Fractured Cylinder Stud. Results are in.

[Freemasm]

Contributor 74-07 started a post regarding a failed cylinder stud(s) found during unrelated maintenance on his aircraft. That thread can be found here:

https://vansairforce.net/community/showthread.php?t=216725

It seemed better to start this thread here (Edit = I had put this in the General Discussion Thread but they moved it here) versus having it buried in the Traditional Aircraft Engines Forum. (Edit = they moved the thread here, oh well I tried). I hopefully don't need to mention how unlikely this discovery was and how bad this could have otherwise turned out.

The fracture surface of one of the studs was inspected by a well respected, very experienced NDE in a world class materials lab; name and company name are withheld. Can't get him in trouble. He's done favors for us before and I'd hope this can continue. Hopefully this is appreciated by the community here as much as I do.

There are zero surprises here but I'd share the "results" of his expert personal opinion below.

Good morning, xxxx. I received your fractured cylinder hold down stud. Stereo microscope examinations at 8X-40X revealed fracture features consistent with fatigue, initiating at a thread root on one side. I couldn’t see any obvious material defects. I can’t tell you much about any potential contributing factors such as microstructure, material properties, thread conditions, amount of preload, etc. I assume....

Down side = it wasn't challenging enough to get examined by the Scanning Electron Microscope or other cool toys at their disposal so no ultra cool pix. There are some images attached below. Hopefully the system here doesn't rotate them. You can at least see where the fatigue cracking started and the direction it propagated. Excuse the limited file size but it's necessary.

Anyway while no surprises, this does reinforce something I've been harping on a for a while here. There appears to be healthy design margins on our aircraft designs but some areas apparently way less than others. While we can't determine the root cause as to why the studs didn't get (or lost) proper preload, the NDE's short summary does not suggest a material issue.

Cylinder installation continues to demonstrate this application is less tolerant of f--kage than other things in our builds. Records suggest this installation was done by an overhaul shop versus a field install. Follow the OEM instructions. It's important.

Cheers boys

 

[AN23]

Scott,

Thanks again for getting professional eyes on this. I couldn't agree more about everything you said. When I found this during an oil change, it sent chills down my spine. My son and grandson fly this airplane regularly. We both know just how bad this could have been. Just one more note. I have heard from several pilots that I could just replace the stud and thu-bolt and "it'll probably be ok". No, a teardown is required as the bearing has seen a release in clamping force. I have no doubt that it hasn't "spun" as the engine turns over very freely with the plugs out BUT there is NO POSSIBLE WAY to know if it shifted. Even a small shift can reduce the flow of oil to the crank and that will eventually cause catastrophic engine failure. In studying this, I've found several NTSB accident reports that this exact scenario was responsible for fatalities. My bank account will be smaller, no doubt, as a result of this IRAN but I'll have confidence in my engine again and my two guys smiling at me after they've enjoyed yet another great RV-8 flight.

Thanks again for your help.

 

[cgarts1]

Scott,
No, a teardown is required as the bearing has seen a release in clamping force. I have no doubt that it hasn't "spun" as the engine turns over very freely with the plugs out BUT there is NO POSSIBLE WAY to know if it shifted.

Many (many, all?) of the wide deck engines have bearings that are doweled to the case. On the four IO360 engines I've done overhauls on they have all had bearing 11021 and they were all doweled. See attached images.

There is no way these kind of bearings are shifting. If I knew that I had these bearings there is no way I would tear down the engine.

 

[AN23]

Chuck,

Very interesting. I talked to multiple well known shops and other highly regarded maintenance personnel. All stated categorically that it needed a tear down. I’m going to call Lycoming tomorrow. I don’t want to spend the money but I can’t handle a “it’s probably going to be ok.” I watched my best friend die in an engine failure on takeoff. I couldn’t handle another.

 

[cgarts1]

I totally don't blame anyone for making sure it's right. Calling Lycoming is a great idea. The overhaul shops also may also be concerned with things other than the bearings.

I also wonder how long has the engine ran since the bolt was broken. We don't even know what would have happened.

If it were me I would send the oil out for analysis. Open the filter and take a look. If all looks good replace the stud and the through bolt and spend the next 20 hours being careful. Cut the filter open every few hours and see whats going on.

Anyway, that's just me. I don't blame anyone for being careful. It just seems that so often flies are being swatted with sledgehammers. A friend q-tipped his prop last year and everyone talked him into a major overhaul and there was nothing wrong inside. The only mandatory thing he had to do was inspect the crankshaft gear.

 

[cgarts1]

BTW, I bet Mike Busch would take a special interest in this topic. Might be worth reaching out to him. Maybe email him a link to this thread.

 

[DanH]

In my opinion, if it broke one of the long through-the-case studs it needs to come apart. The issue is the potential for case fretting at the mating surfaces adjacent to the bearing journal.

The long studs act like very stiff springs. They clamp the case with enough preload that the clamp force exceeds the mechanical stress in operation. Given the fatigue analysis, it was probably running with low preload for some significant period, and if I understand correctly, no one knows when it broke. So, the mating surfaces may have gotten a lot of run time without preload, and when that happens, they rub against each other and lose material from the surfaces.

Here's the rub, literally. If a mechanic drives a new stud and tightens it, the two worn surfaces are pulled together, and the resulting journal isn't round. If it's bad enough the crank journal scuffs the bearing.
.

 

[rv6ejguy]

Agree with Dan. Case fretting can be bad news down the line if it's been happening for a while.

 

[hgerhardt]

Many (many, all?) of the wide deck engines have bearings that are doweled to the case. [snip]

There is no way these kind of bearings are shifting. If I knew that I had these bearings there is no way I would tear down the engine.

I overhauled my IO-360-A3B6D back in 2008 when it had 2100 hrs since new. It has those bearing locating dowels as you showed and yet the center bearing shells were polished on the backside. As in they looked like they were chrome-plated. So they were definitely moving/shifting. And there was fretting at the center bearing support. Findings like that are the biggest reason to overhaul per recommendations. That bearing moving around probably would not have shown up in oil analysis and never showed up in the filter.

As DanH said, that engine absolutely needs to be torn down and the crankcase align-honed to restore roundness of the bearing bores and ensure proper bearing crush.

 

[rv8ch]

... All stated categorically that it needed a tear down. I’m going to call Lycoming tomorrow. I don’t want to spend the money but I can’t handle a “it’s probably going to be ok.” I watched my best friend die in an engine failure on takeoff. I couldn’t handle another.

Rob, your gut is talking to you. You already know what to do. Thanks for sharing this, and sorry you are having to go through it.

 

[AN23]

I talked to Lycoming today. They advised that an IRAN is the prudent course of action. There is much more involved than just the possibility of bearing shift. As mentioned earlier by another poster, case fretting and case cracking are other considerations. Perhaps it was torqued incorrectly from the beginning.....we may never know. Something released the preload on those bolts and allowed the thru bolt to get repetitive loads that exceeded the preload on the bolt.If it now has case fretting, as a result of this preload release, and you simply replace the thru bolt and retorque, you can pinch the bearing. Everything needs to be carefully examined and replaced if required. As another well known expert told me today, "It's all about risk tolerance. There's probably a low chance that something really bad would happen if you just replaced the stud and thru-bolt BUT there was an extremely low chance that your thru-bolt would break to begin with."

The engine is almost ready to pull. It will be going to Triad next week for IRAN. I'll report back on their findings.

 

[Majorpayne317641]

I talked to Lycoming today. They advised that an IRAN is the prudent course of action. There is much more involved than just the possibility of bearing shift. As mentioned earlier by another poster, case fretting and case cracking are other considerations. Perhaps it was torqued incorrectly from the beginning.....we may never know. Something released the preload on those bolts and allowed the thru bolt to get repetitive loads that exceeded the preload on the bolt.If it now has case fretting, as a result of this preload release, and you simply replace the thru bolt and retorque, you can pinch the bearing. Everything needs to be carefully examined and replaced if required. As another well known expert told me today, "It's all about risk tolerance. There's probably a low chance that something really bad would happen if you just replaced the stud and thru-bolt BUT there was an extremely low chance that your thru-bolt would break to begin with."

The engine is almost ready to pull. It will be going to Triad next week for IRAN. I'll report back on their findings.

If you want to never have the case fretting problem again send your case off to DIVCO and ask for the through bolt dowel mod. I had case fretting upon engine disassembly and I discovered the source of a strange through bolt leak was a nicked case seam seal at the through bolt due to case fretting. When I got the case back and dimensioned it without the crank on prior to assembly my case tool almost didn’t get the two case halves off with how snug the dowels are!