<VV> Pin Offset - Description

[edward szuch]

Hugo;
By pin offset, I mean that if you lay the piston and rod assembly on a
bench with the pin vertical to the world, it will not be centered in the
piston.  It is slightly offset side to side in the piston.  This slight
offset compensates for the gas pressure and resulting thrust and cocking
loads on the piston and piston skirt.  You're right, it is not a large
amount but cylinder pressures are high enough to move the piston around in
the bore within the very tight tolerances that are available and create a
tick or slapping sound as they are pushed down the cylinder.  I know that
may boggle the mind the first time you hear about it but there are similar
issues with piston skirt profiles.  Modern skirt profiles go through a lot
of development in terms of computer simulation and dyno/vehicle development
to determine the best profile that minimizes noise yet produces long life
without scuffing.  An operating piston has to be quiet when cold and run
without scuffing (micro-weld to bore wall) when hot.  Pin offset and skirt
profiles are two of the tools used in their design.  It's amazing how just
a thou or two of clearance can affect it all.

Back to the pin offset.  The reason I brought it up is that I wasn't sure
if the after market piston was marked sufficiently or the re-builder paid
attention to the marks.  The Chevy Service Manual assumes you're using
factory pistons and rods where pin offset will be correct if the arrow or
piston mark is pointing forward and the rod numbers are up when installed
in the crankcase.  That's probably the reason it's not discussed more often
during engine discussions.
Gary Szuch