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Mike Busch's cylinder swap scare

scsmith

Well Known Member
I just read Mike Busch's write-up about the factors that can impede achieving proper pre-load in the cylinder hold-down stud and through bolts when installing a cylinder.

He makes it sound like every engine that has ever had a cylinder swap with the engine mounted on the airplane is at high risk for failure and death! How many cylinders are changed on the airplane? How many have led to failures?
A million to one might be an exaggeration, but it is **** rare, for sure. I generally like Mike's discussions and learn from them, but I think this one is a bit over-dramatic.

Reading through his arguments, the real issue, and the real take-away from the story, is that the threads need to be very clean and very well lubricated. If the original assembly was done with generous lubrication, then his other big concern, thread wear from repeated assembly, is minimized. A second assembly, again with generous lubrication, can fairly be expected to produce proper preload. Lycoming's recommendation to mix 10% STP in with the lubricating oil is a very good protection against thread wear, as would other high-pressure lubes. In the car world, I always used an assembly lube that had some moly-sulfide in it.

The other take-away that is useful in his arguments is to be sure to have enough clear swing of the torque wrench to approach the right torque while the nut is turning. It would seem necessary to remove most of the accessories and other things that get in the way to do a cylinder job anyway, so in principle, the access shouldn't be much different between having the engine on an engine stand and having it on the airplane. Not to say that access isn't complicated in any case - there is a whole family of odd-shaped box-end wrenches that aren't called "Lycoming wrenches" for nothing! But fundamentally, it can be done on the airplane to the same degree of care as on an engine stand.

I do agree with Mike that many shops are too eager to pull a cylinder when other options should be explored first. No doubt his overly dramatic approach to a discussion about achieving proper pre-load is motivated by hoping to add some deterrent threat to those that would unreasonably want to pull cylinders off.
 
I see guys pulling jugs without installing torque plates. And, then when the UPS driver fails to deliver that "Next Day Air" package, they go home with just a post-it note warning not to move the prop.

I always thought that was a little dicey. But, I haven't seen anyone admitting that it may have been the cause of a spun bearing.

I guess its to-each-his-own. But, I'd think anything that could potentially disturb the relationship of so many high-stress components should handled very conservatively.
 
This weekend I will be disassembling a field overhaul that I've been chasing a low oil pressure problem for a month. I know the crank was spun on several occasions with no cylinders or any sort of pre-load on the case. Cylinders have been removed and reinstalled. I'll report back next week what I find. I'm betting the bearings haven't budged.
 
Risk analysis

If the outcome of a failure will result in great danger, then caution is advised.
If a failure can lead to an inconvenience or a minor incident, then the risk
may be accepted.
Knowing what you are doing is more important when the consequences can be serious and or the risk is higher.

Good luck
 
This weekend I will be disassembling a field overhaul that I've been chasing a low oil pressure problem for a month. I know the crank was spun on several occasions with no cylinders or any sort of pre-load on the case. Cylinders have been removed and reinstalled. I'll report back next week what I find. I'm betting the bearings haven't budged.

Maybe it’s only some engines, but both my o320 case before overhaul at disassembly had stakes on all main bearings and when my case came back from divco had new stakes set for the new bearings. If the bearings spin and your case has stakes then no amount of preload or no preload at all would have affected those bearings in a staked case. Something else would have to shear the stake.
 
Personally, I take a lot of MB's comments with a grain of salt, this is one of them for sure.
I'd venture to say very few engines make TBO without a top end, most done in the field without torque plates.
 
I have never understood this. Don't the tangs prevent bearing rotation? Do older engines and or Continental not have these?
 

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I have never understood this. Don't the tangs prevent bearing rotation? Do older engines and or Continental not have these?

Its the dowel pin (hole in the middle) that prevents rotation on most Lycomings IF bearing crush is lost. The tangs are for locating the bearing during installation. Some Lycomings (H engines) don't have dowel pins. Continental's dont have dowel pins.

For me Busch's articles on the subject indicate he doesn't have much experience assembling engines.
 
Right there with you Jereme. I've built a bunch of engines in my lifetime and I have hard time seeing how a bearing can spin with hand rotation. The bearing tangs sit proud of the saddle IML by a fair amount. For that "catch" to release, the case halves would have to spread AT LEAST the height of the tang, and frankly, that would be obvious.

Maybe I've just been lucky, but I've topped a few Lycomings in my time and I've never used a torque plate.
 
I see guys pulling jugs without installing torque plates...

Torque plates during initial crankcase assembly makes perfect sense. Their use is detailed in the overhaul manual, section 7.

Is there a Lycoming document describing the use of a torque plate or plates when removing and replacing a cylinder, or multiple cylinders?
 
I have never understood this. Don't the tangs prevent bearing rotation?

Yes they do, in addition to keeping it centered and from sliding side to side. These tangs are pretty standard on engines and generally keep the bearings in place. However, if they get loose (normally oversized and the crush force keeps everything tight), the tang can wear away (they are made from soft stuff like tin and copper) from rubbing and you get a spun bearing. I absolutely do not believe that pulling a cylinder can cause this and think Mike should know better. That said there are several of his conjectures that I simply discard at TLAR style of thought. He reallly shouldn't be running around scarring people from doing what is done every day without incident. That said, I do agree with the thought of taking simpler steps before deciding to pull a cylinder.

It was not that uncommon in the auto world to re use bearings in some tear downs and rarely did this cause spun bearings and that is far more invasive than loosening a hold down bolt.
 
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Okay cool, I figured these tangs (and pins) prevented rotation so I wondered what all the hype was about. After watching many of his webinars I'm under the impression that he is more understanding of a continental than Lycomings but that is just my observation.

I think the guy became popular because of his bringing attention to leaning (which he wasn't the first but he was the most successful) and how he has shattered some old wives tales (also not his work usually but once again, he is successful in reaching the masses).
 
Torque plates during initial crankcase assembly makes perfect sense. Their use is detailed in the overhaul manual, section 7.

Is there a Lycoming document describing the use of a torque plate or plates when removing and replacing a cylinder, or multiple cylinders?

None that I have ever seen and I have looked. When you are squeezing the whole case together they make a lot of sense. However, remember that these through studs are interference fit (which is why you need a torque plate during assembly) and simply removing a cylinder should NOT cause the case to come apart. Let's also not forget the 20 other bolts beyond the through studs that are holding the case together. For caution, I do use something to keep tension on the studs ones the cyl is off. I suspect most A&Ps doing top O/H do not do this.
 
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These tangs are pretty standard on engines and generally keep the bearings in place. However, if they get loose (normally oversized and the crush force keeps everything tight), the tang can wear away (they are made from soft stuff like tin and copper) from rubbing and you get a spun bearing.

My recollection is that the tangs are formed in the steel back shell that the soft bearing material is built up onto. I suppose with a loss of preload, it would be possible for the bearing cap (of a rod bearing) to move enough to start some slight motion in the bearing shell that would start to wear the tang. Do this a few thousand times a minute and it wont take long to wear or carve the tang off. It does happen. But I've always attributed a spun bearing to either an assembly error or loss of lubrication.

It was not that uncommon in the auto world to re use bearings in some tear downs and rarely did this cause spun bearings and that is far more invasive than loosening a hold down bolt.

I've taken engines apart that have run 200,000 miles and found the main bearings showing no sign that they ever touched metal to metal, ever. They look brand new. Used to break my hear to throw them away. Of course that was never true for British engines!

I agree with a lot of the thoughts here about Mike Busch. One thing I really appreciate, however, is his crusade against over-zealous mechanics that hold an airplane hostage against a list of work that they say is necessary to sign off an annual.
 
Bearing tangs

Tangs on bearings are primarily an assembly location aid only,and are not put there to keep a bearing from spinning.Many non aero engines do not have bearing tangs and rely on the crush on the bearing to prevent a bearing spinning in its housing or block.
 
I've taken engines apart that have run 200,000 miles and found the main bearings showing no sign that they ever touched metal to metal, ever. They look brand new.

^^

Makes one a believer in what they say - that the crank spins on a "cushion" of oil. Pretty amazing.
 
Timely topic as we found low compression on #1 of my Titan IO-360 (RV-8). Getting close to 1000 hours. The top ring was worn and the cylinder was smooth. This is my first experience taking the cylinder off after a thorough borescope and examination by my trusted mechanic.

I haven’t turned the prop and don’t plan to; I’ve been advised about the potential negative consequences of doing so. The probability of the bearing spinning after all that work and expense sounds dreadful.

I haven’t read the Mike Busch article but plan to. Always something to learn. Like reassembly, torque, break-in…

Thanks for posting Steve.
 
I split the case yesterday. Big surprise, no bearings spun. :rolleyes:

I've listened to pretty much everything available on youtube from Busch and agree with much of what he has to say. But on this topic it seems like he's never actually done the work himself.
 
Yes they do, in addition to keeping it centered and from sliding side to side. These tangs are pretty standard on engines and generally keep the bearings in place.

This is incorrect. Tangs are only for indexing and proper orientation during installation. Crush its what holds the bearing in place. The tangs are way too small to hold anything. Note there are no tangs on the big bearings since two dowel pins locate them.
 
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Cylinder swaps...
a while back one of his cautions on doing field cylinder swaps is the difficulty in getting a good torque onto the cylinder hold down nuts.

Look at this scenario - you pull only as much baffle, accessories and other junk as necessary to swap one cylinder---with the engine hanging on the airframe of course. So the ability to get a good torque reading is impaired because of stuff in the way. Recall that fastener torque is set with the fastener in motion. Once it stops turning (to get onto the next flat) the torque wrench will likely click when you reapply force, not because it is properly torqued, but because the static force makes it *appear* to be properly torqued.

Result: Under torqued hold down nuts on the cylinder base.
Which happens:

https://www.kitplanes.com/engine-failure/
https://www.ntsb.gov/investigations/accidentreports/reports/aar0403.pdf
https://planecrashmap.com/plane/ca/N299X/

Not saying don't do it. Just saying there are things to watch out for.
 
Mike Busch Disciple - fallen

I read and listened to all the Mike Busch stuff and I'm a firm believer.

That said, when my mid-time O-360 started getting high oil consumption, and borescoping showed some rash / pitting in all the cylinders, I decided to pull all four and have new rings put in and the cyl. walls honed (still within "new" limits).

Since I'd never pulled a cylinder before, I found an A&P with Lyc. engine training to help. But lo and behold, he didn't know about torquing wet with the STP mixture. So he learned something from me since I'd read / listened to all the MB stuff and also read it in the Lyc. O/H manual.

It didn't help as much as I'd hoped with the oil consumption but good enough. And there was definitely educational value in the whole exercise, so I call it a win.

Anyway my take from Mike is, he's overcautious about pulling cylinders, and that's not necessarily a bad thing as it made me that much more diligent about taking care to do it right including getting the help of an A&P (plus teaching him something he didn't know).

Epilogue: I went on Ask the A&Ps afterward and got chastised by MB -- badge of honor!
 
Teaching an A&P

If I had to teach an A&P who was supposedly trained on Lycoming I would have stopped him right there. Sorry but that’s already a red flag. What else doesn’t he know?
 
If I had to teach an A&P who was supposedly trained on Lycoming I would have stopped him right there. Sorry but that’s already a red flag. What else doesn’t he know?

My understanding is that Mike and his team know *lots* of A&Ps, and know their strengths and weaknesses, which is why they recommend an owner avoid pulling a cylinder at an average shop. Or getting an engine overhauled at an average shop.

If you read enough NTSB reports you will find that there are A&Ps that have no business opening up an engine, and probably A&Ps that just had a bad day. This can happen to any of us. If you go to a good overhaul facility, they accept the reality that humans can and do make mistakes, and put in place processes to reduce the likelihood that one moment of inattention or someone having a bad day will result in a mistake deep in your engine.

This topic might be like the "impossible turn" discussion. Of course it can be done, lots of people have proven it, but does it make sense to recommend it to the "average" pilot?
 
This is incorrect. Tangs are only for indexing and proper orientation during installation. Crush its what holds the bearing in place. The tangs are way too small to hold anything. Note there are no tangs on the big bearings since two dowel pins locate them.

I did indicate that the crush is what holds them in place. However the tang will prevent bearing rotation if the two bore halves are loosened and the crush is gone. That was the point of this flawed advice. That loosening the case bolts would allow the bearing to move once the case is loose and that simply isn't possible due to the tangs. Mike also implies that lossening the case will somehow change the fact that the bearing is oversize and crush will not re occur upon tightening. That kind of goes in the face of everything we know about steel.
 
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My recollection is that the tangs are formed in the steel back shell that the soft bearing material is built up onto.

You are correct. I didn't think through this as I wrote the post. The backshell has to be steel in order to maintain the crush due to the interference fit. the solf metals would yield upon the intital torquing and there would be no "crush" force holding the bearing in.
 
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Don’t forget the stakes, you would have to shear off the stake (dowel pin as some call it) and there is on on each main and 2 on the front main. I will also add I have bent a case splitter acme thread on a DIVCO case that had locating pins installed on each through bolt. This was without any sealant on the case halves. I can’t imagine a DIVCO case with the pin mod on the through bolts ever coming apart enough to allow you to spin a bearing with or without stakes.
 
I don't know the article you mean...but lube and cleanliness does for sure matter to get the proper pre-load on threaded fasteners.
Preload can be very different if dirty and dry vs lubed with motor oil vs anti-seize vs oil form the mechanics greasy hair...all torqued to the same spec....and I have never understood why they don't always specify the type of lube on which the toque charts are based on.
 
A&P

Good to remember that an A&P cannot do a major repair on certificated aircraft or engine except under the supervision of an IA. So why in the world would a person building a homebuilt airplane say "my A&P is assembling my engine"??
 
Good to remember that an A&P cannot do a major repair on certificated aircraft or engine except under the supervision of an IA. So why in the world would a person building a homebuilt airplane say "my A&P is assembling my engine"??

An overhaul on a direct drive engine is not considered a major repair by the FAA. See Part 43 Appendix A. An A&P may certainly overhaul one - and return it to service.

Ed Holyoke
 
Good to remember that an A&P cannot do a major repair on certificated aircraft or engine except under the supervision of an IA. So why in the world would a person building a homebuilt airplane say "my A&P is assembling my engine"??

Another reason this statement does not apply to experiential aircraft.

The fact is, he doesn't even need the A&P, he could assemble it himself. The A&P involved is a big plus.
 
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