Are Lycomings externally counter-balanced via flywheel? (2024)

Due to a few conflicting local opinions, I wanted to check in with the Cub collective here:

Are Lycoming four cylinder motors externally counter-balanced via the flywheel? There are holes drilled in an offset pattern in the flywheels of 235/290/320s/etc, and these holes are large enough to (possibly) not be balancing the flywheel itself.

Without an alternator nor starter, is there a need to retain the flywheel on O235/290/320s (outside the need for the prop spacing)? Or, is the proper flywheel an external counterbalance which must be retained?

Specific to my application: O235, no starter or front-mount alternator, Catto prop; large spinning disk of a flywheel up there doing nothing but spacing out the prop from the nosebowl.

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stewartb said:

That's an interesting question. I heard recently how some engines are having harmonic damage to accessory cases and it's related to lightweight props.

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I’d like to hear more about this if you have any more details. It makes a lot of sense, in my layman way of thinking these engines were designed to operate under a load, and a heavy metal prop keeps a load on the internals. If there’s a reduced flywheel effect from a lightweight prop, I can see where damage would come from not operating under a loans and gear chatter. Similar to the feeling of a lawnmower spinning wide open but not cutting, feels like it will tear itself apart until you put a load on it cutting grass.

I’d like to try a composite prop with a weight halfway between a metal and typical composite prop. Something with all the advantages of the newest blade designs and smoothness, and still have some flywheel effect from a heavier weight.

Well, I’ve put quite a few hours on Lyc O-360s with composite props, and I can testify that the composite props are FAR better balanced out of the box than metal props. In fact, installation of an MT or Hartzell composite prop on an O-360 eliminates the restricted operating range required for most metal props. Essentially, the composite prop dampens out many if not most vibration inherent to these engines.

So, maybe there’s something negative related to the accessory case, but the rest of the engine seems to like composite props.

MTV

Just for visual reference, here are the through holes (and partial holes) as found on a O235 flywheel. I suspect 320s might be similar, but I don't have one on hand to measure. (.725", 3 full through at ~.275" thick plate, 2 partials ~.100" deep)

My initial gut reaction that these were to ward off for harmonics for some combination(s) of parts, but I had not thought of the possibility of accessory case interactions as well.

I've been flying an O-320 Hatz with a light mahogany prop and no flywheel for well over thirty years with no problems.

Steve

stewartb said:

The story I was told involved IO-360s. It was offered when I was dealing with my IO-400 recall and questioning why the crank needed to be inspected. I thought it was interesting because another IO-400 owner was having bad luck with Pmags failing. Related? No idea. I know nothing about props and harmonics but since I've got a Whirl Wind CS the comments got my attention.

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Stewart,

I assume those are angle valve engines (there is an IO-360 that’s parallel valve). If so, those engines crankshafts include mass balancers. In other words, they’re internally balanced, unlike O-235, 290, 320 and 360.

MTV

Thanks for the responses everyone.

My flywheel is coming off and will machine up a spacer to retain the correct spacing between the cowl and prop. Solidworks model is complete and hopefully will get cut in time for good weather.

I know it's semantics, but that's not really a 'flywheel' by definition. It's main purpose in life is to hold the ring gear in position for the starter. It's secondary purpose is being scribed with timing marks. A true flywheel has mass to it and smooths out impulses on a shaft or maintains momentum. The Lycoming 'flywheel' is to light to be a 'flywheel' and the balance holes drilled into it are simply used to statically balance the unit. As has been pointed out, many, many Lycomings have been run for a bazillion hours without this piece installed, without harm to the engine.

Also, the Lycoming parts book lists it as the 'starter ring gear support assembly'.

Web

motosix said:

Thanks for the responses everyone.

My flywheel is coming off and will machine up a spacer to retain the correct spacing between the cowl and prop. Solidworks model is complete and hopefully will get cut in time for good weather.

View attachment 41952

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Find someone with a bent wheel. Machine out the center disc and you won't have to do any drilling/fitting.

Web

wireweinie said:

Find someone with a bent wheel. Machine out the center disc and you won't have to do any drilling/fitting.

Web

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That is exactly what I was going to do if I could get my hands on a big enough lathe to turn it down (mine cant chuck up something that large).

The aluminum plate and machine time is not terrible for such a simple spacer however, so it works out about the same for me.

Band saw it down to fit the lathe.

Web

Carey Gray said:

I put timing marks on the edge of my spacer that's made out of an old flywheel. It makes timing mags pretty easy.
What were those ring gear things for anyway? Cubs don't have starters.

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I was going to do just that. The only thing I will miss without the flywheel/ring gear is the timing marks.

Carey Gray said:

I put timing marks on the edge of my spacer that's made out of an old flywheel. It makes timing mags pretty easy.
What were those ring gear things for anyway? Cubs don't have starters.

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Lol, That's what I've been doing.

motosix. Look on the back of your flywheel. There are a second set of timing marks that align with the split line on the crank case. Transfer that set to the edge of your disc and you can still use them for timing.

Don't forget that you have one oddball hole out of six.

Web

wireweinie said:

Lol, That's what I've been doing.

motosix. Look on the back of your flywheel. There are a second set of timing marks that align with the split line on the crank case. Transfer that set to the edge of your disc and you can still use them for timing.

Don't forget that you have one oddball hole out of six.

Web

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I will do just that!

(yup, got that oddball sized hole. noticed it on the end of the crankshaft first as it is difficult to tell by the naked eye on the flywheel itself. .645" vs .675" or so...)

Just to follow up for people searching for this info in the future:

O235s love having their flywheels ("ring-gear") removed. You can cut up a perfectly good flywheel, or you can machine something like this:

Top mark is TDC, two marks to the right are 20 and 25 degrees and are just barely visible above the crank flange when installed. Note the inner chamfer is required for the spacer to sit flat against the crank flange.

If anyone wants the CAD file for this silly little part, feel free to message me.