Re: Piston Slap?????

[Wallace White <wallace@remdesign.com>]

> > In message <001901be0793$6c1428c0$e52296d1@dgrady.nai.net> "Mr. Daniel P.
> > Grady" writes:
> >
> > > What is the milage? What makes him think it is "piston slap"? I assume he
> > > means by "piston slap" that the bores have worn over-size and that the
> > > pistons/ piston skirts are smacking against the bore. That's a rare
> > > phenomenon on audi motors, so far as I know.

Okay, let me see if I understand piston slap correctly, based off what I
learned in an engines class is college--so please correct me if I'm
wrong:

        +-----------------+
        |                 |
        | +-------------+ |
        | |             | |
        |-|             |-|  <- piston ring--point of contact with bore
        | |             | |
        | |             | |
        | |      +      | |  <- where the rod connects to the piston
        | |     /       | |
        | +----/--------+ |
        |     /           |
             /  <- connecting rod          

Say the piston is a little too small for the bore but the piston rings
are making good contact with the bore. When the connecting rod is
pushing the piston up towards TDC (crank rotating clockwise), the bottom
of the piston, or the piston skirt, is being pushed towards the side of
the bore, by the horizontal component of the rod's force on the piston.
So the piston tilts counterclockwise until the skirt hits the cylinder
("slap"). 

Then after TDC on a firing stroke, the piston is getting pushed from the
top, and the rod is oriented like 
 
                  \
                   \
                    \

so the piston rocks clockwise until the bottom of the skirt, on the left
side now, hits the cylinder (another "slap"). The process repeats with
each complete cycle.

This causes wear on the piston skirt, on the sides that have been
striking the bore (perpendicular to the connecting rod bearing, about
which it has been pivoting). With more wear it slaps worse. 

It is also my understanding that the pistons are usually designed to be
undersize for the bore at room temperature, but at steady operating
temperature they have expanded sufficiently to fit the bore. (This
certainly makes sense for iron block / aluminum piston, and I guess it
also works for an aluminum block because the water jacket around the
bore keeps its temperature well below that of the piston?) Thus piston
slap is much more likely when cold.

Is that what we're talking about?

- Wallace
  '87 5kcstq 150k