Re: Oval pistons (was Re: s4 rough acceleration)

[Scott Fisher <sefisher@cisco.com>]

Phil Payne wrote:
> 
> In message <36B8C085.D238F606@cisco.com> Scott Fisher writes:
> 
> > I'm sure the factory made them both as close to
> > perfectly round as possible, regardless of temperature.
> 
> Measure one cold.

As at least one other lister pointed out in a little more detail
(ahem... and thanks to ScottyCBoy@AOL.com for the clue), I was mistaken:
pistons ARE made just a fraction out of round.  (Hey, at least it's for
the REASONS I cited in my original explanation...)

Where the wrist pin passes through the piston, there's more metal in the
piston walls than there is 90 degrees from that axis.  That's because
more metal is required to support the wrist pin in the piston, and also
because space must be provided to give the connecting rod room to pivot
on the small-end bearing up inside the piston.  

When the piston heats up, the area with more metal expands more than the
area with less metal, so pistons are machined a few thousandths out of
round when cold to compensate.  Apparently, air-cooled pistons are
frequently made with this difference more pronounced than pistons in
water-cooled engines, because they tend to operate at higher
temperatures.

As they say in China, 

Huo dao lao, xue dao lao,
Hai you sanfen xuebudao!

("Live to old age, study to old age,
There's still three-tenths you'll never learn.")

--Scott Fisher