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Thread: Broken bottom engine mount bolt on 912ULS Vixen

  1. #11
    Senior Member Dave S's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    Strange marking. I have never seen this type of identification on an aircraft bolt that I recall. However, I believe there is a custom bolt/hardware manufacture called LFC inductries in Arlington TX; and, the "X" is somewhat common as a manufacturer's mark for a lot of bolt manufacturers - what it means is probably coded within the companies literature. It may be intended for aircraft use but if it is custom stuff - who knows and probably hard to dig up. I have some recollection that LFC provided some bad bolts for Hartzell for prop counter weights which is probably memorialized in the FAA's ADs or service bulletins.

    No idea if that helps any with what should be correct; but it might be worthy of a call to Kitfox to see if they have some special bolt for that particular location.

    I am going to be doing my condition inspection pretty soon and will certainly check to see what head markings are on the bolt in our plane.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  2. #12
    Senior Member PapuaPilot's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    I was waiting to see the pictures wondering if the bolt was too long and somebody "shanked" the nut (i. e. tightened the nut to the bolt shank), but that is not the case.

    Shanking a bolt can also happen if we don't use enough washers. The problem is the nut feels like it tightened up, but it is still loose because the nut stopped turning at the shank. Cyclic loads will eventually cause the bolt to break.

    IMO that bolt should be suspected as a bogus aircraft part. It is not an AN bolt:
    1. Unfortunately the markings on the head are correct. Common AN bolts are identified with an X on the head, plus the manufacture's stamp/letters.
    2. What's wrong is the thickness of the head and the amount of threads that are cut on the bolt. Compare it to the new bolt in the picture. It has the correct amount of threads and has a thinner (proper) head.

    I don't see evidence that somebody cut more threads on the bolt. DON'T ever do that to an aircraft bolt, just get the correct length. Most likely it failed due to the repeated stresses that occurred on the treaded part of the bolt where it sheared. A proper bolt will have the shank go entirely through the engine mount and fuselage, and the threaded end will only have washers and the nut on it. The other problem with the threaded part of a bolt is that it is dimensionally smaller then the shank. It doesn't fill the hole and will quickly enlarge the hole.

    Hopefully the new bolt will fix the problem and there isn't any secondary wear in the engine mount and fuselage.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  3. #13
    Senior Member jrevens's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    I wonder if that is possibly a prop bolt (AN76?). They generally have a longer threaded length.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
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  4. #14
    SWeidemann's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    My conclusion is the bolt that broke was not the right bolt for this application. I do not know why this one was used, and hopefully the right one (the one I put in last Friday, torqued and safety wired today) will do the job, although I intend to carefully inspect this part frequently. BTW I ordered the replacement AN bolt by the alpha numeric part designation on the Kitfox engine mount install spec sheet, from Pegasus Racing (a nearby supplier I have used for various parts, including Camloc pieces for the cowl).

    Removing the broken threaded bolt from the engine mount turned out to be a blessing, in that I was able to remove it without totally removing the engine. By using an engine hoist, I loosened the 4 other mounting bolts and moved the engine far enough forward to make space to insert a right angle pick & persistently hammer on the side of the bolt to rotate in the “out” direction, occasionally viewing my project with a small dental mirror, aided by some Mouse Milk (a recommended concoction of what I gather is a solvent, mixed with penetrating oil). I’m a happy man.

    Skot
    Last edited by SWeidemann; 10-11-2021 at 07:43 PM.
    Kitfox Vixen 912 ULS
    N24V at C29

  5. #15
    SWeidemann's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    As a follow up to my follow up, I have discovered that the Vixen engine mount tubing is different from the later model nose wheel equipped Kitfoxes. Through FB Kitfox folks, another Vixen owner has had a failure of the nose shock strut and a collapse of the bottom center engine mount pad (the top, receiving end, support of the rubber stacked shock piston). He provided pictures of the end result, as well as a photo of a different (later model...I assume) engine mount which has an added vertical engine mount tube that obviously has taken up much of the vertical shock load from the shock piston. That (new..different than ours) tube goes right through the space where we currently have a muffler. I assume the later models have the muffler further towards the front. At any rate, I am now convinced that my broken bolt was a result of frequent bending loads of the shock piston pad, causing bolt fatigue from bending. The best solution (I think) would be to completely convert the front FWF to the later model engine mount, muffler locations, and everything else to acquire that beefed up engine mount. In the real world (my world) I am planning on frequent inspection and replacement of the all-important bottom center engine mount bolt.

    Skot
    Kitfox Vixen 912 ULS
    N24V at C29

  6. #16
    Administrator DesertFox4's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    That is good information to archive here Skot. If you have those photos please post here as they will soon disappear off of facebook. Might be very helpful to those flying the same model Vixens.
    A scheduled inspection sounds good along with good nose wheel landing methods, which I’m sure you already practice👍.
    Thanks for the update.


    DesertFox4
    Admin.
    7 Super Sport
    912 ULS Tri-gear


  7. #17
    SWeidemann's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    1156B3FF-2C31-4606-A924-FD6F0D1F0EA0.jpg2A5AE18D-BAAB-4F9B-9DE0-6C5914BEA03E.jpeg96291296-95F3-4F3D-8E9B-1721DA384418.jpeg
    Quote Originally Posted by DesertFox4 View Post
    That is good information to archive here Skot. If you have those photos please post here as they will soon disappear off of facebook. Might be very helpful to those flying the same model Vixens.
    A scheduled inspection sounds good along with good nose wheel landing methods, which I’m sure you already practice👍.
    Thanks for the update.
    Kitfox Vixen 912 ULS
    N24V at C29

  8. #18
    Senior Member Dave S's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    Skot,

    Appreciate you describing these events and differences for the benefit of other nose wheel drivers.

    FWIW, the photo of the newer (post vixen) engine mount is exactly what our engine mount looks like on our early S7 - it has the diagonal tube running up to the starboard upper corner.

    I know there have been a couple service bulletins on the nose gear - one probably not pertinent to this experience as it had to do with the internal structure of the early nose gear legs which were apparently vulnerable to side load cracking with heavy engines installed - not so much with lighter rotax engines.

    The other Sb had to do with the correct fit of the elastomere piston rod through the bearing in the pad at the bottom of the mount. The basis for this SB is where builders may not have verified the fit resulting in binding of the piston rod under compression loads. Your comment causes me to consider that with the earlier engine mount without the extra brace, any binding of the piston rod could result in deflection of the pad due to the bolt or lack of additional brace and further twisting & stressing the parts.

    Anyway, thanks for the information you have provided.
    Last edited by Dave S; 12-09-2021 at 04:23 PM.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  9. #19
    SWeidemann's Avatar
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    Default Re: Broken bottom engine mount bolt on 912ULS Vixen

    Dave, Good point about the possibility of the shock piston binding under actual landing & takeoff conditions (not just in static conditions). I suspect there is more movement of those engine mount tubes when we expect under running conditions. Regardless, the design of the Vixen engine mount is a good reason to practice keeping weight & stress off the nose gear.

    Skot
    Kitfox Vixen 912 ULS
    N24V at C29

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