Surprising Discovery - Failed Piston Ring/Land

[GAHorn]

I am in the middle of Annual Inspection which will be due July 1, and Jamie and I are planning to fly from our central Texas strip to New York to visit our 2 yr-old granddaughter,…and of course our daughter and son-in-law. (Placed those in order of importance)

After a few days in NY, we plan to fly to the 2023 Convention in Michigan, after-which we plan to attend Airventure/Oshkosh featuring the 75th Anniversary of the Cessna 170. (I am particularly interested in making that as I’ve never been to Oshkosh, and our ‘53 B-model was produced by Cessna as the 50th Anniversary of Flight Model. We took N146YS to the 100th Anniversary of Flight Convention and flew it to Kill Devil Hills…. so bringing her to the 75th Anniversary is something to which we look forward.

This engine has 1079 hours Since Major. This (#3) cylinder has never been off since that time in the early ‘90s. Running the compression test portion of the 100 hr inspection provided readings of 72 psi or greater (over 80) … except for Cylinder #3…which gave only 40 psi. Considerable “hissing” was heard at the oil filler cap. This initially caused me to suspect stuck rings (although it is often said that aligned piston-ring gaps can cause serious loss of compression, I’ve never witnessed that.)

Having drained the oil and removed the oil filter (Tempest filter)…I cut open the filter and removed the pleated filter-media and discovered a tiny chip or two…almost invisible…not much to draw attention… However, washing the filter-media in mineral spirits and then dragging a magnet thru the solvent resulted in this :

Filings build-up on magnet. Click to ENLARGE

I had considerable difficulty getting #3 cylinder removed as the cylinder base-nut of the lower/aft thru-bolt refused to come off the bolt without spinning the thru-bolt, possibly loosening the nut on the opposite #2 cylinder base nut.

After almost 3 hours of frustration attempting to put a jam-nut over the #2 cyl base-nut… success was finally achieved stopping that thru-bolt from spinning and getting the #3 cylinder base nut removed. Pulling the cylinder and revealing the piston demonstrated the piston-rings were NOT stuck…they rotated quite freely.

I was beginning to fear I’d pulled a cylinder needlessly in believing it might be source of the metal filings. It wasn’t until I got the piston pin drifted aside and removed the piston that the following,… very surprising conditon was found. I have never seen or heard of this type of ring-failure…. The ring was not broken….(surprisingly)….despite the piston ring-land having “disappeared” Below the ring (between the top and 2nd ring).

But even MORE Surprising…. is the area Above ring #1 is hammered by the #1 ring creating a “recessed” area toward the piston crown. This #1 ring had to flex SIGNIFICANTLY for this to occur…. many times each minute…. Yet, despite being a brittle item, it did not break!

Click to ENLARGE SUNLIGHT VIEW

Click to ENLARGE FLASH-PHOTO

The cylinder bore itself has no detectable damage from 5 o’clock to 7 o’clock … but at the 6 o’clock region near the bottom of the piston-stroke range is a smooth “ridge” which can be felt with the naked finger. Inside calipers show the ridge to be about .003” … I believe this is where the top ring must have flexed during piston-travel reversal. It cannot be noticed by eyeball. It can barely be detected with a finger.

LESSONS LEARNED :

1) TEMPEST filters, which have a magnet incorporated in their media-base (near the by=pass spring)…. the Tempest magnet has no special benefit. Zero metal was caught by the Tempest magnet….Yet washing the media and then putting my magnet into the solvent resulted in Significant Metal Filings attaching to my magnet, which prompted me to investigate further.

2) Rings can contribute to cylinder-wall damage without breaking, especially when piston-lands are damaged or missing.

3) Never Fail to cut open filters and WASH the media in Mineral Spirits and Check with a Magnet for ferrous metal. Run the media-wash thru a coffee filter to discover other types of metal or contaminates.

I am ordering a new cylinder assy and will install it later this week.

I hope to bring this piston to convention for “show and tell” at the maintenance seminar, and hope to see you guys/gals there!

[mmcmillan2]

Nice find! Will you get her back together in time for your trip?

[GAHorn]

Nice find! Will you get her back together in time for your trip?

Hopefully! New Superior Cylinder Assembly ($1130) ordered from J & J AirParts, Pleasanton, Tx and should be here before this upcoming weekend.

[ghostflyer]

You were very fortunate to catch that cylinder when you did . The new superior cylinders are a work of art . But saying that some of the superior rings are a little soft compared with continental and lycoming rings . I would be check torquing all the cylinder base nuts also . The through bolts stretch over time . I am very interested in seeing what your final tappet clearances ended up being on the new cylinder fittment.

[mit]

So you think the steel was ring wear? Looks like much more Al missing?

[GAHorn]

I believe the steel found by my magnet was mostly ring and cylinder wear. This was a Continental Steel cylinder installed back when the engine was restored by the previous owner. He spent 17 years restoring this airplane and admitted one mistake he made was to restore the engine (in the 1980s) before completing the airframe (1997). This required him to place the engine in storage for a number of years before installing it and starting it for the first time….kinda got the cart before the horse.

It also required him to disassemble the magnetos due to Service Bulletins/ADs/etc which came-out while it was in storage….before he could return it to service.

Anyway, the point is this is an early Contenental cylinder assy…. it’s unknown if that ring were somehow dislodged during assembly…but it’s inconceivable to me that it would have lasted 1079 hours if it’d been “crushed” into the upper piston land/crown during installation. I have to believe the 2nd piston-land failed between the top and 2nd ring…which allowed that top ring to “flutter” at each reversal of the piston travel.

While not “absolute”…(it’s not known if the metal found in the filter is completely that of the #3 cylinder)…. it is what I consider the culprit…. as the engine has 72/80 compressions on all others, and there are no other known defects outside the usual Continental oil leaks.

(I’m not devout, but in a full confession, I’ll admit I am not the “example” to follow as regards recommended practice.)
Since we bought the 172 a few years ago for a trainer the 170 has sat in the back corner of the hangar. This engine has had little exercise over the last decade. It has not been flown as frequently as is recommended by most authorities. It sits long periods between flights.
I DO plug the exhaust tailpipes with upside-down “Dixie-cups”, Close the Throttle, Use cowling plugs, and don’t live on the sea-coast. Otherwise this engine might be a prime example of “idle-ness”… it has been known to sit for months at a time…. And as I bought the airplane 20+ years ago when it had 220 hrs on the engine… it only just-now has 1079 hrs. The last long-flights it had were TX-VT-TX for the 2022 Convention, and two weekends ago the 3-Hr Round Trip for the TX Fly-In at Graham, TX. There are a couple annuals between which less than 10 hrs are recorded.

This cylinder has No visible Rust or Corrosion in the bore, and there is none seen on the crank or cam via the open hole created by removing cyl #3.
Typical CFI advice: Do as I say and not as I do. (except keep the idle engine openings plugged and stored in a “dry” hangar… one not with “dirt” floor… like our grandfathers did.)

[Bruce Fenstermacher]

George, we have found loosening the opposite nut slightly, on the through bolt will release the nut you are trying to remove without turning the bolt. It does not matter that you loosen that opposite nut as you will be following TCM procedure and torquing BOTH sides anyway.

[GAHorn]

George, we have found loosening the opposite nut slightly, on the through bolt will release the nut you are trying to remove without turning the bolt. It does not matter that you loosen that opposite nut as you will be following TCM procedure and torquing BOTH sides anyway.

Thanks for that suggestion…. but that is what occurred quite naturally…as we held the opposite nut while loosening the #3 cyl nut….which turned the thru-bolt and naturally loosened the opposite nut on #2 cyl.

The problem was that “someone” had apparently attempted to seal a leak at the #3 cyl base-nut with black “pooky” …(RTV)… which also got onto the exposed threads of the thru-bolt outside the nut. When the nut was loosened…it absorbed/over-ran some of that RTV which made it into a kind-of “nylock” nut…. increased the friction on the #3 cyl nut with the #2 side being free to loosen. (I tried to scrape that stuff off as much as possible when it was discovered…but was impossible.)

We solved the problem by installing a “jam nut” over the #2 base nut….but it was difficult to access, requiring removal of the #2 cyl lower cooling baffle. (Whew!) And, of-course,…this sort of thing always happens to the most-difficult nut to see/access….the bottom ones.

Isn’t Aviation fun..?

<I wonder if it wouldn’t be a good technique to install thin jam-nuts on all thru-bolts … It would help prevent this issue…as well as insure against loosening of base nuts. At one time “Pal” nuts were used as common practice on cylinder base-nuts…but that seems to have been abandoned in recent years. We first tried a pal nut for a jam nut…but it didn’t have sufficient strength and popped off right away. We (son-in-law, student pilot/helper) ended up using a thin nut. NAS671.

IMG_1995.jpeg (25.72 KiB) Viewed 14979 times

IMG_1996.jpeg (36.87 KiB) Viewed 14979 times

[GAHorn]

Overnight delivery:

[Bruce Fenstermacher]

Thanks for that suggestion…. but that is what occurred quite naturally…as we held the opposite nut while loosening the #3 cyl nut….which turned the thru-bolt and naturally loosened the opposite nut on #2 cyl.

The problem was that “someone” had apparently attempted to seal a leak at the #3 cyl base-nut with black “pooky” …(RTV)… And, of-course,…this sort of thing always happens to the most-difficult nut to see/access….the bottom ones.

Isn’t Aviation fun..?

All bets are off when black "pooky" is involved

I have a custom 9/16 wrench which allows the bottom cylinder nuts to be accessed from the top for removal, reinstall and torquing without removing the lower baffling, only loosening its retainer and dropping it a little in place.

Custom cylinder base nut wrench. The socket is welded into the open end which allows the wrench to be used with a ratchet and an extension of a torque wrench for torquing all from the top of the cylinder.

Yes my new found fun in aviation is conceiving, then building tools to make PITA jobs much easier and faster.

[johneeb]

Last night here at the home we had Black Pooky for dinner, not to be confused with Monday nights when we have Green Pooky.

[GAHorn]

Bruce..what “fudge-factor” do you use when deciding how much torque to place on the nut when using that tool?

(I once had to re-torque some prop-nuts on a 175 that were difficult to access. We used a long open-end wrench…which by happy-fortune was exactly 12” long….making the calculation pretty simple.)

[Bruce Fenstermacher]

George, it's not a "fudge factor", it is math. I'm sure you know, but others may not, that the explanation and formula can be found in the very first pages of TCM M-O in the torque specifications page as well as the Lycoming manual not to mention 43.13.

I have that formula in a spread sheet on my phone and can calculate any torque given extension length, torque wrench length and angle of the extension to the torque wrench.

In this application of this extension I use it straight out of the torque wrench so torque set on the wrench will be lower that the torque you're trying to achieve. The straight line length of the extension is 5.75". the length of my torque wrench to the center of the handle is 14" Desired torque, center of range for an 0-300 corner cylinder nut is 41 ft Lb. The formula would be (41/(14+5.75))x14 = 29 ft lb. (rounded)

[GAHorn]

Yep.

And I’m taking this opportunity to remind folks that such tools should be calibrated… and not by Harbor Freight.