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Brad Wilkinson, Bearhawk #1062
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Date: 10-4-2009
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Number of Hours: 3.00
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Manual Reference:
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Brief Description: Boot Cowl, Left Side - Firewall attachment
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Installed countersunk nutplates for boot cowl attachment at firewall.
Because the engine cowling will fit over the boot cowling (or fuselage cowling), the boot cowling's attachment to the firewall has to be made with flush rivets and screws. Since my boot cowling will be removeable (I don't know why - I just like it that way - it could be permanently riveted in place), I am using #8 countersunk nutplates (or 'Dimpled Anchor Nuts' - K1100-08) and #8 stainless flush (100deg countersunk) head machine screws (MS24693C8) to accomplish this. These nutplates are spread somewhat evenly around the firewall flange at about a 3" spacing.
The bolt holes are dimpled, and the boot cowl skins are too, so they nest (provided you did your match-drilling correctly). I used a dimple-die set for #8 holes in my TATCO stainless steel rivet squeezer (a fine piece of equipment that I really like using, and it makes me happy with my purchase everytime I pick it up - made in USA! by Townend Aerospace Tool Co). The holes were drilled to 11/64" before dimpling. I found by experimenting on scrap before dimpling the skin that the hole will fracture around the edge if it is too small. Although the dimpling die's nipple will go through a 5/32" hole (it will center on and squeeze through a 1/8" hole), when it stretches the sheet for the dimple the sheet will tear around the edges of the hole. So I found that the dimple works best with the slightly larger hole. Since the machine screw is going to find the center of the nutplate, this gives a little tolerance on the holes, and there is still plenty of material for the screw to hold onto.
The nutplates are riveted in with 3/32" flush rivets, which have to be in countersunk holes as well. Lots of drilling and riveting - but a lot less than the RV guys! :-))
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Dimple-die for rivet squeezer
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Dimpled and countersunk
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Attached C'sink Nutplate
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