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Fuel tank covers and inspection covers screw lenght

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  • #16
    I use stainless fasteners almost ever day and also break loose by hand then use power tools to remove and reinstall, I only use known good fasteners and will verify the thread quality with a high quality tap and die set when we change suppliers.when possible source all of your fasteners from a single source reputable manufacturer, as they will use a thread standard across their product line. If I am noting -any- drag or resistance on screws I will run each one of them through a die to clean up the major diameter and thread pitch. Two things I always have on my bench are a stick of door-ease type stick lube ( regular bees wax works well also ), and loctite 242 ( blue ) which actually helps lube the threads when installing screws.
    https://www.dkhardware.com/tube-wax-...SABEgLS1_D_BwE
    Last edited by quadra; 05-17-2019, 01:40 AM.

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    • #17
      John - those screws in your hardware package should have been countersunk/flush head screws. I would return them. Around the tank bay cover, this is a place for some countersinking as the thickness of the aluminum there (skins + rib flange or spar flange) make dimpling not easy or impossible. The tank cover itself can be dimpled. Mark

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      • svyolo
        svyolo commented
        Editing a comment
        Thanks Mark. That is what I was probably going to do anyway. Just wondering if there was some preference for not dimpling/countersinking the covers.

        When I get close to being done with this thing a vender is definitely going to get a decent sized return package.

    • #18
      I was trying a few combinations of dimple/countersink that I am going to have to do. The tank covers are .032, #6 flush screws. The skin is .025 and the aft spar is .032. To get the #6 dimple to sit in the countersink, requires countersinking too deep, and the hole is enlarged. This is permissible?

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      • #19
        You just have to C/S deep enough for the rivet head (or screw head) to be flush. Mark

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        • svyolo
          svyolo commented
          Editing a comment
          If I dimple the cover, the screw head will always be flush. But the dimple has to sit down in the countersunk skin/spar flange. Or do I C/S the cover/skin/spar flange combination? I I read that was not really acceptable. Would it be acceptable because it is just a tank cover, and not primary structure?

        • jaredyates
          jaredyates commented
          Editing a comment
          The counter sink restrictions are in cases where the fastener could pull through the hole. The intent is to provide a shoulder of material to prevent this. But in this case, the screw is the primary fastener, and it is subject to the dimple. The over-countersunk hole it's just one inner layer in the sandwich; don't over-counter sink the bread, but in this case we are dealing with cheese.

      • #20
        Would you like to dimple it those holes?
        Aircraft Tool Supply has a dimple die block set that accepts any dimple die from a rivet squeezer. I wonder about fabricating this tool for myself.

        Shop aircraft-tool.com for all of your aviation and aerospace maintenance tools and test equipment. We have the best prices and outstanding customer service.
        Brooks Cone
        Southeast Michigan
        Patrol #303, Kit build

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        • svyolo
          svyolo commented
          Editing a comment
          I have one but the block is huge. I don't think I can get any combination of dimples to work. I think 3 layers totaling .090 thick is to heavy to dimple together. The #6 screw dimple requires too deep of a countersink for .057 thickness. Unless doing so is acceptable for this application.

      • #21
        The buck is very simple. The .401 shank is a little more effort. I also would like the .401 driver heat treated. It will probably cost $30.00 with shipping. Look for the driver alone and make a buck to fit your needs. I have this tool but I usually put the dimple dies in my squeezer and complete the dimples that way unless I am in the middle of a field. I have made several different shaped bucks to hold my dies.

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        • #22
          You can dimple the holes but, at the moment, you can't lay your hands on dimpled #6 nutplates - plus they are listed at over $1 each......

          032 + 025 should be enough to take a #6 C/S without knife-edging the hole. In any case, the best way to do it is to wait until you have fitted the nutplates. Then use a #40 C/S bit - the guide pin fits nicely into the nutplate and keeps the cutter centred even if the hole gets slightly enlarged.

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          • Bcone1381
            Bcone1381 commented
            Editing a comment
            Paul; I ordered my nut plates in February. They say they'll arrive next month before Oshkosh.

          • svyolo
            svyolo commented
            Editing a comment
            I think I saw a video where the guy mentioned using the nut plate as a countersink guide. I kind of had a feeling that is what he was referring too.
            .057 might take a #6 CS, but it won't take a #6 dimple. I tried a couple of times to get the dimple to fit, but it wouldn't sit flush without blowing out the bottom of the CS hole and chattering the CS bit.

        • #23
          Originally posted by PaulSA View Post
          You can dimple the holes but, at the moment, you can't lay your hands on dimpled #6 nutplates - plus they are listed at over $1 each......

          032 + 025 should be enough to take a #6 C/S without knife-edging the hole. In any case, the best way to do it is to wait until you have fitted the nutplates. Then use a #40 C/S bit - the guide pin fits nicely into the nutplate and keeps the cutter centred even if the hole gets slightly enlarged.
          Regular nut plates work fine on countersunk holes. I used this solution everywhere. You can machine countersink nut plates a tiny amount, but you have to be careful not to weaken them too much around the barrel, so it's not always advisable.
          .
          At the end of the day, you are just holding the nut in place, it's not a highly stressed part. If there is a dimple underneath, it doesn't effect it much.

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          • #24
            Did some searching on VAF in the Building Techniques section on dimpling, CS, and how dimples sit in CS holes. I guess I have some reading to do. And mounting nut plates, including the use of Oops rivets to mount them with only the tiniest of CS in metal as thin as .020. Never heard of that.

            I was going to take a couple of weeks off building to fix a couple of mechanical problems with - me. I guess I get to study up.

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            • #25
              Please report back here with what you find. Those ops rivets are called NAS1097 rivets. Wicks and Vans sell them. I countersink a NAS1097 rivet by hand with my deburring tool.
              Brooks Cone
              Southeast Michigan
              Patrol #303, Kit build

              Comment


              • James
                James commented
                Editing a comment
                I'm not a sheety, but my understanding is NAS1097 rivets are used in shear applications - with the smaller head, they won't have the same tensile strength as a AN426 type rivet, pound-for-pound.

                That said, my mate reckons you can put the odd one in here and there in tensile applications, if you need to go up a size - e.g. the hole's too big for 3/32, so you drill it out to 1/8th with the same size head in the countersunk hole. There's still lots of meat holding the skins together.

              • Bcone1381
                Bcone1381 commented
                Editing a comment
                I agree. No tension forces are placed on these small rivets when they hold down a nut plate. A little shear is experience when tightening down the machine screw into the nut plate.

            • #26
              Apparently it is not unusual practice to use 3/32 NAS1097 to install nut plates. Even countersinking .025 (and someone claimed .020) Very little head to it.
              I am not advocating it.

              Getting started with this project I had to learn aircraft standards, I thought. Instead I have to learn aircraft "gray area", and different opinions. Interesting, but also daunting as that means different DAR's also have different opinions.

              I did see some great info on countersinking to accept a dimple above it. Some advocated softening the edge of the CS, as the dimple is slightly curved and the hard edge will cause you to over CS the hole.

              The best info might have been something Vans did everywhere on the 14. If he had to CS to accept a dimple, the kits are to CS to take the fastener flush, and then 7 more clicks on the countersink cage. It is claimed you can see a bit of light, but when you drive the rivet or tighten the screw, it cinches up tight. I haven't tried it. But I will when i get back.

              I wouldn't mind posting a couple of links to those threads, but I am not sure if Jared wants that on this forum. If he says its OK, I will post them.

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              • jaredyates
                jaredyates commented
                Editing a comment
                Fine with me, as long as we are posting links to their content and not copying/pasting from their forum onto ours.

              • svyolo
                svyolo commented
                Editing a comment
                OK, thanks.

            • #27
              I started putting BoeLube on my stainless fasteners on my Citabria and no longer have a problem with galling.

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              • #28
                I had problems with SS hardware galling on my Citabria but I started putting BoeLube on the threads and the problem went away.

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