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Thread: 3 of 8 studs sheared off???

  1. #1
    Guest
    Guest

    3 of 8 studs sheared off???

    Folks, especially you wrench turners, please read and comment. I'd
    sure appreciate it. Please bare with the history. I think it might
    be relevant.


    In the spring, I told my mechanic that my engine (IO-360, Beech
    Sierra) wasn't making peak RPM. He strobed the prop, and made some
    adjustments based on the strobe reading vs the actual RPM of the
    spinning prop. All seemed well enough and I kept flying.

    In the summer, while it was really hot, my engine, at idle was running
    at a very low RPM and seemed to be on the verge of stopping. My
    mechanic adjusted the fuel flow and that seemed to take care of the
    problem at idle; however, my engine was now idling at 900 RPM. I
    asked him to adjust that, which he didn't get around to. I kept
    flying because I thought the only problem was not one of not being
    able to reduce the prop rpm on landing necessitating a go around. I
    had no concerns for the engine's health. That was a few weeks ago.

    This weekend, I put 3.6 on the airplane. In the first 1.5 hours, I
    noted that the EGT seemed to be behaving a little funny. I'd lean and
    see temperature fluctuations between 1400 and 1587 almost instantly.
    The normal behavior for my EGT is to trend in a direction; but, never
    to have wild fluctuations like that. I also know that when I"m
    between 6 and 9 thousand feet, my EGT will read 1415F when my engine
    is running at 100 ROP which is where I run my engine. To achieve
    that, I was needing to run the engine at 13 GPH fuel flow which is 2
    GPH higher than normal. I thought I was dealing with a sensor
    problem, leaned to 11 GPH, the EGT finally settled in at 1485 and I
    was still ROP of peak by maybe 30 degrees. I thought I would get the
    problem looked at when I got back to my home field. The flight back
    was a 2 hour flight and the EGT behaved normally, fuel flows were as
    expected. Nonetheless, I asked my mechanic to look into it.

    The number 3 jug is where my EGT probe is and my mechanic found 3 of 8
    studs sheared off. He indicated that the engine shop recommends a
    tear down at 3500 to investigate because it would be scary to just
    replace the studs. If I understand right, they indicated their
    concern is that the backplate could be cracked....and might be the
    cause???

    I authorized the tear down. Do you think that the events over the
    spring and summer could have been early evidence of this? Can you
    help me understand what would cause 3 of 8 studs to shear off? Do you
    agree with the engine shops recommendation for a tear down? Any
    thoughts you have would be most appreciated.




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  2. #2

    3 of 8 studs sheared off???

    >The number 3 jug is where my EGT probe is and my mechanic found 3 of 8
    >studs sheared off. He indicated that the engine shop recommends a
    >tear down at 3500 to investigate because it would be scary to just
    >replace the studs. If I understand right, they indicated their
    >concern is that the backplate could be cracked....and might be the
    >cause???
    >
    >I authorized the tear down. Do you think that the events over the
    >spring and summer could have been early evidence of this? Can you
    >help me understand what would cause 3 of 8 studs to shear off? Do you
    >agree with the engine shops recommendation for a tear down? Any
    >thoughts you have would be most appreciated.

    There are 8 nuts on the cylinder base. 4 big ones and 4 little ones.

    2 of the big ones hold the engine CASE together and are threaded on what
    are called "through studs", as in they go all the way through the case to
    the other side.

    The other 2 big ones are just threaded into the case about 1" deep.

    All 4 little ones are just threaded into the case flange and are there to
    keep the base sealed and not distorting.

    Depending on which 3 studs you have left, you most certainly could need a
    complete teardown and inspection. Should EITHER of the 2 big through studs
    be untorqued or broken, the crankshaft bearings could move in the crankcase
    bore. That could mean instant failure should the oil passages not line
    up. This is something that Lycoming warns about in their service literature.

    Even in the case of the 2 through studs being good, I'd want to tear it
    down and have the case inspected. You may have cracks from the stress,
    etc. Most certainly ALL the case hardware needs to be tossed and new parts
    installed.

    As for the EGT/Fuel Flow issues, I can't say that a loose cylinder is going
    to make the combustion process change.

    I do recall a Piper Arrow run by one of the local check flying operations a
    number of years ago that had a cylinder COME OFF in flight. Punched a
    really nasty hole through the side of the cowl and the piston and
    connecting rod beat the rest of the case up as it flopped around at 2500
    RPM. It was impressive.

    While the engine is off, why not have your injection system bench
    checked. A blocked or restricted fuel nozzle will cause the fuel flow
    meter (actually a pressure gauge) to read high because of the reduced
    orifice size. So possibly the cylinder moving caused the injection line to
    be pinched?

    Glad you made in down safely. It COULD have been your most exciting
    flight, ever.

    Bob Steward, A&P IA
    Lycoming Alumnus, Class of '99
    Birmingham, AL



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  3. #3
    Guest
    Guest

    3 of 8 studs sheared off???

    Bob,

    I completely appreciate your response and will take that route. I'm
    curious what would make 3 studs depart the aircraft though? Do I need
    to look deeper for a cause? Is it possible/likely that I could install
    the newly refurbished engine only to have the same thing happen again?
    Is it possible to detect things like this before they happen? When you
    say case hardware, I imagine you're referring to the studs and not the
    pistons, rings, rods, crank, etc., unless indicated, correct?

    Thank you very much for your response,

    RJF


    Bob Steward wrote:
    >
    >
    > >The number 3 jug is where my EGT probe is and my mechanic found 3 of 8
    > >studs sheared off. He indicated that the engine shop recommends a
    > >tear down at 3500 to investigate because it would be scary to just
    > >replace the studs. If I understand right, they indicated their
    > >concern is that the backplate could be cracked....and might be the
    > >cause???
    > >
    > >I authorized the tear down. Do you think that the events over the
    > >spring and summer could have been early evidence of this? Can you
    > >help me understand what would cause 3 of 8 studs to shear off? Do you
    > >agree with the engine shops recommendation for a tear down? Any
    > >thoughts you have would be most appreciated.
    >
    > There are 8 nuts on the cylinder base. 4 big ones and 4 little ones.
    >
    > 2 of the big ones hold the engine CASE together and are threaded on what
    > are called "through studs", as in they go all the way through the case to
    > the other side.
    >
    > The other 2 big ones are just threaded into the case about 1" deep.
    >
    > All 4 little ones are just threaded into the case flange and are there to
    > keep the base sealed and not distorting.
    >
    > Depending on which 3 studs you have left, you most certainly could need a
    > complete teardown and inspection. Should EITHER of the 2 big through
    > studs
    > be untorqued or broken, the crankshaft bearings could move in the
    > crankcase
    > bore. That could mean instant failure should the oil passages not line
    > up. This is something that Lycoming warns about in their service
    > literature.
    >
    > Even in the case of the 2 through studs being good, I'd want to tear it
    > down and have the case inspected. You may have cracks from the stress,
    > etc. Most certainly ALL the case hardware needs to be tossed and new
    > parts
    > installed.
    >
    > As for the EGT/Fuel Flow issues, I can't say that a loose cylinder is
    > going
    > to make the combustion process change.
    >
    > I do recall a Piper Arrow run by one of the local check flying
    > operations a
    > number of years ago that had a cylinder COME OFF in flight. Punched a
    > really nasty hole through the side of the cowl and the piston and
    > connecting rod beat the rest of the case up as it flopped around at 2500
    > RPM. It was impressive.
    >
    > While the engine is off, why not have your injection system bench
    > checked. A blocked or restricted fuel nozzle will cause the fuel flow
    > meter (actually a pressure gauge) to read high because of the reduced
    > orifice size. So possibly the cylinder moving caused the injection
    > line to
    > be pinched?
    >
    > Glad you made in down safely. It COULD have been your most exciting
    > flight, ever.
    >
    > Bob Steward, A&P IA
    > Lycoming Alumnus, Class of '99
    > Birmingham, AL
    >
    >


    [Non-text portions of this message have been removed]



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  4. #4
    Guest
    Guest

    3 of 8 studs sheared off???

    This sounds somewhat similar to an incident I had about 1.5 years ago with an O-320. The major difference is I experienced a dramatic loss of power while in flight and the engine ran rough. My IA discovered cylinder #1 (or was it #2 - I don't remember) was attached by fewer bolts that required, even though a compression check on all cylinders appearred normal. Upon further inspection, the piston was burned and the cylinder was toast.

    In my case, Don found a hole in the intake manifold that led to increased cylinder head temperatures and eventually predetonation of the fuel. It was the predetonation that snapped the bolts.

    In 20/20 hindsight I could have avoided this mishap with:
    a.) more careful pre-flight with more attention to the integrity of the intake manifold, and
    2.) more careful monitoring of the EGT and CHT. (You can read this as an advertisement for an 8-probe system if you wish!) That cylinder had to have been running hotter than the other three for some time and I missed it.

    I'd take a good hard look at all the intake manifolds and certainly get that jug and piston off and have them checked. And have your A&P contact Bob Steward for information so that he can order the special stud puller, if he doesn't already have one. You are going to need it. Keep your fingers crossed when you try to remove the old through studs. Even the studs that didn't break should be replaced, and if one of those through studs snaps ... Good Luck!!!

    Paul Edwards
    2388Z still at ERI



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  5. #5
    Guest
    Guest

    3 of 8 studs sheared off???

    I got my first look at the wounded engine. My mechanic was mistaken.
    It was 4 bolts sheared off (all the smaller internal ones) and 2 of the
    larger outside bolts were loose. I think a jug was getting ready to try
    it's hand at flying without the benefit of the rest of the airplane.

    Cost:

    $1300 for engine removal and replacement
    $3500 to crack the case open, send out the case halves and crank to have
    them inspected and reassemble with all the same parts.

    Along the way, My mechanic found the rim gear was horribly worn and the
    Bendix on the starter needs another overhaul (last done 11/05).
    Warranty is out on the overhaul (6 months)

    $700.

    So far, I'm at $5500 and that is if everything inside the engine is
    fine. I'm at 1611hrs on a 2000hr engine. Tomorrow will be telling.
    The engine shop will have cracked open the case and be able to provide
    some thoughts on the health of the bottom half.

    Wish me luck!!!





    Paul Edwards and Sheril Kolenda wrote:

    > This sounds somewhat similar to an incident I had about 1.5 years ago
    > with an O-320. The major difference is I experienced a dramatic loss
    > of power while in flight and the engine ran rough. My IA discovered
    > cylinder #1 (or was it #2 - I don't remember) was attached by fewer
    > bolts that required, even though a compression check on all cylinders
    > appearred normal. Upon further inspection, the piston was burned and
    > the cylinder was toast.
    >
    > In my case, Don found a hole in the intake manifold that led to
    > increased cylinder head temperatures and eventually predetonation of
    > the fuel. It was the predetonation that snapped the bolts.
    >
    > In 20/20 hindsight I could have avoided this mishap with:
    > a.) more careful pre-flight with more attention to the integrity of
    > the intake manifold, and
    > 2.) more careful monitoring of the EGT and CHT. (You can read this as
    > an advertisement for an 8-probe system if you wish!) That cylinder had
    > to have been running hotter than the other three for some time and I
    > missed it.
    >
    > I'd take a good hard look at all the intake manifolds and certainly
    > get that jug and piston off and have them checked. And have your A&P
    > contact Bob Steward for information so that he can order the special
    > stud puller, if he doesn't already have one. You are going to need it.
    > Keep your fingers crossed when you try to remove the old through
    > studs. Even the studs that didn't break should be replaced, and if one
    > of those through studs snaps ... Good Luck!!!
    >
    > Paul Edwards
    > 2388Z still at ERI
    >
    >




    [Non-text portions of this message have been removed]



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