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Ring Gear replacement?

airguy

Unrepentant fanboy
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In reading through various threads about starter kickbacks and such I got to thinking about possible damage to the ring gear (mine has a few nicks in it, no missing teeth) and it made me wonder about a replacement. The ring gear itself appears to be pressed on, can it be pulled off and replaced or does the entire flywheel have to be replaced?

Is this one of those "freeze the flywheel and bake the ring gear" assembly techniques? What about removing a damaged one without destroying the flywheel?

EDIT - nevermind - enough GoogleFu and I found my answers, along with part numbers.
 
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Yeah, I'd like to hear the answer. I have the Lycoming parts catalog so I know the gear and support assembly (aka the fly wheel) are in fact 2 separate parts, but that doesn't really answer the question as to the best way go about replacing the ring gear. Obviously the easiest way (although I suppose not necessarily the cheapest) is to buy the 2 already mated, but I'm curious as to whether pulling an old gear off and putting a new one on is even possible outside a full blown engine shop.
 
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I have done a few ring gears on cars trucks and big dieses, cut the old one off with tool of choice, heat new ring gear with torch, not glowing, with hot mitts, place over flywheel and as it cools it will shrink fit. old timers use to just light the box on fire with the gear on top when working out on a remote site :)
 
Yep.

I have done a few ring gears on cars trucks and big dieses, cut the old one off with tool of choice, heat new ring gear with torch, not glowing, with hot mitts, place over flywheel and as it cools it will shrink fit. old timers use to just light the box on fire with the gear on top when working out on a remote site :)

Yep, Bret has it. There is a book way of doing it and it would be the same for a car as well. I do not remember the tem. to heat them to but we fine a space heater work good and dose not over heat the ring. Most of us just take a cut-off wheel and put a couple of notches in the old one to let the old one slide off. If you take the assembly off to do the job, cut the old one off, clean the flange, put the hub in the ice-box for a while, heat the new one and very quickly slide the new one on. It will contract quickly, so have a small hammer and a brass drift to tap it down and make sure it is seated in the first few seconds after you slide it on. But the correct answer is do it by the book. Hope this helps. Yours, R.E.A. III #80888

P.S. just what I may do.
 
Greg, curious about what you found. If you have time, post a few observations.

I found several sites that describe it, but SkyTech has the official Lycoming procedure linked on their site, and that document contains all the part numbers.

The TL;DR version is to notch the old one with a grinder and break it, it will spring off. Clean up the flywheel, heat the new ring gear to ~450F and drop it in place, it will shrink to an interference fit as it cools.

http://www.skytecair.com/images/PDF/SI1141_A.pdf
 
KitPlanes did a good write up not long back, essentially, to remove it, get your Dremel and cut a slot almost all the way through the ring gear, then whack it with a cold chisel to split it.

To reinstall the new ring gear, use the bake & freeze technique - making sure you get the alignment correct.
 
I've done two of these and you guys are on the right track.

You will have the prop off and the alternator belt tension eased off. You might match mark the prop flange to the starter ring disk and the disk to the crank. Thank you Sharpie for all your help!

Use a die grinder with a radiac blade and notch into the starter ring. You will need to go farther in than you want but the ring is very strong. Don't go into the disk though. Pop a chisel into the notch to complete the break.

Put the disk into the freezer for a few hours and heat the ring to about 350 or so. It will not stink up the oven. When ready, drop the ring onto the disk with the notches/teeth bevels to the engine side. Make sure it is on all the way and sits flat. (easy) You will want some gloves like welding gloves for this step.

Make sure the replacement ring has the correct amount of teeth.

It is more trouble dealing with the prop than changing the ring.
 
And just in case you've been married long enough to be finding it hard to impress, make sure your wife is around while you accomplish this "Amazing" feat on her airplane. She may be impressed thereby buying you a little time to think up the next way to amaze her. Hopefully not involving drilling a finger or some other "impressive" act done my Man.
 
Ring gear replacement

Done this process numerous times and it goes like this.

Removal - Clamp up the assembly in a vice. With a hacksaw cut between 2 teeth of the steel ring gear as deep as you can toward the aluminum flywheel. Angling the blade will allow a deeper cut about 80% thru ring gear. Do not cut into aluminum flywheel. Next I put a chisel between the teeth where the cut is and tap it with a hammer. Chisel is fairly large but small enough to fit between the teeth. Hammer tap forces the teeth apart and will break the ring gear at the saw cut. Ring gear will spring open a bit and is now very loose and easily removed from flywheel.

Installation - Verify the orientation of the new gear on the flywheel, it will fit either way, but the teeth with one corner machined off go on the rear side of flywheel. (This machined corner works like a on ramp for the starter gear when it engages) Now flywheel goes in the freezer and ring gear in the oven for 30-45 min (I put them in correct nose down orientation) Here is the only critical part, you have about 10-15 seconds to place ring gear correctly on flywheel before it cools, shrinks & locks into place. When it's hot it easily slips into place on the chilled flywheel. It sits on a little ledge and needs to be placed correctly. I once saw a guy drop the new gear onto the flywheel and it bounced up a little and siezed out of place and had to be cut off. I take the flywheel out of freezer and place on a table nose down then grab the ring gear out of the oven with a pair of channel lock pliers and put in place and in seconds it cools a little and locks on.


Don Broussard

RV 9 Rebuild in Progress
 
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Follow up question for the knowledgeable A&P types...

Reading through all the ring gear and flywheel literature I have found, there are obviously many different part numbers for the flywheel/ring assembly - like maybe 3 dozen different ones - but it appears to me that there are only 2 different rings gears, a 122-tooth and a 149-tooth, is that right?

Are there various different models of the 149 (or 122) ring gear itself? Or is this more of a generic gear that fits any of the flywheels intended for that size gear?
 
Like Magic!

I did this to my RV6 O360 engine years ago. Followed all the instructions and went according to plan. I was impressed at the precision of the fit.
If I remember right there are only two ring gears, each with a different tooth count. Just make sure you put the gear on the correct side - you won't get it off in one piece if it cools on the wrong side.
Have fun!
 
Not all ring gears are the same. I purchased a 149 tooth from Superior for my Lycoming flywheel. It is about 1/32 wider than it should be. It should sit flush to the back of the flywheel when installed.
 
Not all ring gears are the same. I purchased a 149 tooth from Superior for my Lycoming flywheel. It is about 1/32 wider than it should be. It should sit flush to the back of the flywheel when installed.

+1 on this. Skytech has a nice paper explaining the different styles of rings and how to identify them.

Larry
 
Follow up question for the knowledgeable A&P types...

Reading through all the ring gear and flywheel literature I have found, there are obviously many different part numbers for the flywheel/ring assembly - like maybe 3 dozen different ones - but it appears to me that there are only 2 different rings gears, a 122-tooth and a 149-tooth, is that right?

Are there various different models of the 149 (or 122) ring gear itself? Or is this more of a generic gear that fits any of the flywheels intended for that size gear?

Here is a link to Lycoming Service Instruction 1141A that lists all the different ring gear supports and how to change the ring gear. As near as I can tell, there are only two different ring gears.

SkyTec published a web page that gives a warning about changing from one to the other. Their NL starter is manufactured to a very close tolerance and may not work when changing from a 122 tooth gear to a 149 tooth gear.
 
I had some emails exchanged this morning with Richard Condy of Skytecair, he confirmed that there are multiple part numbers for the flywheel itself and these translate to multiple part numbers for the flywheel/ring gear assembly, but the actual ring gear itself is only available in 2 variants - the 122-tooth and the 149-tooth. They are not interchangeable on the same flywheel due to differences in the mounting depth of the support.

The upshot is that if you need to replace a 149-tooth ring gear, you can buy one from any of the sources selling them, they are all the same.
 
Tooth count

I had some emails exchanged this morning with Richard Condy of Skytecair, he confirmed that there are multiple part numbers for the flywheel itself and these translate to multiple part numbers for the flywheel/ring gear assembly, but the actual ring gear itself is only available in 2 variants - the 122-tooth and the 149-tooth. They are not interchangeable on the same flywheel due to differences in the mounting depth of the support.

The upshot is that if you need to replace a 149-tooth ring gear, you can buy one from any of the sources selling them, they are all the same.

I didn't know the 122 and 149 were not interchangeable. Mine is a 122 and I have planned on changing to a 149 as I have an overhauled 149 starter on the shelf.

If someone could post the width of their 149 tooth gear I will measure and post the width of my 122 tonight.
 
I didn't know the 122 and 149 were not interchangeable. Mine is a 122 and I have planned on changing to a 149 as I have an overhauled 149 starter on the shelf.

If someone could post the width of their 149 tooth gear I will measure and post the width of my 122 tonight.

You'll need to exchange the flywheel also, for one intended to take a 149-tooth ring gear, since the depth of the ring gear support is different.
 
I had some emails exchanged this morning with Richard Condy of Skytecair, he confirmed that there are multiple part numbers for the flywheel itself and these translate to multiple part numbers for the flywheel/ring gear assembly, but the actual ring gear itself is only available in 2 variants - the 122-tooth and the 149-tooth. They are not interchangeable on the same flywheel due to differences in the mounting depth of the support.

The upshot is that if you need to replace a 149-tooth ring gear, you can buy one from any of the sources selling them, they are all the same.

Swapping from a 122 to 149 may or may not work with your starter. There may be an issue doing this with the Skytec NL starter but it may work. I had the 122 tooth ring gear swapped with a 149 gear 20-years ago. I used a Skytec PM starter without issue for a number of years. It worked perfect. When switching to the Skytec NL starter a year ago, there was minimal clearance between the starter gear and the ring gear when rotating the propeller one or more revolutions.

IF you change from a 122 tooth to a 149 tooth ring gear, make sure you rotate the prop at least one complete revolution before trying to use the starter or fly the airplane.
 
Swapping from a 122 to 149 may or may not work with your starter. There may be an issue doing this with the Skytec NL starter but it may work. I had the 122 tooth ring gear swapped with a 149 gear 20-years ago. I used a Skytec PM starter without issue for a number of years. It worked perfect. When switching to the Skytec NL starter a year ago, there was minimal clearance between the starter gear and the ring gear when rotating the propeller one or more revolutions.

IF you change from a 122 tooth to a 149 tooth ring gear, make sure you rotate the prop at least one complete revolution before trying to use the starter or fly the airplane.

The profile of the gear teeth is very different between the 122 and 149 ring gear. The starter gear should match the profile of the ring gear. Skytecair specifically calls out which starters should be used for 122 and which ones for 149 ring gears, I would suspect all the other starter manufacturers do as well. Go look at the link on post 15 above (Skytec link), the document linked there shows the difference.
 
The profile of the gear teeth is very different between the 122 and 149 ring gear. The starter gear should match the profile of the ring gear. Skytecair specifically calls out which starters should be used for 122 and which ones for 149 ring gears, I would suspect all the other starter manufacturers do as well. Go look at the link on post 15 above (Skytec link), the document linked there shows the difference.

The word CONVERT from 122 to 149 teeth may be a better description. It has been done.

Not all conversions from 122 to 149 will work. Using a 149 tooth starter ring gear on a support that had a 122 tooth ring gear may or may not work with all 149 tooth starters. The 149 tooth Skytec NL starter is the one that MAY have problems when used with a 149 tooth ring gear when it is on a support that had 122 teeth as delivered by Lycoming. Front to back clearance is where there may be a problem when converting from one to the other.

Go back and look at the links in MY posts that points you to the Lycoming Service Instruction (it lists many of the different support part numbers) and the link I gave to the Skytec documentation.

Yes starters are the same price that work on the 122 or 149 tooth ring gear. One can buy either at the same price. Some people have multiple airplanes and engines therefore desire to have common spare parts inventory. Other people have a starter but the starter ring gear for a different starter. The price of a new ring gear and the small amount of work is less than a replacement starter.
 
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