druidjaidan Posted Sunday at 08:40 PM Report Posted Sunday at 08:40 PM Just discovered a broken cylinder bolt during an oil change. Pretty yikes. We had our alternator go bad at the end of an 8 week vacation with like 80 hours flown. When I picked up the plane I noticed they had topped up the oil to 8qts for whatever reason. At the end of the first leg home of course I noticed a fairly substantial amount of oil in the gear door near the breather, but chocked it up to the 8qts blowing out some. Flew the rest of the way home and got around to the oil change today. Immediately noticed the lower cowl and everything had more oil than I expected, started looking for leaks. And I found a sheared off bolt hanging out next to the governor. 1
N201MKTurbo Posted Sunday at 09:28 PM Report Posted Sunday at 09:28 PM I’m not real familiar with those engines, so I can’t tell if that is a through bolt or not. If it is, I would be very concerned. It might be worth a call to the factory for advice.
redbaron1982 Posted Sunday at 10:15 PM Report Posted Sunday at 10:15 PM Sorry to hear this. What part of the engine the first picture shows? I can't tell. I mean I do see the nut backed off, but I can tell where that is.
druidjaidan Posted Sunday at 10:18 PM Author Report Posted Sunday at 10:18 PM 1 minute ago, redbaron1982 said: Sorry to hear this. What part of the engine the first picture shows? I can't tell. I mean I do see the nut backed off, but I can tell where that is. Sheared side of the bolt is cylinder 6 toward the propeller. The backed off side is in front of cylinder 5
druidjaidan Posted Sunday at 10:20 PM Author Report Posted Sunday at 10:20 PM A bit more context on the engine.
IvanP Posted Sunday at 10:21 PM Report Posted Sunday at 10:21 PM Looks like a through bolt. How many hours are on the engine? Any recent work on the cylinders?
druidjaidan Posted Sunday at 10:33 PM Author Report Posted Sunday at 10:33 PM (edited) 5 hours ago, IvanP said: Looks like a through bolt. How many hours are on the engine? Any recent work on the cylinders? No recent work, at least since I've owned it. Around 1550 hours on the engine. I'll check logs for any work before I owned it. Edited Monday at 03:54 AM by druidjaidan
jetdriven Posted Sunday at 11:25 PM Report Posted Sunday at 11:25 PM If it is not torqued to spec, or not torqued at all, it has low or no preload on it, it will fail in cyclic fatigue. That looks like maybe what we have here. 1
N201MKTurbo Posted Sunday at 11:58 PM Report Posted Sunday at 11:58 PM 27 minutes ago, jetdriven said: If it is not torqued to spec, or not torqued at all, it has low or no preload on it, it will fail in cyclic fatigue. That looks like maybe what we have here. It could be the case was fretting, or some sealant got on the case halves where the through bolt went through. This can cause a lack of stress (torque) leading to what you describe. Lycoming has a service bulletin warning people that the use of any type of sealant around the through bolts can lead to this failure. Some were putting sealant around the through bolts to stop oil leaks. Lycoming says that is a very bad idea.
Utah20Gflyer Posted Monday at 02:50 AM Report Posted Monday at 02:50 AM During an oil change last year I found a broken cylinder hold down stud. Fortunately it wasn’t a through stud which I think would complicate the process of fixing the problem. Fixing mine required removing the cylinder and an interesting process to remove the broken stud piece that remained in the case - but was broken off below the surface. I’d imagine the fix for a through stud will require removing two cylinders. I would certainly contact the manufacturer for their input on the proper procedure for removing and replacing a through stud. Hopefully there is a precedent for doing so while the engine is still installed. 1
druidjaidan Posted Monday at 03:56 AM Author Report Posted Monday at 03:56 AM (edited) Double checked the logs. Last cylinder work was quite a while ago 10/14/2014 at 962 TSMOH. Last annual 6/19/2025 we were at 1567 and we've probably flown 90ish hours since then. Calling the local mechanic on the field tomorrow. Unfortunately mechanic availability is absolute crap at KPAE so I'm likely down for quite a bit for something as severe as this =( Edited Monday at 03:57 AM by druidjaidan
EricJ Posted Tuesday at 12:01 AM Report Posted Tuesday at 12:01 AM Often through-bolts require the case to be split to replace them. I don't know whether that's the case with this one or not, but it would be nice if not. Even if not it might be worth consulting with Continental or a relevant shop on what inspections might be a good idea if the engine ran for a while with no tension on that bolt.
druidjaidan Posted Tuesday at 09:28 PM Author Report Posted Tuesday at 09:28 PM Continental Tech Support says: Given this through bolt provides clamping pressure to the main bearing they recommend IRAN and I might as well overhaul it since it's been more than 12 years. 1 1
Ragsf15e Posted Tuesday at 09:47 PM Report Posted Tuesday at 09:47 PM 18 minutes ago, druidjaidan said: Continental Tech Support says: Given this through bolt provides clamping pressure to the main bearing they recommend IRAN and I might as well overhaul it since it's been more than 12 years. That’s probably reasonable based on your hours as well. Glad you found it, but sorry this happened to you.
MikeOH Posted Wednesday at 01:42 AM Report Posted Wednesday at 01:42 AM 4 hours ago, druidjaidan said: Continental Tech Support says: Given this through bolt provides clamping pressure to the main bearing they recommend IRAN and I might as well overhaul it since it's been more than 12 years. Sorry to hear this. But better than having an engine failure! At least you got some hours and over 12 years out of it. Thread drift warning! I've heard that the worry with loss of the clamping pressure runs the risk of spinning a bearing insert. But I've never quite understood that. Don't the inserts have tangs that 'locks' them in place in the case like automotive bearing inserts? Or, do they only rely on the friction provided by sufficient clamping? 1
DXB Posted 19 hours ago Report Posted 19 hours ago More thread drift alert My engine rebuilder told me that it's worth the hassle of retorquing the through bolts and then everything else linking the case halves about 1000hrs into the life of the engine since the bolts do stretch a bit over time . Supposedly it reduces risk of events like this in addition to a preventing oil leaks and case fretting? Any insights on best practice on this?
druidjaidan Posted 17 hours ago Author Report Posted 17 hours ago On 9/9/2025 at 6:42 PM, MikeOH said: Sorry to hear this. But better than having an engine failure! At least you got some hours and over 12 years out of it. Thread drift warning! I've heard that the worry with loss of the clamping pressure runs the risk of spinning a bearing insert. But I've never quite understood that. Don't the inserts have tangs that 'locks' them in place in the case like automotive bearing inserts? Or, do they only rely on the friction provided by sufficient clamping? I've gotten quite a few opinions from various engine builders and A&P/IA's over the past few days. Continental was the only one that felt IRAN (and by default overhaul due to age) was needed. The rest basically were like: if the bearing was spun it would block/restrict oil flow and you'd know very quickly (as well as ruin the engine). All said they would/had safely replaced thru bolts in similar situations and didn't have concerns about doing so with mine. The specifics varied between just replacing the bolt to re-torquing all, to replacing the bolt and all studs on that cylinder + re-torquing all. The biggest fear is that the loss of that bolt caused the main bearing to spin and the engine is toast as it is. Assuming I trust the engine builders and A&P's are correct, if that happened we would know during a nice thorough run up. If that run up destroys the engine, so what we would overhaul anyway (and I'm likely to reman this time since it's GB engine). The balance of that is if they are failing to mention some risk of near future catastrophic failure, it isn't really worth risking my life over a few hundred hours more we'll likely get out of the engine. They were pretty adamant that wasn't a risk though, the risk was the next time we start the engine the oil won't flow and we'll toast the engine within minutes. 2
EricJ Posted 17 hours ago Report Posted 17 hours ago 48 minutes ago, druidjaidan said: The balance of that is if they are failing to mention some risk of near future catastrophic failure, it isn't really worth risking my life over a few hundred hours more we'll likely get out of the engine. They were pretty adamant that wasn't a risk though, the risk was the next time we start the engine the oil won't flow and we'll toast the engine within minutes. I'll generally concur with the gist of your post, but I think it's worth pointing out that if you do a ground run and the bearing is spun or does spin, there is significant risk that it'll ruin the crankshaft, which is the most expensive part in the engine. It's just another consideration in planning your path forward. That said, I think the risk that the bearing is spun is pretty low.
MikeOH Posted 14 hours ago Report Posted 14 hours ago 2 hours ago, EricJ said: I'll generally concur with the gist of your post, but I think it's worth pointing out that if you do a ground run and the bearing is spun or does spin, there is significant risk that it'll ruin the crankshaft, which is the most expensive part in the engine. It's just another consideration in planning your path forward. That said, I think the risk that the bearing is spun is pretty low. I'd further make the point that the plane flew for a while with the through-bolt loose...so, if the bearing has spun then the crankshaft has already been compromised. Therefore, I don't see much added risk to now replace/tighten the through-bolt and ground-run the engine aggressively. IOW, the odds that the bearing spun JUST before the plane was last shut-down without damaging the crankshaft, and now after tightening the through-bolt the previously spun bearing will now damage the crankshaft seems pretty unlikely. 1
GeeBee Posted 14 hours ago Report Posted 14 hours ago If the bearing spun it likely would show up on oil analysis. What is that telling you?
N201MKTurbo Posted 14 hours ago Report Posted 14 hours ago To test for a spun bearing, pull the prop forward, then push it backwards. You should feel the crank move about 0.020”. This is the thrust clearance of the crank and bearings. I’ve been told by an old A&P (humm, what and I) that if a bearing has spun, it won’t move for and aft. Could just be an OWT. 1
druidjaidan Posted 11 hours ago Author Report Posted 11 hours ago 3 hours ago, MikeOH said: I'd further make the point that the plane flew for a while with the through-bolt loose...so, if the bearing has spun then the crankshaft has already been compromised. Therefore, I don't see much added risk to now replace/tighten the through-bolt and ground-run the engine aggressively. IOW, the odds that the bearing spun JUST before the plane was last shut-down without damaging the crankshaft, and now after tightening the through-bolt the previously spun bearing will now damage the crankshaft seems pretty unlikely. That's kinda exactly where I am at for sure. I think the odds that it spun are low. The engine builders I spoke to both felt that if it did spin we'd have a rapid (within minutes) engine failure due to restricted oil flow. My current opinion is that I'm sitting on Schrodinger's cat. Either the bearing spun when I warmed up the oil for the oil change or it didn't. If this was a low time engine it would be worth doing the IRAN to look inside that box and making sure it was all in order so as to not risk more damage. This is a high time engine though, and a GB. I'll be sending it in to Continental to exchange for an LB within a few years anyway. If all I'm risking by fixing it is that the cam might get further damage on startup, that math seems to point toward fix-it, run it up hard, and fly it with maybe a few extra oil changes to monitor for metal. If that bearing spun on last operation that cam is likely unserviceable anyway. The biggest uncertainty in my mind (as I'm not an expert): is there another failure mode that isn't as obvious as "bearing spun, oil flow obstructed, engine die". I have no interest in taking a risk that this engine is a timebomb and would rather just overhaul if there is any reasonable chance of that.
Slick Nick Posted 11 hours ago Report Posted 11 hours ago Call Savvy. I bet Mike Busch will tell you to replace the bolt and run it, which is what I would do. His wisdom on stuff like this is worth the small price of admission.
IvanP Posted 10 hours ago Report Posted 10 hours ago Here is an interesting video of Lycoming engine running for some time without any oil. Not intended as advice - for entretainment purposes only
EricJ Posted 9 hours ago Report Posted 9 hours ago 2 hours ago, druidjaidan said: The biggest uncertainty in my mind (as I'm not an expert): is there another failure mode that isn't as obvious as "bearing spun, oil flow obstructed, engine die". I have no interest in taking a risk that this engine is a timebomb and would rather just overhaul if there is any reasonable chance of that. I don't know where the usual case cracks happen in those engines, but if it is anywhere near where the bolt broke, it may be worth it to dye pen both sides of the case to make sure they didn't crack due to flexing around where the bolt broke. This is probably also a low-probability, and a crack may be latent if it hasn't propagated to the outside of the case yet. I only offer that since you asked for other possible latent failure modes.
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