>I am in the process of getting my engine overhauled. Should
>I spend the money to get the engine balanced? Is it worth
>the money? Will it make my engine last longer or will it
>just run smoother (more pleasant)? The engine is an
>0-235-C2C mounted in a Grumman AA1B. I realize that
>I have to decide this now. I was thinking of getting a 
>dynamic prop balance sometime. Would that be a waste
>of time if I do not do the engine balance?

What, exactly, are you thinking of when you talk about balancing?  Are you
talking about matching piston and rod weights in excess of the manufacturer's
specifications?  Micro balancing the crankshaft?

In general those kinds of practices have very little return - especially
in a four cylinder engine like your O-235.  The opposed fours have a
fundamental second order secondary moment imbalance that cannot be
eradicated without adding a balance shaft to the engine.  They're
just natural shakers to begin with.  No attempt to "blueprint"
that engine is going to make it seem smoother or alter its longevity.

To the contrary actually.  An incompetent shop could easily damage your
engine in the quest for balance by removing material from connecting rods
and pistons.

I would not recommend going beyond Lycoming's recommendations regarding
balance.  Specifically, they do not recommend mixing Lycoming and
older Superior SL-13623 pistons in the same engine as the weights differ
for O-235 engines (see Service Letter L215).  They also recommend that
all replacement pistons be checked and compared for weight (see Service
Instruction 1243).

In general, get the pistons in opposite bays to match, weight-wise.
Do this by ensuring that pistons in opposite bays are of the same weight
code or, if they are not marked with a weight code, that they actually
weigh within a few grams of each other.  Whatever you do, do NOT try and
"micro balance" pistons by removing metal!

Connecting rods are a different animal.  It is especially important that
the big ends of the connecting rods be in balance in order that
the crankshaft be in dynamic balance.  The easiest way to do this is
to LEAVE THEM ALONE.  You will almost certainly be putting the same
connecting rods back into the engine with the same crank.  The factory
already took care of balancing them together.  Just make sure that
each con rod goes back on the crankpin from which it was removed and
there should be no problem with balance.

In the event that a connecting rod or two needs to be replaced then make
sure the  replacement is of the same weight code (A, B, D, or E) as the
original or is the "catch all" intermediate S weight code.

I AM a big fan of a "prop balance" when the engine and prop are all
back together and running.  This is the kind of balance you get
with a Chadwick or MicroVib machine and adding weights to the
prop spinner.  You should do this.

In sum:  If your shop is trying to push you to have your engine
"balanced" during overhaul then ask them how they intend to do this.
If the method involves removing metal from ANY internal engine
part then WALK AWAY.  If it simply involves sound overhaul practice such
as weight-matching pistons in opposite bays they say "that should
be standard practice, why are you charging me extra for it?"  In
general there should be no need to rebalance your crankshaft itself
unless the bearing journals are being ground undersize.  In that case,
make sure the shop has the proper equipment to dynamically
balance the crank.  Also, unless a problem is found with a connecting
rod (which is unusual) then all the original connecting rods will
simply go back in as they were.  No balancing required here.  If you
do have to replace a connecting rod then it should be WEIGHT MATCHED
with the other rods in accordance with Lycoming's instructions.



For further reading, here is what Lycoming has to say on the subject:

	"After certifying hundreds of flat, opposed cylinder aircraft
engines, and after building more than a quarter million engines for
general aviation, is it possible that Lycoming engineers have not
leared what it takes to produce an engine which is properly balanced?
			...
	To put it simply, the subject of engine balance is well understood
by Lycoming and is a consideration in the design and manufacturer [sic]
of all Lycoming engines.
			...
	These statements are contrary to what has been implied by several
advertisements and magazine articles.
			...
	The implication is that the manufacturer knows nothing
about balancing an engine and simply throws the
parts together without consideration of the importance of
balancing.  This is absolutely false.  Lycoming engines are very
carefully balanced to the degree which is necessary.  They are not
balanced to a point of absolute perfection because they
run at relatively low speeds (compared to some automobile engines) and
therefore do not require the degree of balancing being advocated by
shops which deal in this specialty.
			...
	There are occasions when dynamic balance of the prop/engine
[such as with a Chadwick or MicroVib machine - grt] combination can
provide reduced first order vibration, but additional internal balancing
of Lycoming engines is not required nor recommended.
			...
	ADDITIONAL INTERNAL BALANCING CONTRIBUTES LITTLE TO ENGINE
SMOOTHNESS AND IT MAY EVEN BE HARMFUL WHEN MATERIAL IS REMOVED FROM
HIGHLY STRESSED PARTS OF THE ENGINE [their emphasis]
			...
	To summarize, these are the points we have attempted to
communicate by providing information about the balancing of Lycoming
reciprocating engines.  First, Lycoming engineers have acquired vast amounts
of data through years of engine testing.  This knowledge is used to insure
that Lycoming engines are carefully balanced during manufacture.  It is
also used to formulate a system which allows satisfactory engine balance
to be maintaned when weight matched parts are replaced during
the operational life of the engine.  The engine balancing done by Lycoming is
part of an organized system which is intended to provide a quality product.

	The second point is that additional internal custom balancing is
not recommended by Lycoming.  It simply is not necessary, and therefore
engine owners would be well advised to save their money."

greg