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Thread: Spar Mounting Doubler Holes

  1. #1

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    Default Spar Mounting Doubler Holes

    I have noticed that the holes in the spars once you remove the doubler's measure .375 instead of .3125, which is the size of the fastener. My experience with heavier air frame structures has me wanting to bush all of these holes. Can anyone tell me why these mounting areas had such a large holes? This just does not seem right.

    Cheers.Spar Mounting Holes.jpg

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    Administrator Steve Pierce's Avatar
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    Default Re: Spar Mounting Doubler Holes

    Isn't that the hole that gets a bushingand the bolt for the jury strut?

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    Default Re: Spar Mounting Doubler Holes

    All of my wing attach holes have .375 diameter holes. .3125 diameter fasteners go through these holes. I am extending the spars so the new strut attach holes will be drilled and reamed for the .3125 fasteners.

  4. #4
    Southern Aero's Avatar
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    Default Re: Spar Mounting Doubler Holes

    Quote Originally Posted by chicobushmaster View Post
    I have noticed that the holes in the spars once you remove the doubler's measure .375 instead of .3125, which is the size of the fastener. My experience with heavier air frame structures has me wanting to bush all of these holes. Can anyone tell me why these mounting areas had such a large holes? This just does not seem right.

    Cheers.Spar Mounting Holes.jpg
    I'm a little confused........... All the holes in this pic are .375 holes...... and also the strut attach fittings (for 3/8 bolts). The attach fitting at the end of this spar in the pic has a .375 D bushing that has an I.D. of .312 for the wing attach bolt. The Bushmaster spars splice and extend outboard the strut attach point, but all the splicing is done with rivets.

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    Default Re: Spar Mounting Doubler Holes

    This has nothing to do with the splice on the opposite end. All of the fasteners that go through the end of the fuselage spar attach plates are .3125 diameter. These holes that are shown in the picture of the root end of the spar all measure .375 in diameter. I have seen the attach hole, located at the top, with a bushing before. However the remaining holes in the spar root are .375 in diameter. My inclination is to bush the remaining holes to support the installation of the .3125 diameter ID X .375 OD bushings, and .3125 fasteners through the spar root attach doubler fittings. I have seen this in other spars as well. What I am asking is why are the spars this way. Ideally, I would like to install press in Hiloks. My experience in the air frame modification and design business is that you would never put a .3125 fastener into a .375 diameter hole in any primary structure.

  6. #6
    Southern Aero's Avatar
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    Default Re: Spar Mounting Doubler Holes

    The bolts you reference are 3/8 D (AN6-15A) not 5/16 D except where the compression strut might mount which is a bushed 3/8 hole with a 5/16 bolt. Check Piper drawing 10710. I think that is the rear spar.................. but only looks like 3 holes. ???

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    Southern Aero's Avatar
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    Default Re: Spar Mounting Doubler Holes

    Chico

    Looks like the 4th hole is hiding behind the spar cap in the pic ................. A little clarification from post #4. The strut fittings in the spar do use 5/16D bolts in the drawings. I went to 3/8D in mine plus one hole for gross weight increase. I believe the 3/8 bolts in the root fittings is for shear loads in the web .......... bushed 5/16 should work too

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    Default Re: Spar Mounting Doubler Holes

    Thank you so much. This makes sense now.

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