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  #21  
Old 06-27-2018, 04:58 PM
Turbo69bird Turbo69bird is offline
 
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Location: CT
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I?d be willing to bet that the guys on vans airforce are not the target audience for this article. Your average pilot of a certified would never even think of removing a cylinder themselves and would rely on the local A&P . Let?s face it most people here do things themselves, because if they brought it to someone else it probably wouldn?t get done to their liking, 😂


I remember an old merc outboard mechanic watching a guy pick up a set of needle bearings with a magnet just cringing. Everything has its idiosyncrasies the difference is the guys here do research ? before ? they do anything.

Now maybe, I read a different article but the one I read stated most of his info comes from continentals. Right off the bat.

It also said that many mechanics don?t use wet torque, went into detail on differences of frictional losses, most don?t use all new nuts and bolts with perfect plating. (Having built car race engines you always use new bolts And nuts on reciprocating parts and fasteners.) the cheapest insurance you can buy.
He then goes on to say even if the mechanic THINKS they did it right that the MFGs own manuals say nothing of replacing the bolts,nuts etc, toque to click in one continuous motion, or disassemble / remove the through bolts to replace/ retorque the bottom nuts properly. So even the best intentioned likely will not get proper preload even if they try to do it 100% right.

I found the article to be a real eye opener and from what I?ve seen for work from A&Ps I doubt most A&Ps are doing this correctly, I?d bet 9 out of 10 get the clamping force wrong. Heck I?d bet half the over haul shops get it wrong, 😂


Many mechanics I see when in the field, (construction equip, race cars etc) when they get a click stop then re click with a slight move even after the click, that?s probably saving them. I always thought it was a bad practice, but now I?m not so sure.
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  #22  
Old 06-27-2018, 05:05 PM
jrs14855 jrs14855 is offline
 
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Default Torque

How many hundreds of times on this forum have we heard the advice-get an A&P to help with that engine or with a pre buy. Not all A&P's are created equal and there are far to many that are not experienced enough or trained properly to remove and replace a cylinder. I would surmise that there are also too many that do not have access to a quality, properly calibrated torque wrench.
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  #23  
Old 06-27-2018, 06:27 PM
rackley16 rackley16 is offline
 
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Location: Oceanside, California
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I can say from years of working in Q.C. and having my A&P rating, all of my measuring tools and any shop measuring tools are calibrated at a regular intervals. Period. A torque wrench is a precision measuring device, so......If people are not calibrating them and keeping certs on file I don't see how they could pass an audit by the FAA or any customer. Ask your mechanic if he has calibration schedules and certs for calibrated tools. Ask if the IA that stamps it off has any idea of the calibration standards.

They should.

I don't see how they could look at themselves in the mirror after assembling an engine and not knowing, for sure, that every fastener that was supposed to be torqued, was in fact torqued properly.

That said, there are many "mechanics" in the world that are really just parts changers. That goes for all fields that have mechanics. Heavy equipment, automotive, aircraft, it's all the same. A good mechanic measures, installs, and tests using the proper practices and techniques. Wet torque is not a new thing to anyone that builds engines. Whether it is a diesel engine from Caterpillar, or a standard Lycoming.

Are there guys out there that are missing regular torque wrench calibration? Do they not take extra care of precision tools? Do some do improper torque technique?

I am sure of it. I personally don't know of one, but I am sure they are out there.

I can honestly say I am not the least bit afraid to R&R any or all cylinders without splitting the case. If you use proper technique and procedure, there is no reason to fear this. But. Is YOUR mechanic using proper technique and procedures?

And new fasteners on critical components and where applicable should not even be a question.
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  #24  
Old 06-27-2018, 08:47 PM
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RV6_flyer RV6_flyer is offline
 
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This was beat to death once before.

http://www.vansairforce.com/communit...er+replacement

I appear to be the only one that says a lot of caution must be used if more than one cylinder is removed from an engine. I have seen one four cylinder Lycoming engine that did NOT have a warped case and NO case fretting have one dowel pin rotate in the case. I believe that the bearing rotated when one or more cylinders was off the engine and the crankshaft rotated. Torquing the replacement cylinders pushed the bearing down on the dowel pin pushing it to one side.

I am the first one in this thread to agree with Mike Bush's warning about pulling a cylinder being potential harmful.

BTW, said engine that had the dowel pin rotate ended up with a crack in the case ~800 hours after 4-cylinders replaced. I am of the option that the 2,200 hour engine should have had a complete disassembly instead of just four cylinders replaced.

I respectfully disagree with all the experts saying pulling a cylinder cannot cause any damage. Pulling a cylinder or all cylinders can and have been done successfully but I caution anyone that does not have experience that there can be issues. I have seen the issues caused on one engine. IS the next engine that this issue is going to be yours when you pull the cylinders?
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  #25  
Old 06-28-2018, 05:53 AM
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This is crazy, I'd venture to say that easily 90% of engines have top ends replaced prior to TBO. If replacing cylinders routinely caused engine failure the FAA would be all over this and we'd have aircraft engines failing daily.
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  #26  
Old 06-28-2018, 05:55 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by RV6_flyer View Post
I am the first one in this thread to agree with Mike Bush's warning about pulling a cylinder being potential harmful.
The other thread dealt with crankshaft rotation while the case is unclamped, and its potential for bearing shift. Some say the chance of bearing shift is very limited, others don't, including Lycoming. Not the same discussion at all.

Regarding how to obtain sufficient stud tension upon reassembly, we all agree there is a potential for harm. Mike takes it a bit further, by arguing it is very difficult to obtain accurate stud preload, even for a diligent mechanic. It's not a silly argument. The mechanics who do the work successfully have a grasp of the two key underlying physical issues.

(1) If stud tension due to preload is higher than tension applied by the anticipated load, the studs are not subject to stress cycles. Cycles are key to fatigue.

(2) Static friction is always higher than dynamic friction. Put another way, it requires more torque to start the nut rotation from rest than it does to keep it moving. It is why final torque is to be applied in one smooth rotation, without stop and start.
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Last edited by DanH : 06-28-2018 at 07:26 AM.
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  #27  
Old 06-28-2018, 01:08 PM
isosceles isosceles is offline
 
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Quote:
The unofficial feeling around RAM is that anytime a cylinder is removed and replaced the odds of case cracking due to uneven through-bolt torquing increases dramatically.

https://www.aopa.org/news-and-media/all-news/2000/august/pilot/airframe-and-powerplant-(8)
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  #28  
Old 06-28-2018, 01:11 PM
isosceles isosceles is offline
 
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From https://mooneyspace.com [hearsay]

Quote:
Lycoming said they had put considerable research into this and could say that much of the cracking was due to improper torquing. They said one of the contributing factors was removing more than one cylinder and then retorquing them using the single cylinder torque sequence instead of the multi cylinder sequence. They said the multi cylinder sequence was developed by mounting strain sensors in areas prone to crack and developing a sequence that minimized strain in these areas.
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  #29  
Old 06-28-2018, 02:51 PM
lr172 lr172 is offline
 
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This article attributes the cracking associated with cylinder removal to improper torquing of the through bolts and studs upon reinstallation. Maybe Mike's article should have centered around finding a competent mechanic willing to follow proper torquing procedures instead of advocating a full overhaul, which can also be subject to cracking due to improper torquing of studs. I still struggle to see how field replacement makes it any harder to properly torque the studs, beyond the added difficulty of using torque plates in an installed engine.

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Last edited by lr172 : 06-28-2018 at 02:53 PM.
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  #30  
Old 06-28-2018, 03:25 PM
BillL BillL is offline
 
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Quote:
Originally Posted by DanH View Post
The other thread dealt with crankshaft rotation while the case is unclamped, and its potential for bearing shift. Some say the chance of bearing shift is very limited, others don't, including Lycoming. Not the same discussion at all.

Regarding how to obtain sufficient stud tension upon reassembly, we all agree there is a potential for harm. Mike takes it a bit further, by arguing it is very difficult to obtain accurate stud preload, even for a diligent mechanic. It's not a silly argument. The mechanics who do the work successfully have a grasp of the two key underlying physical issues.

(1) If stud tension due to preload is higher than tension applied by the anticipated load, the studs are not subject to stress cycles. Cycles are key to fatigue.

(2) Static friction is always higher than dynamic friction. Put another way, it requires more torque to start the nut rotation from rest than it does to keep it moving. It is why final torque is to be applied in one smooth rotation, without stop and start.
And to add, if not lubricated well, the applied stretch can vary by 20% plus or minus due to thread, nut and washer friction. Hardened washers are required.

Development of (production) torquing methods (and calibration) by my former employer went on for years. They made 50,000 engines a year with dozens of head bolts in each engine.
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