn a deaf ear to
your complaint. Of course, I’m only hearing
one side of the story, there’s almost
always “he said-she said”—and the truth
usually lies somewhere in between.

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