Posted for "John Barrett" <2thman@cablespeed.com>:

The answer to the problem is two. You weigh 13 of the pistons and log the
weight. Then weigh 13 more. If both groups of 13 weigh the same, the
piston you did not weigh is the heavy one.

"""
You think you are good at diagnostics? Here is an opportunity to
demonstrate your alacrity. You will need some of all the materials and
information listed. You have 27 Continental pistons that appear identical.
Twenty six of the pistons weigh 623 grams and one piston weighs 624 grams.
The pistons have a specific density of 0.09 pounds per cubic inch. You have
a five gallon bucket of water, a four liter clear plastic container (that
will accommodate one piston), a knife, two dimes, a penny, half a roll of
duct tape, 3 feet of 0.020" safety wire and a simple balance beam scale that
has a 50 Kg capacity and can indicate a 0.1 gram difference between the
items placed on the pans. What is the fewest number of measurements needed
to identify the heavy piston with certainty?
"""