Boot cowl fastener philosophy: Rivet vs nutplate

[Zzz]

I recall that this was somewhat of a debate, but in starting the boot cowl on my quickbuild kit, I see the potential to use rivets for a permanent fastener, or nutplates to allow disassembly.

I tend toward flexible solutions, so even though the nutplates are more work, should I use them for EVERY fastener in the boot cowl? Do any of the joints just work better as rivets alone?

I'm sort of paralyzed by planning and understanding of this phase and I'm terribly afraid to drill any unnecessary holes in my nice firewall flange.I also HATE drilling out rivets.

I imagine that most of these parts will be painted on my bird and that will necessitate removal of the panel for the painting process. I really don't like spraying assemblies as paint gums up screws and coverage is almost always not as good.

Current progress:

Stainless tunnel clamped in place. I'm ready to poke some holes in the bottom flange of the firewall, but unsure if I should use rivets alone or nutplates for that joint. I stuffed a 1/4" thick piece of wood between the firewall flange and the fuselage tubing to block it for drilling.





The tunnel fit mostly perfect out of the box from B.A. I only had to trim the forward flange edge to prevent it overlapping the firewall flange when it's turned upward.



I'm obvious a newbie at this and sheet metal work in general. Any tips or pointers are welcome.

[aerolite]

I will be riveting the tunnel to the firewall flange. I see no reason to remove it when the plane is done.
I make the side boot cowls removable though. They attach at the bottom with nutplates along the sides of the tunnel. The tunnel is held at the back with screws, along with the belly pan aft of it.
I also rivet the top boot cowl to the firewall, and the aft part is riveted to an aluminum angle which the top of the instrument panel gets screwed to. Again, I don't see the need to ever remove the top boot cowl unless something got damaged. At that point drilling out the few rivets along there would probably be the least of my worries. With the side's removable and the panel easy to remove, I can get to anything behind the panel quite easily. Just don't rivet anything until you absolutely have too, as during the building process you will likely want, or need, to remove the part to do something else.
I don't have time right now, but I can take some pics for you of how I've done it and post them.

[jaredyates]

Steve, will you be able to buck the rivets at the bottom of the firewall/front of tunnel junction, or will you use some sort of blind rivet? I used #8 screws along the front, and found barely enough room for nutplates and a thread or two of the screw sticking through.

[Zzz]

Removal of the firewall at this point is pretty simple. It's just cleco'd on. I would likely drill the bottom flange in place then cleco. Later when everything else has been drilled and cleco'd, come back, remove firewall and the tunnel, drill the tunnel for nutplates and rivet them on, then move back into place and fasten with short #8's like Jared suggested. There is about 5/16" between the top of the bottom firewall flange and the bottom tube of station A.

However, I also see the point about the practicality of disassembling this structure in the future, there just isn't much point; there's nothing to access that isn't accessed more easily by removing the floorboards. It just seems weird to have one end of the tunnel riveted and one end screwed.

[Battson]

I don't like removing those floorboards once you've got a few things completed and attached to them / overlaps. Things like the flap handle, carpet, and fuel selector get in the way, so I go through the belly every time.

I used nutplates and screws in every single location, and there's not one place where I have regretted it. I have opened every single panel and part at some point, either during a re-design or for a repair / maintenance.

There ends up being lines and wires running all over the show, and I like having all the design flexibility that ample access allows me.

If you go this way, just be sure to use flush screw heads, countersunk or dimpled as necessary. You never know where you might need to overlap some other panel for a change in design or to deal with a pesky issue. And be careful where you put the screws on the firewall flange, as this somewhat dictates where your cowl doors can and cannot go.

[Zzz]

What's a good rule of thumb for fastener spacing/density?

[Mark Moyle]

These are 3-3/8" counter sunk. The Pacer uses both counter sunk and flush nut plates to secure the bottom cowl and center top cowl .


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[Mark Moyle]

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[Mark Goldberg]

Zane, I like to be able to remove the tunnel. So it is installed on my planes with nutplates. I think there are a couple sheet metal screws also. It all depends on what you want removable for inspections or accessing things. Many builders do not think in the future about accessing areas for maintenance. My boot cowl is a combination of screws/nuts and nutplates. I don't figure I'll be removing the entire boot cowl. If I needed to I could get a hand from someone else to get a wrench on nuts in some spots.

Spacing - it is not a bad idea to walk around and look at certified planes on the ramp. I would say in general 4-6" is plenty close spacing in most cases. Mark

[aerolite]

Steve, will you be able to buck the rivets at the bottom of the firewall/front of tunnel junction, or will you use some sort of blind rivet? I used #8 screws along the front, and found barely enough room for nutplates and a thread or two of the screw sticking through.

Will use SS blind rivets. The floor comes off quite easily with the flap handle up and fuel selector is not in that area. I did however screw the tunnel in place on the Patrol I did so I still might change my mind yet

[jaredyates]

I've come across one good reason for tunnel removal, which is to have a solid place to set the fuselage when there is need to remove the landing gear.

[Zzz]

Thanks guys. Nutplates it is.

[Zzz]

Hey all, I'm doing some drilling tonight. I've sunk a few holes in the lower flange of my firewall and cleco'd it to the SS tunnel. I'm just drilling the center holes with a #30 for now of course. On the lower flange of the firewall, I plan to use #8 screws (#6 everywhere else), for which I currently have no nutplates (I only have #6) so I went shopping. I'm embarrassed to say I didn't realize (or had just never thought about it) that nutplates come in 2 basic varieties: MS21047 / K1000 (flat) and MS21049 / K1100 (countersunk.)

I assume the countersunk versions are meant to be used with a dimpled sheet? That begs the next question: On your boot cowl fastening, what did you do for screws? Countersunk or trusshead/buttonhead? Seems like anything in the airstream should be countersunk (even though there wasn't a single flush screw or rivet on my old 170 cowling ) but it would be nice I guess to have the glareshield smooth.

.032 and .025 must be dimpled, right? Too thin to countersink with a bit. That dimpled offset has to go somewhere... into the countersunk nutplate?

/n00b

[Battson]

You can use either nut plate if needs be, just be careful not to drastically weaken the nut plate when reaming material off it.

Look at the clearance for the nutplate and screw above that bottom flange of the firewall. There can be a clearance issue there - larger size nut plates may not be a good choice, I used the smaller size on the bottom flange, and most everywhere else EXCEPT supporting the main cowl to the firewall on the main flange. Of course the screws which hold the main cowl in place, can be different or the same as the ones holding the boot cowl to the firewall, depending on your philosophy. But be sure to leave enough room in the right spots, if you are using different screws. That flange gets covered in nut plates really fast.

I used countersunk everywhere, and I used the same screws (and nutplates) to secure the boot cowl and the main cowl. So the screws go through up to three pieces of metal, which all have to have matching countersinks / dimples. I think I used #8 in most places, and #10 top and bottom edges of the lower main cowl. I used #6 in the boot cowl. I hope I am not misremembering the numbers by one size all-round.

For me, the bottom edge screws are critical for keeping the cowl in position as I remove it for maintenance, to keep from massively scratching paint off. So they need to come out last. But my install is complicated with cowl flaps and a flexible air intake connection up front.

[Zzz]

You can use either nut plate if needs be, just be careful not to drastically weaken the nut plate when reaming material off it.

Do you mean by dishing out the mouth of a non-dimpled nutplate to make it into one? Or drilling through the wing holes when mounting?

If it's the former, why not just spend a few bucks to buy purpose-built dimpled nutplates?

Look at the clearance for the nutplate and screw above that bottom flange of the firewall. There can be a clearance issue there - larger size nut plates may not be a good choice, I used the smaller size

Yes, I'm aware of that tight fit below the station A tube. I'll fit what I can. There are "L" series nutplates that have a lower profile, of which i am easily able to fit the 10-32 version in there, so I imagine the 8-32 should fit. By "smaller size" what exactly do you mean? -06?