Re: Balancing Flywheels for 345/392

[jlandry@domain.elided (John A. Landry)]

At a recent NATO press briefing, "David Willmon" <dwillmo@domain.elided> was heard
threatening Yugoslav President Slobodan Milosevic...

>I'm confused after Greg Hermann's comments on changing flywheels between
>304/345/392s.

David,

Just my $0.02 on this.  The flywheel is part of the overall balancing
mechanism for the IH engine (along with the other reciprocating parts
including the harmonic balancer).  You cannot properly balance one single
component by itself, as they all work together.  If you were to try and
dynamically balance the flywheel alone, you would find it's apparently way out
of balance.  That's because the IH engine is "externally" balanced, meaning
such items as the flywheel and harmonic balancer help to offset other internal
engine components.  So while it's true that as others have stated, the
flywheel is interchangeable, you cannot *properly* balance it without
disassembling the engine.

That being said, every engine component is mass produced to some acceptable
tolerance.  In a perfect world, every flywheel, flex plate, crank and harmonic
balancer would weigh exactly the same, etc.  Of course that's not going to
happen, but if IH (or their sub-contractors) did things right, every component
will fall within some allowable range.  All the reciprocating engine
components assembled during production should therefore have an acceptable
balance.  I have no idea how much IH balanced each engine as it was being mass
produced, but I'd assume they didn't spend much time doing much of a custom
job on each one.  Rather, I'd guess they mass produced all the parts and
possibly had test machinery to balance the individual components to some
acceptable standard.  When the whole assembly was bolted together, the balance
would of course not be perfect, but it would be good enough for normal
designed use.

The resulting out-of-perfect-balance engine would not be vibration free, but
more than acceptable for normal use.  When a person does a custom rebuild, the
opportunity is there to actually verify the total balance of all the
reciprocating engine components.  A very smooth nearly vibration free engine
can be realized by a good machine shop.  Mike Ryan said a 345 he was
rebuilding and balancing was found by his machine shop to be more than 100
grams out of balance.  My 392 I just rebuilt was found to be 85 grams out of
balance when first checked.  In both our cases the overall out-of-balance was
reduced to zero grams.

So the bottom line is that if you need a new flywheel, as long as is the
correct component for your engine, just bolt it on and forget about it.  You
can't really do anything about it anyway.  You engine balance won't be so
dramatically affected as to cause any problems.  But if your doing a rebuild,
it's money well invested if you go ahead and have the engine balanced...
especially if you start changing components like the pistons, connecting rods,
harmonic balancer, flywheel, flexplate, etc.

Again, just my $0.02.

Regards,

John L.
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