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Broken piston ring....

bret

bret

Well Known Member
So....last year # 2 cylinder was at 72/80 not too bad but I kept an eye on it, well this year it was at 65/80, dang it! time to pull a jug, so there it is, now I need to find some parts, anyone know where to start to find the correct part # and vender? its narrow deck, IO 360 A1A angle valve with chrome cylinders. Thanks. sorry for the pic size...

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with the superior charts piston ring for chrome cylinders it looks like SL74671A?
 
Sorry I can not help with the part number, but wonder what other indicators you had other than the cylinder leak test. Was the ring end butting to cause this?
 
I measured the second ring that was not broke, .060 seams to be ok. my experience for auto stuff was .004 per one inch of bore for a NA engine, this seams like a large end gap? this is my first rodeo with an aero engine so not too sure about the recommended specs. this was burning 1 qt of oil every 10 hr. ordered some cast iron rings for the chrome cylinders, what grit should the cylinders be honed with, and what cross hatch angle for these engines? Thanks, Bret.
 
Hey Bret, the overhaul book will address the cold gaps. They are quite a bit higher in the air cooled aircraft for several reasons. Continuous high power operation that yields higher piston/ring temps than an intermittent engine (class 8) is one reason. The biggest reason, though, is possibility of hitting a rain shower and quenching the barrels on the ring/piston. That is a key design criteria for the engines. It results in sloppy piston/bore clearances (by car/truck standards) and thicker skirts to handle the slap.

You might want to call some reputable shops about the hone surface finish that works. I understand the chrome bores have some different needs than standard nitride bores with plasma faced rings. Ring face scuffing is the failure mode to be avoiding. Sorry, I have forgotten most of my ring/bore knowledge.

You can look at the butt ends of the broken ring under magnification to see if there are polished areas that result from contact. If there are none, it was likely an installation mistake, and you are good to go. Really good you caught it before the piece ruined the ring groove and scored the bore.
 
Interesting thought on the polished ring ends, kinda looks like they are? this is a 300 hr SMOH engine, 1964 variety and going back through the logs this #2 has had low comp test numbers from the last rebuild, the pistons were new at last OH and look pretty good. I've got a ring gap filling apparatus and will definitely make sure they are at specs going back together, also got a look at the cam, good to go, was worried with all the stories I seen here :rolleyes:
 
.06" is big. I remember the low end of the spec being in the mid 30's. My kit had the wrong rings (choke bore vs. straight bore) and they had gaps in the upper 40's or 50's I believe (just outside the tolerence range). At 100 hours, I honed and re-ringed with the proper rings. My blow-by went down a decent amount.

If you have choke bores (you may not if the jugs are from the 60's), be sure you are measuring the ring gap in the correct part of the cylinder. The ring has to be pushed quite close to the top of the cylinder to get into the smaller bore area. The Lycoming manual gives a distance from cylinder top for doing the measurement.

I would also be sure that they didn't put choke bore rings in a straight bore if you have already been measuring the gap in the proper location. You will need a bore gauge to check. I believe the choke taper is only around .020 - .030.

The choke thing is tough to work around, as you only typically see straight barrels manufactured in the 60's and re-worked. The terminology used by the PMA's is very unclear and they don't talk about the choke, only cylinder material. It took me a while to figure out which rings were which. Also, if not careful, the overhaul shop can take out the taper when they bore or hone a choke barrel..

I don't know what the protocol is for chrome jugs. For steel, I honed with 220 grit stones and I believe that that is the old standard. I would guess the chrome layer is relatively thin and don't want to over do it. I would have finished off with 400 grit, but didn't have the stones on hand and just decided to do it the old way. I burned 1 quart in 20 hours for a couple of hundred hours and it has settled at around 1 quart in 15+ hours.

Good luck,

Larry
 
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If the polish on the original gap ends looks the same as the polish on the cracked ends, that is likely not from too narrow of a gap or excess heat, just wear from moving around in the land. Given the log data, it is more likely that they were cracked on installation. It's easy to do if you are not experienced with the installation procedure.

Larry
 
I've only done a bit over 50 aircraft cylinders. I have seen a good amount of broken rings, but never one broken that close to the end. Makes me wonder if it was partially cracked before it was ever installed, and came apart after..
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Well done. Good video that was informative and entertaining.

On a side note, I noticed the flag hanging in your hanger. It's a little early but Happy Birthday. ;)

Semper Fi
 
Hey Bret, What did you find out about honing a chrome cylinder? Any particular stones necessary? Re-honing is OK I guess. I have chrome cylinders with 10:1 pistons and have considered going back to stock compression and have wondered about re-honing the chrome.
 
I am no expert on these, I just did the bare minimum on the hone with the chrome layer in mind, I am just going to keep an eye on it real close and report back here, after looking at the cylinder I could see a lot of wear at the top, so I would like to replace them when I win the lotto :rolleyes: the other cylinders were 76/80. This engine was on its third rebuild when I bought it used with 250 SMOH, but no leaks and runs strong, 10 hr per qt oil consumption, I just hate to fix what is not broke. It was a good deal for 16k engine and new OH CS Hartzel but you get what you pay for. The repair was 40.00 for two rings so I did this repair instead of a new 33K engine :D
 
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