Boot cowl fastener philosophy: Rivet vs nutplate

[Mark Goldberg]

Zane, I have very rarely used the special nutplates. If I am using a dimpled skin into a normal nutplate, I will take the countersink cutter and run it a bit into the hole where the screw goes. It works fine. .025 sheet must be dimpled. .032 can be countersunk but it leaves minimal material. That changes if there is more than one sheet underneath. Like in the areas around your gas tank bay cover.

Caution about using many different sizes of screws - there is no problem using them except when you start pulling screws out it makes maintenance confusing sometimes if you (or the person helping you) doesn't realize there is a different size screw required for that one special hole. Keeping the same size screws does have advantages. Mark

[Zzz]

Caution about using many different sizes of screws - there is no problem using them except when you start pulling screws out it makes maintenance confusing sometimes if you (or the person helping you) doesn't realize there is a different size screw required for that one special hole. Keeping the same size screws does have advantages. Mark

That makes a lot of sense. What size have you guys used for engine cowling? My old 170 was 8-32 (actually shitty sheet metal screws, but about that size) but the CubCrafters aircraft I fly uses 6-32 all along the cowling.

6-32 seems adequate for boot cowling.

[Mark Goldberg]

I used all #6 screws. It has held up well. Mark

[Zzz]

The nutplate saga... Thanks for allowing me to think out loud.
I figure if all the boot cowl and engine cowl fasteners are to be countersunk...then I need to stock up. I have zero countersunk nutplates from my big hardware order.

K1100 is the nutplate with countersunk shape for the countersunk screw (AN509) but it has flat rivet holes in the wings. I learned you can just dimple these with the squeezer for a perfect fit behind dimpled sheet.

K1100-6 (6-32) for the bootcowl.
K1100-8 (8-32) for the engine cowl and tunnel and stuff that might routinely be removed for maintenance.

Two more dimple dies to buy now... #6 and #8 screws.

[svyolo]

I just finished up fitting all the bottom boot cowl panels, and took most of the panels back off to de-burr and fit all the nut plates. I thought I was going to countersink the firewall flange (dimple the skin), which would require I countersink it too much (using the nut plate as a guide). But I have a piece of .050 5052, so aI drilled it and tried dimpling it with my pneumtic squeezer. I thought generally you don't dimple anything thicker than .040, but the squeezer put a nice, crisp dimple on the panel.

Is there any structural reason not to dimple the firewall flange, which is .050, if my squeezer will do it? I was surprised, as my squeezer did a poor job dimpling 2 .025 pieces together. I ended up doing them separately the first, and then dimpled them together.

[Mark Goldberg]

John, the .050 (5052H32) is a much softer aluminum than the .025 wing skins (2024T3). You did good testing it on scrap to see how it behaved. Those boot cowl panels are just panels that cover the fuselage. Non structural. Not like wing skins which are structural. You should be fine. Mark

[svyolo]

I picked up a couple of sheets of .050 5052 last week. My aluminum welding is coming along and I wanted to try to make a small header tank. Before I countersunk the flange I figured I would give it a shot. It dimpled really well.

[davzLSA]

Hey guys there is a lot of good info here and I am about to need it myself but I wanted to ask if anyone has considered the weight penalty of using a lot of nut plates? I know they are absolutely necessary but not in all places. I think it might be a consideration worth mentioning.

[Mark Goldberg]

Nutplates are required where you will be removing panels for annual inspection. Like on the tunnel underneath. Mark

[svyolo]

I was about to start installing nut plates on the firewall flange for the boot cowl, but then thought about it for a while. It seems it might be a better idea to hold the that sheet metal together with clecos until the engine cowling is also fabricated so I can match drill it all at the same time.

Is that a better approach, at least for the firewall flange? Door formers and tunnel to sheet metal should be no problem to do now.

[Nev]

I was about to start installing nut plates on the firewall flange for the boot cowl, but then thought about it for a while. It seems it might be a better idea to hold the that sheet metal together with clecos until the engine cowling is also fabricated so I can match drill it all at the same time.

What did you end up doing ? Has anyone used #6 sized nutplates to attach the main cowling to the firewall ?

[Mark Goldberg]

That's what I have used Nev. Flat head #6 screws. Mark

[svyolo]

I haven't done it yet but I thought it called for #8s?

[Nev]

The reason I ask is that in my very OCD inventory spreadsheet it shows that I’ve got several hundred #6 nutplates in the workshop, and exactly 22 of the larger #8 nutplates. I’m sure I had a reason to order the 22 nutplates at the time, but now I have no idea where they were destined for.

[svyolo]

I ended up using more #8's than I thought, and stuff like the firewall and boot cowl, and a few other places, some corner and 1 lug #6's are also pretty handy. I try to keep some on hand so I don't have to wait on an order to finish something. An order for me is 3-5 days away.

And I am not on the bottom side of the equator.