REFLECTOR: Piston

[John Dibble]

Thanks everyone.  I appreciate all comments as I'm in unknown territory
here.  I have a set of used pistons from a non-PZL Franklin
engine (mine is PZL).  After removing the deposits, the weights vary in
a range of 3.5 grams.  The deposits amount to around 10
grams.  My Franklin mechanic told me that the pistons are sold in
batches of similar weights.  The wrist pins have end caps which
would prevent anything I put inside from going anywhere.  My only
concern about a machine fit is any possible expansion effect on
the tolerance between the pin, and the piston or connecting rod.  I'm
wondering if a spring that fits snuggly in the pin (but
shorter than the pin so it does not exert force on the end caps) would
eliminate any expansion issues.  If the pistons were
unbalanced, could this be addressed by balancing the prop?  I've had my
IVO
prop balanced several times (after blades were replaced).
Each time the amount and location of the weights have been identical
which suggest that something in the engine is what is getting
balanced.  Keep the comments coming.

John

gstull at tampabay.rr.com wrote:

> As an A&P, I've changed many cylinders with new pistons. Many times
just 1 cylinder and piston combination was replaced. I'm sure
there were differences in the weight between the old and new pistons. It
was never a problem. These engines turn at such low RPM
that I can't imagine the slight difference ever being a problem. At
least in my experience it never was, since it is never adressed
when you change a cylinder. Many times serious problems are introduced
by trying to fix a problem the doesn't exist. The old saying
"if it ain't broke, don't fix it" applies. As I said I've never heard of
this being a problem working on light aircraft for over 30
years now. I'm sure if you removed the old pistons and weighed them you
would find different weights between all of them. Just a
thought. ----