The incredible (non-)shrinking holes

[Scott Fisher]

Having just explained nuclear fusion to my
seven-year-old by using mini chocolate chips ("You
see, Charlie, when you pressed them together, the heat
from your hands fused them into a single bigger chip;
the difference is, in the interior of a star, it's
like you press two chocolate chips together and they
turn into a peanut -- they become a completely
different element"), I guess it's my role to be Dr.
Science today... :-)  And this is one I can even
answer with an automotive example.

The first time I installed interference-fit wrist pins
into a set of pistons, the answer to the question of
whether holes shrink or expand when you heat the
surrounding metal was answered graphically,
categorically, and unambiguously.  

The instructions that came with the pistons said to
heat the pistons in order to fit the wrist pins into
the tapered bores.  Sure enough, they would not fit
when both items were room temperature -- the pins
would only get about halfway.

We used a hair dryer set on full heat and held the
piston in a welding glove -- the aluminium does get
quite hot with a 1000W hair dryer.  Then I'd grab a
wrist pin and slip it into place (after passing it
through the small end of the con rod, of course).

With the pistons heated up, the pins slipped into
place easily, sliding all the way through.  The holes
had clearly become larger after heating the material
around them.

The final touch for these particular pistons (for my E
Production MGB) was to install circlips at either end
of the wrist pin to keep them from rattling loose when
the pistons heated up again from combustion.  (Sadly,
it appears I muffed a clip because that engine
developed a massive knock a year or two later, which
turned out to be a wrist pin that had come loose and
gouged the cylinder wall.)

So -- you can do all the "reasoning" you want, but
practical experience demonstrates that holes do NOT
shrink when you heat the surrounding metal, they get
larger.  (Well, unless you MELT the metal and it fills
up the holes.  A friend did that by mistake once using
a torch to loosen a stuck fitting in what turned out
to be pot metal; he ended up with a nice shiny puddle
with a brass fitting investment-cast into the middle
of it...)

And of course, the ancient Celts were doing the same
thing with bronze wagon wheel rims three thousand
years ago, heating up the rims to slip over the wooden
wheels and knowing they'd contract when cool, but
that's a subject for Dr. History, not Dr. Science.

--Scott Fisher
  Laboratory, Pennsylv--er, Tualatin, Oregon :-)





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