flywheel balance question

Good day
Should flywheel be neutrally balanced or balanced with crankshaft?

I have 2 engines with removed flywheels IO-390 Thunderbolt
and Superior IO-360 parallel

checked them both on the static balance stand and both out of neutral balance and casting looks absolutely horrible

they both have drilling from factory for balance and have "O" mark but no 'O' stamped on any of the crank shafts

Can some one shed light on this
plz include info source if you have it

Thank you

Lug

The "O" goes on the crank lug with the raised step on it. The o mark hole is actaully larger than the other holes.
Needs to be balanced as a whole assembly, prop spinner, flywheel

BobRv4 said:

The "O" goes on the crank lug with the raised step on it. The o mark hole is actaully larger than the other holes.
Needs to be balanced as a whole assembly, prop spinner, flywheel

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The question is more about the balance !
Positioning of flywheel should be only one way to respect timing markings

While I can't say for certain, I am pretty confident that the ring gear (it is NOT a flywheel) is neutral balanced. The ring gear is VERY light. It is the prop that is acting as a flywheel. Usually when a designer goes to the vibration damper or flywheel for balance, they are adding like 10-25 oz. You would notice weights on it if that was the design. It is a very symmetrical part with no weights, therefore highly improbable it is anything but neutral. That is not to say that Lyc doesn't have VERY loose tolerances on the balance. But they do balance them. Many have drill spots from the balancing.

Larry

BobRv4 said:

The "O" goes on the crank lug with the raised step on it. l

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This is incorrect. It goes on the lug that is 10 or 20 thou oversized at the base (sorry but forgot the spec). SOMETIMES this is a stepped lug, but NOT always. It is called the index lug and often it is a short one that doesn't protrude past the ring gear. It does not do this for balance, but instead to be sure the TDC and ADV markings are in the correct spot.

lr172 said:

While I can't say for certain, I am pretty confident that the ring gear (it is NOT a flywheel) is neutral balanced. The ring gear is VERY light. It is the prop that is acting as a flywheel. Usually when a designer goes to the vibration dampner for balance, they are adding like 15-25 oz. You would notice weights on it if that was the design.

Larry

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I am too 99.9% sure rung gear is neutrally balanced
I when say flywheel I am talking about ring gear support that looks like flywheel on cars

additional info

on IO-360
had much better balance through whole range
it had 2 screws on flywheel added by dynamic balance.
with screws it was almost neutral balanced
wiht screws removed it went off balance
that is logically say that dynamic balancing corrected casting ugliness of flywheel
and not propeller.

on IO-390 Thunderbolt
also 2 screws but it was absolutely not neutral and was very heavy
but with screws removed almost neutral
and in this case we can say for sure that propeller was balanced more them flywheel

in both cases we actually creating 2 or more masses on rotating disk
located at different ofset from center and trying to balance it
yes you can balance it certain RPM range but when staring to change RPM you start filling this vibrations

would like to figure out how to get best results in whole RPM range
if flywheel is neutrally balanced I would reface casting that have bulges on it
and balance

There are two pieces here, in Lycoming speak, ring gear (toothed part) and ring gear support. The second part is commonly called flywheel as that's well understood.

AFAIK Lycoming balances ring gear support and crankshaft separately otherwise if balanced as an assembly, you'd have to remove the crank to re-balance the whole assembly when changing just the ring gear support- impractical and expensive.

There's one larger OD master lug and matching hole in the RGS. There are 5/8 and 3/4 lug pieces and the ones for O-235s have a smaller center hole. The master lug is in a different location on the small and large lug parts.

rv6ejguy said:

Lycoming balances ring gear support and crankshaft separately

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Thank you for confirmation that is exactly what i was thinking!
but could not find any confirmation in any lycoming documentation
if you have any and could share would be grate